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”- , LIBRARY
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University

This is to certify that the
thesis entitled

RESIDUAL ACTIVITY OF AT-PLANTING .
APPLICATIONS OF IMIDACLOPRID AND
THIAMETHOXAM TO CONTROL COLORADO POTATO
BEETLE, LEPTINOTARSA DECEMLINEATA (SAY),
ADULTS

presented by
Eduardo Espitia-Malagén

has been accepted towards fulfillment
of the requirements for the

MS. degree in Entomology

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RESIDUAL ACTIVITY OF AT-PLANTING APPLICATIONS OF IMIDACLOPRID
AND THIAMETHOXAM TO CONTROL COLORADO POTATO BEETLE,
LEPTINO TARSA DECEMLINEA TA (SAY), ADULTS
By

Eduardo Espitia-Malagén

A THESIS
Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Entomology
2006

ABSTRACT
RESIDUAL ACTIVITY OF IMIDACLOPRID AND THIAMETHOXAM TO
CONTROL COLORADO POTATO BEETLE, LEPTINOTARSA DECEMLINEA TA
(SAY), ADULTS
By
Eduardo Espitia-MalagOn
This research was aimed to study the effect of soil type (organic vs.
mineral), application method (in-furrow vs. seed-treatment), water level (normal
vs. excessive), active ingredient (imidacloprid vs. thiamethoxam), and rate (full
vs. reduced) on the residual activity of at-planting treatments to control Colorado
potato beetles. Treated potatoes were grown in the field, in 2002 and 2003, at
two locations differing in soil type, and in greenhouse in 2004. Feeding
bioassays were conducted on a weekly basis until affected beetles proportion
was 20% or lower. Field studies Showed: sustained high efficacy of both active
ingredients until d 40 in mineral soils, and d 40 or less in organic soils; a steep
efficacy drop after that date; reduced imidacloprid activity due to resistant strain;
low evidence of cross resistance, late effect of resistant strain on thiamethoxam
in mineral soils and sustained effect in organic soils; no effect with thiamethoxam
activity at lower rate although imidacloprid activity was hampered; most factors
responded in organic soils. For greenhouse, when total affected beetles
recorded, the following was observed: sustained efficacy curve until d 80 even in
organic soil; low effect of excess water (partly affecting thiamethoxam activity).
When only dead beetles registered, efficacy was exhibited by thiamethoxam and;

there was a strong effect of organic soil. Special attention should be paid under

commercial conditions to potential hampering effects of all factors studied here.

Copyright by
EDUARDO ESPlTlA-MALAGON

2006

To the memory of two of my most
loved ones and never-ending
source of inspiration:

Ana Maria MalagOn
Guillermo Rondén Carvajal

May God have them at His side

ACKNOWLEDGMENTS

My deep gratitude to Dr Ed Grafius for all the guidance, encouragement
and teaching he gave me during my marvelous years at Michigan State. His way
of teaching me and letting me explore my own potential devoted to the discovery
of the science behind the integrated management of insect pests, I will never
forget. To my committee members: Dr. Doug Landis, M. Sc. Beth Bishop, Dr.
Dave Douches, Dr. Walt Pett and Dr. Larry Olsen for all their suggestions,
encouragement, patience and contributing guidance.

l greatly thank all the people in the Entomology lab: Adam Byme, Nick
Bramble, and the several students in the lab who were my technical support,
companion and co-leamers during my field and lab work with the interesting
Colorado potato beetle.

All that I learned at the Department of Entomology in MSU is my most
valuable professional treasure. To all my instructors (professors and staff of the
Department) my eternal gratitude for the invaluable teaching and learning
experiences I have gained.

I thank the support of the MSU Office of lntemational Students and
Scholars, the Academic and Professional Program for the Americas -LASPAU-,
the Fulbright Commission at Bogota (Colombia), and the Colombian Corporation
of Agricultural Research (Corpoica), for their support during almost three years to
make this dream of getting a Master Degree in an American Land Grant

University come true.

Thanks to my friends at Michigan State, you guys are part of my life now
and forever, and for you I have got a sense of how rich the human heart can be.

I want to deeply thank my wife and my daughter. Linda: you know you
have been here always; Laura Juliana: you have been the most precious
inspiration for my work at all times. Thanks for giving me your souls and love in

your material and spiritual support, I love you forever.

vi

TABLE OF CONTENTS

LIST OF TABLES .................................................................................. ix
LIST OF FIGURES ............................................................................... xiv
CHAPTER 1: INTRODUCTION .................................................................. 1
Potato production .......................................................................... 1
Colorado potato beetle ................................................................... 2

Host plant range ........................................................................... 6
Diapause .................................................................................... 7
Management ................................................................................ 8
Chemical control and insecticide resistance ...................................... 13
Neonicotinoid insecticides and control ............................................. 14
Insecticide resistance .................................................................. 14
Insecticide resistance management ................................................ 18
Factors affecting soil-applied neonicotinoids .................................... 19

Soil characteristics .............................................................. 19

Water ............................................................................... 21

Application method ............................................................ 22

Insecticide uptake ....................................................................... 23
Research objectives ..................................................................... 25

CHAPTER 2: THE EFFECT OF APPLICATION METHOD AND RATE ON THE
RESIDUAL ACTIVITY OF THIAMETHOXAM AND IMIDACLOPRID FOR THE
CONTROL OF THE COLORADO POTATO BEETLE,

Leptinotarsa decemlineata (SA Y) ................................................................... 28
Introduction ............................................................................... 28
Objectives ........................................................................ 32
Materials and Methods ................................................................. 33
Field Planting .................................................................... 33
Bioassays ......................................................................... 34
Beetle strains ..................................................................... 38

Data analysis ..................................................................... 38
Results and Discussion ................................................................ 41
Mineral soils site, year 2002 .................................................. 41
Weekly results .......................................................... 41

Treatment residual activity ........................................... 45
Overall season results at the mineral soils site in 2002 ....... 49

Organic soils site, year 2002 ................................................. 52
Mineral soils site, year 2003 .................................................. 54
Weekly results .......................................................... 54

Season results .......................................................... 63

Treatment residual activity ........................................... 67

Overall season results at the mineral soils
Site in 2003 .............................................................. 72

vii

Organic soils site, year 2003 ................................................. 74

Weekly results .......................................................... 74

Season results .......................................................... 79

Treatment residual activity ........................................... 85

Discussion Organic soils site 2003 ................................. 88

Overall analysis 2002 and 2003 ............................................. 88
Conclusions ................................................................................ 93

CHAPTER 3: THE EFFECT OF SOIL TYPE, EXCESS WATER, AND
APPLICATION METHOD ON THE RESIDUAL ACTIVITY OF
NEONICOTINOIDS FOR THE CONTROL OF THE COLORADO POTATO
BEETLE LEPTINOTARSA DECEMLINEATA (SAY) UNDER GREENHOUSE
CONDITIONS ........................ 95

Introduction .............................................................................. 95
Materials and Methods ............................................................... 101

Beetle strain ..................................................................... 101

Plants ............................................................................. 102

Watering ........................................................................ 1 02

Insecticides .................................................................... 105

Bioassays ...................................................................... 106

Recovery from intoxication ................................................ 108

Data processing and statistical analysis .............................. 108

Results and Discussion ............................................................ 109
Evaluation for total affected beetles .................................... 109

Weekly results ...................................................... 1 10

Overall results ...................................................... 122

Foliage consumption ......................................................... 126

Recovery of poisoned beetles ............................................. 129

Discussion of “total affected beetles’ variable ..................... 131

Dead beetles (no movement, darkened color ........................ 132

Weekly results ...................................................... 133

Overall analysis for the ‘dead beetles’ variable ............ 140

Treatment residual activity ............................................. 145

General discussion ............................................................... 148
Conclusions Greenhouse studies ............................................. 152
CHAPTER 4 OVERALL CONCLUSIONS ................................................. 153
CHAPTER 5: RESEARCH PROJECTION ............................................. 156
LITERATURE CITED .......................................................................... 160
APPENDIX 1 ..................................................................................... 168

viii

LIST OF TABLES

Table 1. Field treatments with neonicotinoid insecticides, made to mineral
(Montcalm Farm) and organic (Muck Soils Farm) soils for control of Colorado
potato beetle in 2002 ............................................................................ 35

Table 2. Field treatments with neonicotinoid insecticides, made to
mineral (Montcalm Farm) and organic (Muck Soils Farm) soils for control
of Colorado potato beetle in 2003 ............................................................ 36

Table 3. Main effects and interactions analyses on day 28 after planting

for Colorado potato beetles affected in bioassays by foliage from plants treated
with neonicotinoids at the mineral soils site in 2002 (linear regression

statistics for Type 3 Analysis)......... 43

Table 4 Main effects and interactions on day 42 after planting for

Colorado potato beetles affected in bioassays by foliage treated

with neonicotinoids at the Mineral soils Site in 2002 (linear regression

statistics for Type 3 Analysis .................................................................... 44

Table 5 Main effects and interactions on day 56 after planting for Colorado
potato beetles affected in bioassays by foliage treated with neonicotinoids

at the Mineral soils site in 2002 (linear regression statistics for

Type 3 Analysis) .................................................................................. 46

Table 6 Main effects and interactions on day 63 after planting for Colorado
potato beetles affected in bioassays by foliage treated with neonicotinoids

at the Mineral soils site in 2002 (linear regression statistics for

Type 3 Analysis) .................................................................................. 47

Table 7. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids

at the Mineral soils site in 2002 (linear regression statistics for

Type 3 Analysis) ................................................................................... 50

Table 8. Probit analysis for the evaluation of neonicotinoid insecticides
in the control of Colorado potato beetle for the mineral soils site 2002 ............. 51

Table 9 Main effects and interactions on day 28 after planting for Colorado
potato beetles affected in bioassays by foliage treated with neonicotinoids
in a organic soil site in 2002 .................................................................. 53

Table 10. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids in a
mineral soil site on day 26 after planting in 2003 (LR Statistics

for Type 3 Analysis) .............................................................................. 56

Table 11. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids

at the Mineral soils site on 33 days after planting in 2003 (linear regression
statistics for Type 3 Analysis ................................................................................ 58

Table 12. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids

at the Mineral soils site on 40 days after planting in 2003 (linear regression
statistics for Type 3 Analysis ................................................................................ 60

Table 13. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids

at the Mineral soils site on 47 days after planting (linear regression statistics

for Type 3 Analysis) ............................................................................................. 62

Table 14. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids

at the Mineral soils site on 54 days after planting in 2003 (linear regression
statistics for Type 3 Analysis) .............................................................................. 64

Table 15. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids

at the Mineral soils site in 2003 (linear regression statistics for

Type 3 Analysis) .................................................................................................. 65

Table 16. Probit analysis for the evaluation of neonicotinoid insecticides, at two
rates and for two beetle strains in the control of Colorado potato beetle,
Montcalm 2003 .................................................................................................... 68

Table 17. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids at the
organic soils site on day 26 after application in 2003 (linear regression

statistics for Type 3 Analysis) .............................................................................. 75

Table 18. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids at the
organic soils site on day 33 after application in 2003 (linear regression

statistics for Type 3 Analysis) .............................................................................. 78

Table 19. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids at the
organic soils site on day 40 after application in 2003 (linear regression

statistics for Type 3 Analysis ................................................................................ 80

Table 20. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids at the
organic soils site on day 47 after application in 2003 (linear regression

statistics for Type 3 Analysis) ............................................................................. 81

Table 21. Main effects and interactions analyses for Colorado potato beetles
affected in bioassays by foliage from plants treated with neonicotinoids at the
organic soils site overall season 2003 (linear Regression Statistics for

Type 3 Analysrs)82

Table 22. Probit analysis for the evaluation of neonicotinoid insecticides, at two
rates and for two beetle strains in the control of Colorado potato beetle, in the
organic soils site 2003 ......................................................................................... 86

Table 23. Greenhouse treatments with neonicotinoid insecticides, for control
of the Colorado potato beetle ............................................................................ 107

Table 24. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and

active ingredient, on day 24 after application (linear Regression Statistics For
Type 3 Analysis) ................................................................................................ 112

Table 25. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 31 after planting (linear Regression Statistics For

Type 3 Analysis) ................................................................................................ 113

Table 26. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 31 after planting (linear Regression Statistics For Type 3
Analysis) ............................................................................................................ 1 15

Table 27. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 45 after planting (linear Regression Statistics for Type 3
Analysis) ............................................................................................................ 1 16

Table 28. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 52 after planting (linear Regression Statistics for

Type 3 Analysis) ................................................................................................ 118

xi

Table 29. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 59 after planting (linear Regression Statistics for

Type 3 Analysis) ................................................................................................ 120

Table 30. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and

active ingredient, on day 66 after planting (linear Regression Statistics for

Type 3 Analysis) ................................................................................................ 121

Table 31 . Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 80 after planting (linear Regression Statistics for

Type 3 Analysis) ................................................................................................ 123

Table 32. Effect of neonicotinoid insecticides on percentage affected beetles
under differing conditions of soil, water level, application method and active
ingredient (linear regression statistics For Type 3 Analysis) .............................. 124

Table 33. Effect of neonicotinoid insecticides on mortality percentage of beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 24 after planting under greenhouse (linear regression
statistics for Type 3 Analysis) ........................................................................... 135

Table 34 Effect of neonicotinoid insecticides on mortality percentage of beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 31 after planting under greenhouse (linear regression
statistics for Type 3 Analysis) ............................................................................ 136

Table 35 Effect of neonicotinoid insecticides on mortality percentage of beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 38 after planting under greenhouse (linear regression
statistics for Type 3 Analysis) ........................................................................... 138

Table 36. Effect of neonicotinoid insecticides on mortality percentage of beetles
under differing conditions of soil, water level, application method and active
ingredient, on day 45 after planting under greenhouse (linear regression
statistics for Type 3 Analysis)... 139

Table 37 Effect of neonicotinoid insecticides on mortality percentage of beetles
under differing conditions of soil, water level, application method and active
ingredient, day 52 after planting under greenhouse (linear regression

statistics for Type 3 Analysrs) 141

xii

Table 38 Effect of neonicotinoid insecticides on mortality percentage of beetles
under differing conditions of soil, water level, application method and active
ingredient, day 59 after planting under greenhouse (linear regression

statistics for Type 3 Analysis) ............................................................................ 142

Table 39. Effect of neonicotinoid insecticides on mortality percentage of beetles
under differing conditions of soil, water level, application method and active
ingredient (linear Regression Statistics for Type 3 Analysis)................. ......143

Table 40. Probit analysis for the evaluation of neonicotinoinds for control of
the Colorado potato beetle under greenhouse conditions. Mineral soils only....147

xiii

LIST OF FIGURES
Figure 1. lmidacloprid and Thiamethoxam structure ................................... 15

Figure 2. Percent of Colorado potato beetles affected by neonicotinoid
insecticide-treated foliage from two experimental sites in 2002 ...................... 42

Figure 3. Log ‘days after planting’ probit lines for affected Colorado potato
beetles when fed with neonicotinoid-treated foliage from the mineral soil site
in 2002 ............................................................................................... 48

Figure 4. Percent of Colorado potato beetles affected by neonicotinoid
insecticide-treated foliage from the mineral soils experimental site in 2003 ....... 55

Figure 5. Log “days after planting’ probit lines for affected Colorado potato
beetles when fed with neonicotinoid-treated foliage from the mineral soil site
in 2003 ............................................................................................. 71

Figure 6. Percent of Colorado potato beetles affected by neonicotinoid
insecticide-treated foliage from the organic soils experimental site in 2003 ...... 77

Figure 7. Log ‘days after planting' probit lines for affected Colorado potato
beetles when fed with neonicotinoid-treated foliage from the organic soil site
in 2003 ............................................................................................... 89

Figure 8. Accumulative weekly amounts of water applied on two soil types for
potato development for the evaluation of neonicotinoid insecticides in
greenhouse ..................................................................................... 103

Figure 9. Percentage affected beetles by neonicotinoid insecticides
from foliage treated in greenhouse under differing conditions of soil,
watering, and application method ........................................................... 111

Figure 10. Percentage consumption of neonicotinoid-treated foliage
treated in greenhouse under differing conditions of soil, watering, and
application method, by Colorado potato beetles ........................................ 128

Figure 11. Percentage recovery of adult Colorado potato beetle on day

14 after being poisoned by neonicotinoid-treated foliage from

greenhouse under differing conditions of soil, watering, and

application ........................................................................................ 130

Figure 12. Percentage beetle mortality by neonicotinoid insecticides from

foliage treated in greenhouse under differing conditions of soil, watering,
and application method ........................................................................ 134

xiv

Figure 13. Log ‘days after planting’ probit lines for mortality of Colorado potato
beetles when fed with neonicotinoid-treated foliage from greenhouse under
differing conditions of watering, soil, and application method ........................ 146

Figure 14. Root growth of potted potatoes grown under greenhouse conditions
to study residual activity of neonicotinoid insecticides in mineral soil
(A) and organic soil (B) ....................................................................... 150

CHAPTER 1:
INTRODUCTION

Potato production

Potatoes, Solanum tuberosum L., have a long history as a cultivated food
plant. This crop was introduced from its center of origin in South America, to
different areas worldwide (Hijmans and Spooner 2001). Potato is a major food
source and is fourth following wheat, maize and rice as the most produced crops
in the world (Sattaur 1989). Potatoes are an important part of a typical American
diet, with a consumption rate of 64 kilograms per person per year, exceeded only
by wheat flour (Anon. 2003a). The United States is fourth in the world in potato
production (Anon. 2004d) with annual yields of 21.0 million metric tons from
about 445,000 ha planted. In 2001, the US. potato crop provided $ 2.5 billion in
income for farmers, representing 15% of all annual income in the vegetable
sector (Anon. 2003a). Michigan potato production corresponds to an annual
average of 18,600 ha harvested (Anon. 2003b), yielding 629,495 metric tons in
2002 (Anon. 2003c) and 662,245 metric tons in 2003 (Anon. 2004a). In 2002
Michigan produced 3 % of the total US. potato crop (Anon. 2003c) and ranked
10th in the nation in potato production. Potato production adds almost $93 million
to the state’s economy each year and it is one of the state’s top 10 crops (Anon.

2004a).

Colorado potato beetle

The Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera:
Chrysomelidae), is the most devastating defoliator of potatoes worldwide
(Radcliffe 1982, Jacques 1988, Hare 1990, Ferro and Boiteau 1993) and is the
most important arthropod pest of potatoes in North America. It is present
throughout the US. except California, but it is most economically important in the
northern regions of the midwestern and eastern US. (Ferro and Boiteau 1993).
Two factors have established Colorado potato beetle as a key pest of potatoes:
1) its highly voracious feeding habits and 2) its ability to rapidly develop
insecticide resistance. If uncontrolled, Colorado potato beetle can completely
defoliate a potato field prior to tuber initiation (Hare 1990). By the early 1990s,
Colorado potato beetle had developed resistance to virtually all the insecticides
used to control it (Bishop and Grafius 1996).

The Colorado potato beetle probably originated in Mexico (Hare 1990,
Casagrande 1987) and from there it invaded potato production areas in North
America and, later on, in Eurasia. Potato crop damage was first reported in 1859
in Nebraska, apparently after a host shift to potatoes (Jacques 1988,
Casagrande 1987). Populations in southern Texas, Arizona and Mexico still do
not accept potatoes as a host (Hare 1990). The pest rapidly spread in the US.
after shifting to the cultivated potato as a host plant and reached the US. East
Coast from Connecticut to Virginia by 1874. It entered Canada in 1870 (Ferro

and Boiteau 1993) and all of eastern Canada was infested by 1878 (Capinera

2001). It gradually spread southward, reaching Florida and Louisiana by 1900
(Capinera 2001). It was first introduced into Europe in 1875 (Jacques 1988),
became established in Germany in 1914 (Jacques 1988), in France in 1921
(Webber and Ferro 1994), in Eastern Europe in the mid to late 19505 (Radcliffe
1982). It is also found throughout the former Soviet Union (Jacques, 1998). The
Colorado potato beetle was reported in Iran in 1984 (Anon. 2004b) and reached
China in 1992 (Xie and Zhang 2002, Jacques 1998).

The Colorado potato beetle’s life cycle on potatoes is completed in as little
as 3 wks under optimal conditions (Ferro and Boiteau 1993). Optimal
temperature range for development is 25 — 33 °C, with a threshold of 10 °C (Hare
1990). Overwintered adults emerge in mid to late spring, generally at the time
potato plants are emerging. ln continuous potato cropping systems, the beetles
may overwinter directly in the field (Caprio and Grafius 1993); however, they may
also arrive from adjacent cultivated fields or distant wooded areas (Ferro and
Boiteau 1993). After about 1 wk of feeding, mating occurs and females lay eggs
in clusters of 25 — 40 on the underside of the foliage (Ferro and Boiteau 1993).
Eggs are anchored to the leaf surface with a small amount of yellowish adhesive
(Capinera 2001). Each female can lay up to 450 eggs, which hatch in 3 -10 d
depending on temperature (Ferro and Boiteau 1993). All the eggs in a cluster
hatch more or less simultaneously (Hare 1990); the newborn larvae consume the
egg cases and after 1-2 d disperse and start feeding on foliage (Ferro and

Boiteau 1993).

Larvae go through four instars. The first instars are bright red with black
heads, pronotums and legs and older instars are light orange with black markings
(Mahr et al. 1995). At the end of each stadium, from first to fourth, larvae will
reach average lengths of 2.6, 5.3, 8.5 and 15 mm, respectively (Capinera 2001).
Leaf consumption by larvae is estimated at 35 - 45 cm2 (Capinera 2001). The
first three instars cause minimal damage to the foliage, while the fourth instar is
responsible for 77% of foliar consumption by the larvae (Hare 1990, Ferro and
Boiteau 1993). The duration of the larval stage can be as short as 8 -11 d at 30
°C or as long as 31-35 d at 13 °C (Ferro and Boiteau 1993). Mature larvae stop
feeding, fall to the ground, and burrow to a depth of 5 - 15 cm to pupate
(Hartcourt 1971, Mahr et al. 1995).

Colorado potato beetle pupae are oval shaped and orange, about 9.2 mm
long and 4.4 mm wide. After about 6 d pupae transform into adults, which spend
their first 3 — 4 d in the soil before digging to the surface. Total time spent in the
soil by the mature larva, the pupa and the newly emerged adult varies from 8.8 to
22.3 d at 28 to 15 °C, respectively (Capinera 2001).

Adults of this generation are referred to as “summer adults”, and their
emergence in the midwestern U.S begins in July. These beetles feed more than
the overwintered adults and can do extensive foliar damage in a very short time if
populations are high. Foliar consumption by adults is 7-10 cm2 per day (Capinera
2001). They start laying eggs approximately 1 wk after emerging from the soil. At
the end of the season, beetles burrow into the soil for ovenlvintering at depths of

8 to 13 cm (Hare 1990). Colorado potato beetle larvae and adults feed on stem

tissue and even on exposed tubers if all foliage has been consumed (Capinera
2001).

In the midwestern US, Colorado potato beetle populations complete one
to three generations in a season, although the climate in the northernmost
portion of its range usually allows only a single generation per year. In Maryland
and Virginia three generations per year may occur, but many second generation
adults go into diapause rather than laying eggs for a third generation (Capinera
2001). In the lower peninsula of Michigan one or two (rarely three) generations
occur each season.

Males and females mate more than once and with different partners: at
least three mating events are required to fill the female’s sperrnatheca completely
and multiple mating may enhance fertility (Boiteau 1988, Alyokhin and Ferro
1999). In studies of sperm utilization the importance of the second male in
fathering offspring has been reported as 32-53% (Boiteau 1988) and 72% with a
range of 16.7 - 100% (Alyokhin and Ferro 1999).

Colorado potato beetles disperse by walking, flight, and human-assisted
transport. Flight is of variable importance among populations and can be of three
types: short-range or trivial flight, long-range or migratory flight and the diapause
flight (Hare 1990). Each one of these serves different purposes. Short range
flight enhances mating and food resource use; migratory flight enables the insect
to find new areas when searching for food; and diapause flight allows beetles to
locate tall vegetation and wooded areas contiguous to the crops, which are then

used as diapausing sites (Weber and Ferro 1994). Flight capability increases the

area of dispersion of Colorado potato beetles up to several km (Radcliffe 1982,

Weber and Ferro 1994).

Host plant range

The Colorado potato beetle is an oligophagous insect that feeds
exclusively on Solanaceae, and primarily on about 20 species in the genus
Solanum (Radcliffe 1982, Hare 1990). Different adaptation degrees to host in the
United States are reflected in differential beetle body weight as observed in
beetles that have fed on potatoes compared with those feeding on less suitable
hosts (Hsiao 1978). The major factors determining the host plant range of this
insect are: 1) feeding behavior, related to the insect’s response to plant
compounds, 2) host plant location, and 3) host geographical distribution (Hare
1990). In most of North America, in addition to potatoes, Colorado potato beetle
feeds on its original hosts: buffalo bur (Solanum rostratum Dunal), silver-leafed
nightshade (Solanum elaeagnifolium Cav.) and narrow-leaved solanum
(Solanum angustifolium Mill). Most-preferred hosts also include climbing
nightshade (Solanum dulcamara L.) and the cultivated eggplant (Solanum
melongena L.) (Hare 1990). Horsenettle (Solanum carolinense L.), Solanum
tn'quetrum Cav., Physalis spp., and tomato (Lycopersicon esculentum Mill.) are
also reported as important hosts (Jacques 1988).

Preference towards these hosts varies geographically. ln north eastern
US. and central states potato is the main host, in southern states (Colorado,

New Mexico, Oklahoma, Texas) buffalo bur is the main wild host, followed by

silver-leafed nightshade. This pattern may vary: in Arizona the predominant host
is silver-leafed nightshade and horsenettle is an important host in central and
southern states. In Mexico, it is found mainly on buffalo bur and narrow-leaved
solanum. In Michigan, an important alternative host is horsenettle (Mena-
Cobarrubias et al. 1996). A host in Central America (Honduras) is lanoeleaf
nightshade (Solanum Ianceolatum Cav.), on which the insect completes two
generations per year at that latitude (Caflas et al. 2002).

The diversity in host range as the insect colonized new areas represents a
broadening of range rather than shifts from previous to new hosts (Hare 1990),
but populations in the US. have evolved to prefer potato as a host (Hsiao 1978).
Alkaloids and glycoalkaloides are the crucial semiochemicals determining host
suitability. Due to the insect’s ability to adjust to locally abundant hosts, its host

range continues to expand within the Solanaceae (Hare 1990, Capinera 2001).

Diapause

Aestival and reproductive (hibernal) diapause occurs in Colorado potato
beetle (Tauber et al. 1988). Univoltine traits and changes in food are related to
aestival diapause. Late summer adults and second-generation adults that
emerge in mid-August or later, delay development of the reproductive system
and their flight muscles are underdeveloped (Ferro and Boiteau 1993). When the
beetles are ready to overwinter, they fall to the ground or search for
ovenNintering sites and then enter diapause in the soil. The most important

factor that induces diapause is photoperiod; Colorado potato beetle is a long day

insect with a critical photoperiod of ca. 15 h (De Wilde & de Loof, cited by Voss et
al.1988). In addition to photoperiod, cool temperature and poor food quality also
induce diapause (Hare 1990). These factors together trigger the physiological
preparation for cold winter conditions. At higher temperatures, sensitivity to
photoperiod is reduced; thus, southern populations generally require shorter
photoperiod as a diapausing stimulus. Physiological condition of potato foliage
also affects diapause induction; senescing foliage can induce diapause and
reduce beetle reproductive functions (Hare 1990). Less suitable hosts also
increase the incidence of aestival diapause (Hare 1983). If a combination of the
above factors is in place, substantial variation in the proportion of beetles

entering diapause occurs within generations and within populations (Hare 1990).

Management

Crop defoliation by Colorado potato beetle may occur even before tuber
formation, leading to total yield loss if no control measure is taken (Hare 1990).
Susceptibility to defoliation varies with cultivar and growth stage, but in general,
plants just beginning to bloom and to form and fill tubers are most susceptible to
yield reduction by Colorado potato beetle (Ferro and Boiteau 1993). Therefore,
Colorado potato beetle management programs emphasize the need for early and
midseason pest suppression with higher beetle densities tolerated later in the
season.

Integrated management practices are recommended for Colorado potato

beetle control. These practices include: monitoring, judicious use of synthetic

insecticides, and cultural, physical and, to a limited extent, biological control.
Throughout the history of Colorado potato beetle control, many insecticides have
been used heavily, resulting in severe insecticide resistance problems
(Casagrande 1987).

The backbone for any management program is monitoring. Through
monitoring, Colorado potato beetle population levels can be estimated in the
field, thence monitoring constitutes the main tool for decision-making regarding
insecticide use. Both pest density and damage can be quantified.

A key element for Colorado potato beetle management is the
understanding the pest’s life cycle and to know the factors that affect it,
especially temperature. Changes in temperature induce variation in stage length.
Predicting pest development is based on quantification of degree-days (DD). In
the Midwestern US, a base of 10 °C is commonly used (Mahr et al. 1995).

Monitoring consists of weekly scouting of five adjacent plants at 5 - 40
sites throughout the field. Total plants sampled in a field would be 50 — 200,
covering transects of 90 m that intersects 50-100 rows (Capinera 2001, Mahr et
al. 1995, Ferro and Boiteau 1993). Scouting should start at plant emergence,
examining leaf undersides for adults and egg masses. Initial infestation is often
detected on field edges. Detected egg masses sites should be flagged and the
hatch rate monitored. In the following weeks, scouting for larvae is important to
detect young larvae which are more sensitive to insecticides. Head width is the
instar-identifying character. Average head widths are 0.5, 1, 1.5, and 2 mm for

first to fourth instars, respectively. Fourth instars need to be controlled before

they drop to the ground for pupation. Summer adults can be detected by
scouting as well. However, they can cause severe foliar injury and are more
difficult to control than the overwintered adults. Newly emerged (less than 2 wks
old) summer beetles have clear hind wings compared with the smoky orange
wings with bright orange veins of ovewvintered adults or older summer adults
(Mahr et al. 1995, Ferro and Boiteau 1993).

Crop rotation is a cultural practice that reduces and delays beetle
infestations in the spring. Some beetles may overwinter for a year or more
(Biever and Chauvin 1990, Peferoen et al. 1981, Ushatinskaya 1978 cited by
Webber and Ferro 1994) or arrive from other fields after diapause (Weber and
Ferro 1994). Early or very late potato varieties are recommended to reduce the
effect of second generation beetles (Weber and Ferro 1994) although late
varieties are not recommended where three insect generations occur (Ferro and
Boiteau 1993). Trap crops can also be used; they consist of early-planted
potatoes (for trapping ovenrvintered adults), or late-planted potatoes (for trapping
summer adults at the end of the season). Beetles concentrated on either type of
trap crop can be confined and controlled by physical or chemical methods
(Weber and Ferro 1994).

Several physical control techniques are effective on Colorado potato
beetle but their feasibility is variable, therefore many of them have not been used
commercially. Plastic lined trenches placed along the row sides to capture
beetles seeking overwintering sites have shown efficiency of 95% (Misener et al.

1993). Disturbing ovenlvintering sites significantly reduces adult survival (Milner

10

et al. 1992). Use of steam has shown to inflict up to 50% mortality on beetles in
field plots (Pelletier et al. 1998). Flaming is suitable for early-season control of
adults on plants, especially for plants < 3 — 4 cm tall. Flames must be directed to
the plant at an angle at the base of the plants, adjusting flame, distance and
tractor speed to avoid damage to the plants. Mortality of beetles can reach more
than 80% (Lague et al. 1999). Flaming may reduce egg hatch by 30% (Kuepper
2003) and it is comparable in cost to insecticide application. This method can
also be used at the end of the season to kill both vines and beetles. Crop
vacuums, which remove up to 70% of small larvae and adults, have also been
used. The removal rate is variable and has been reported to be 34% - 48% of
adults, 9 - 42 % of small larvae, and 29 — 50% of large larvae. Many larvae drop
to the soil and are not picked up by the equipment (Hodik et al. 1999, Boiteau et
al. 1992). The high cost of vacuuming makes its use limited. Combination of
tactics, such as enhancing the value of crop rotation by means of early trap-
cropping combined with diapause disruption may provide the greatest benefits
from cultural control (Weber and Ferro 1994).

No Colorado potato beetle control program relies heavily on biological
control. However, several biocontrol agents have been studied and their use may
be more important in the future. Predators, parasitoids and entomopathogenic
microorganisms have been identified as natural enemies, but none has been
shown to consistently suppress beetle populations (Capinera 2001, Hare 1990,
Mahr et al1995). Predators: The spined soldier bug (Podisus maculiventn's

Say); and, the two spotted stinkbug, (Peril/us bioculatus Fabr.), are effective for

11

egg and larval control, they can reach 80% control when an appropriate ratio
predatorzprey is established (Capinera 2001). The pink spotted lady beetle
(Coleomegilla maculata De Geer) feeds extensively on Colorado potato beetle
eggs and early instars (Ferro 1994). Carabids are reported as predators on
Colorado potato beetle. Lebia grandis Hentz is the best known. Adults consume
ggs and small larvae, and Lebia larvae parasitize prepupal stages. It appears to
be one of the most promising endemic natural enemies (Groden 1989).

Parasitoids: The tachinid flies, Myiopharus doryphorae (Riley) and M.
abenans (Townsend), are larval-pupal parasitoids. M. doryphorae has been
reported to kill about 50% of the season’s last beetle generation (Casagrande
1987). The South America-originated wasp Edovum pultlen’ Grissell is a
parasitoid of Colorado potato beetle eggs but its limitations for ovenivintering and
adult food sources prevent use in commercial situations. Nevertheless,
differences in its parasitism rate found at low temperatures show a potential for
future use in temperate regions (Acosta and O’Neil 1999).

Neither predators nor parasitoids are practical for high levels of biological
control under commercial conditions: multiple releases of E. puttlen' and the
spined soldier bug, despite effective control, are not feasible for large scale use
due to high costs (Tipping et al. 1999).

Microorganisms: The entomopathogenic fungus, Beauven'a bassiana
Bals., is the most important of the identified fungi with activity on Colorado potato
beetle. Factors related to B. bassiana deployment techniques and synergistic

effects with imidacloprid, antifeedants and Bacillus thuringiensis var. tenebn‘onis

12

have been studied and show compatibility and enhanced activity of the fungus
(Fernandez et al. 2001, Furlong and Groden 2001, Furlong and Groden 2003).
No adverse effects on biodiversity from B. bassiana applications have been
found (Lacey et al.1999) but incompatibility with the fungicides chlorothalonil,
mancozeb, maneb, thiophanate-methyl, metalaxyl+mancozeb and zineb has
been reported (Jaros et al. 1999, Todorova et al. 1998). In contrast, the use of B.
t. tenebn'onis, in foliar sprays is recommended and effective to control early
instars under commercial conditions and reduce resistance build-up to chemical
insecticides (Mahr et al. 1995). B. t. tenebn'onis can be used for larval control at
150 DD as an early spray or after suppressing beetle populations by other means

(Mahr et al. 1995).

Chemical control and insecticide resistance

Because the Colorado potato beetle has developed resistance to most
insecticides used for its control; only the novel neonicotinoid insecticides exhibit
satisfactory control in areas where insecticide resistance to other chemical
groups is widespread. Two neonicotinoids, imidacloprid and thiamethoxam, are
registered in the United States for use against Colorado potato beetle and
currently constitute the best tools available for the control of this pest. In 1995,
imidacloprid was the first neonicotinoid to be registered for use in Colorado
potato beetle control; thiamethoxam was registered in 2001. Other
neonicotinoids are currently under evaluation. For Colorado potato beetle control

imidacloprid and thiamethoxam are recommended as at-planting applications

13

(Boiteau et al 1997) and, to a lesser extent, foliar sprays. Due to their systemic
and residual activity, a single at-planting application of neonicotinoids may
eliminate the necessity for additional chemical control of beetles during the
season; Boiteau et al. (1997) reported lasting control from a soil application at a

rate of 0.03 g/m of imidacloprid.

Neonicotinoid insecticides and control

Neonicotinoids have been the fastest growing chemical class of
insecticides world-wide in terms of volumes produced and use. This success has
been favored by characteristics like broad-spectrum activity, low application
rates, excellent uptake and translocation in plants, a new mode of action, and a
favorable safety profile (Maienfisch et al. 2001). Broad commercial use of these
insecticides has increased in recent years world-wide for control of sucking and
chewing insect pests.

Of the three sub-classes of neonicotinoids known up to date, imidacloprid
(Figure 1) belongs to the chloronicotinyls and thiamethoxam (Figure 1) to the
thianicotinyls. The first neonicotinoid under commercial use, imidacloprid, was
introduced in Japan and Europe in 1990 and first registered in the US. in 1992.

Currently, it may be the most widely used insecticide globally (Ware, 2000).

Insecticide Resistance

The Colorado potato beetle ranks fourth among insects with the most

reports of insecticide resistance worldwide (Anon. 2004c). Historically,

14

\ ll
N
C1 N '-'— NO 2
lmidacloprid
"mo,
stJLN’CHa
CI \
‘1. w
Thiamethoxam

Figure 1. lmidacloprid and Thiamethoxam structure

15

populations of Colorado potato beetle have developed resistance to every
compound used for control (Bishop and Grafius 1996). However, chemical
control remains a necessary component for Colorado potato beetle management
program. In the 1900s, differences in rate effectiveness of Paris green (copper
acetoarsenite), the earliest product used for chemical control of Colorado potato
beetle, provided the first historic signal of insecticide resistance. Colorado potato
beetle was the target of the development of arsenical insecticides and prompted
the first large-scale use of insecticide in an agricultural crop for its control
(Capinera 2001, Hare 1990). With the use of these insecticides in the 1940s the
impact of the Colorado potato beetle was temporarily reduced (Radcliffe 1982).
Signs of resistance to arsenicals appeared in Long Island, New York, just
before the introduction of DDT, the first chlorinated insecticide (Bishop and
Grafius 1996). Resistance to different organochlorine insecticides was
widespread by the late1950$ and early 60$ (Bishop and Grafius 1996). At that
time, organophosphate and carbamate insecticides began to be used for
Colorado potato beetle control. After great initial control, these products followed
the fate of organochlorine insecticides, inducing pest resistance. Resistance to
azinphosmethyl, oxamyl, phosmet and other organophosphates and carbamates
became widespread in the northeastern US. by the 19803 and was confirmed in
other regions of the US. during that decade. Resistance to pyrethroids,
registered in the late 19703 and 1980s, evolved extremely rapidly. Before the
mid-1980s, pyrethroids did not provide control of Colorado potato beetle

populations in the northeastern US. In the 1990s, Colorado potato beetle

16

resistance to all available insecticides was so prevalent that virtually no
insecticide was effective for controlling the pest in most production areas of the
eastern US. Based on this history Colorado potato beetle can be expected to
develop resistance to any new insecticide deployed against it (Bishop and
Grafius 1996).

Resistance has already been reported for imidacloprid in the United States
including field populations from Long Island NY (Olson et al. 2000, Zhao et al.
2000), and Delaware (Bishop et al. 2003). Populations selected in the laboratory
show high levels of potential resistance: a NY strain has developed more than
100-fold resistance (Bishop et al. 2003) and a Michigan strain has developed,
more than 100-fold resistance (Bishop et al. 2003). In Canada, no resistance to
imidacloprid had been found by 2002 (Brent and Gavloski 2002) but susceptibility
level decreased in some populations during the period 1998 - 2001 (Hilton et al.
2002). No imidacloprid resistance has been found in Poland (Wegorek 2002).

Low levels of resistance (<20x) to thiamethoxam also occur in Colorado
potato beetle populations from Long Island, NY (Bishop et al. 2003), Delaware
and a Massachusetts strain that was 9-fold resistant to thiamethoxam (Bishop et
al. 2003, Grafius et al. 2004). Since thiamethoxam had not been used at any of
these sites, this low level resistance is likely due to cross resistance
betweenthiamethoxam and imidacloprid. Since imidacloprid and thiamethoxam
are chemically related (Figure 1), the chance for cross-resistance is high. Unlike
most neonicotinoids, which bind on nicotinic—acetylcholine receptors,

thiamethoxam has low affinity to these receptors (Nauen et al. 2003). However,

17

thiamethoxam’s metabolite, clothianidin, acts with high affinity on the imidacloprid
binding site like other neonicotinoids, making thiamethoxam not an option for
insecticide rotation with imidacloprid (Nauen et al. 2003). Cross-resistance
between the two insecticides is less than 1:1 but is high enough to suggest that
alternating between the two insecticides will not be an effective resistance

management tactic (Grafius et al. 2004).

Insecticide Resistance Management

Intensive use of insecticides for control of Colorado potato beetle places
great selection pressure on pest populations. With high resistance levels
occurring in many areas, choosing the appropriate insecticide is very important.
Techniques must be adopted wisely, when using chemical control, to reduce the
risk of resistance build-up. Insecticide rotation is very important; insecticides
from the same chemical group should not be used on more than one Colorado
potato beetle generation in the same season. Since the chance of cross-
resistance between imidacloprid and thiamethoxam is high, rotation must be
done with other products. Crop rotation is the most effective option to reduce
insecticide selection pressure, because other selection factors would take place
like migration in search for food.

Maintaining susceptible populations in refugia can lower resistance build-
up. With most resistance management programs relaying on the refugia/high-
dose strategy (Alyokhin and Ferro 1999), the sperm mix resulting from the

polygamous habits, enhances the occurrence of heterozygotes in the offspring

18

coming from the mating between resistant beetles from the main crop area and
susceptible beetles from the refugia. However the chance for matings taking
place among resistant mates is high, because Colorado potato beetles often
mate with conspecifics emerging close to the pupation site (Gould 1998 cited by
Alyokhin and Ferro 1999). Early-planted refugia and the slow growth and
development of resistant larvae may enhance the cross mating of susceptible
and resistance populations (Alyokhin and Ferro 1999).

Criteria used to choose an insecticide include: degree of beetle resistance
to the insecticide, stage of the pest to be controlled, and population density.
Most insecticides are not effective on eggs, whereas larvae are easily controlled.
When resistance is present or suspected, sprays to control larvae should start
when 15 -- 30 % of egg masses have hatched. Other products may be used for

additional sprays, especially Bacillus thun'ngiensis, as described above.

Factors affecting soil-applied neonicotinoids

Soil characteristics

Because imidacloprid and thiamethoxam are recommended for at-planting
applications for Colorado potato beetle control, the nature and characteristics of
the soil are an important influence on compound activity. Soil (topography,
physical conditions) interacting with water, can affect insecticide movement and
half-life because the organic matter content and physical and chemical
characteristics interact with the insecticide’s intrinsic characteristics (chemical

and biological) (Krohn and Hellpointner 2002).

19

The available amount of insecticide in the soil after application may vary
since insecticides can interact with soil characteristics. In aerobic soil
metabolism studies, the 1st order half-life of imidacloprid averaged 156 d (Krohn
and Hellpointner 2002). Field dissipation studies conducted in the US. under
crop conditions show a half-life for imidacloprid of 7 - 146 d. In studies in Europe
a half-life range of 83 to 124 d has been reported (Krohn and Hellpointner 2002).
Ground cover is one determining factor for half-life of imidacloprid (48 d for soil
covered with grass vs. 190 d without cover) (Scholz and Spiteller 1992) in
greenhouse studies. In field studies, on bare soil in Europe, the half-life
averaged 174 d (Krohn and Hellpointner 2002). Dissipation studies in soil
indicate that during its lifetime, imidacloprid remains in the upper 30 cm of the
soil and is only partially translocated into soil beyond that depth (Krohn and
Hellpointner 2002). Active imidacloprid sorption in the soil, especially by organic
carbon, has been proposed to be the cause of the low mobility of the compound
in the soil despite its high water solubility. However, in aqueous solutions the
persistence of imidacloprid is also affected by pH (higher half life of the
insecticide under alkaline compared with acidic conditions), and type of
formulation (powder formulations have higher half life than liquids) (Sarkar et al.
1999). Organic soils may reduce insecticide activity compared with mineral soils
because imidacloprid binds to the organic fraction of the soil (Krohn and
Hellpointner 2002). A higher potential for binding in organic soils is supported by
the finding that humic acids have higher sorption capacities than Ca-clay

minerals (Liu et al. 2002). Also the differential effect of organic matter aging on

20

half-life has been confirmed. Aged cow manure induced longer persistence of
imidacloprid in soils than recently-applied manure (Anon. 2006). Many studies
indicate that imidacloprid does not accumulate in the soil over the long term
(Krohn and Hellpointner 2002); however, half lives of more than 365 d have been
reported in the US. in crop situations (Anon. 2001)

Thiamethoxam has been studied mostly in laboratory conditions. In
laboratory soils, thiamethoxam degrades at moderate to low rates. Its half-life
ranges from 34 to 75 (I under favorable growing conditions but may increase by a
factor of three under unfavorable growing conditions (Maienfisch et al. 2001).
Also in soil, thiamethoxam's half life is reported to be 101 d; photolysis in soil: 47
--54 d and in water 3 — 4 d (Anon. 2003d). Sensitivity studies for pH show that
thiamethoxam is very stable at pH 5 and pH 7, but labile at pH 9 (Maienfisch et
al. 2001).

Water

Reduction in activity of imidacloprid and thiamethoxam by excessive
rainfall or irrigation is a concern because potatoes are generally grown on sandy
loam or loamy sand soils. Water solubility for imidacloprid, 0.5 gll at 20°C and pH
7, is considered moderate and places the compound as a hydrophilic substance
and medium in mobility in soil (Krohn and Hellpointner 2002). Water solubility for
thiamethoxam is 4.1 gll and is ranked as medium to highly mobile in soil (Anon.
2003d). These products could potentially move through sensitive soil types
including porous, gravelly, or cobble soils, depending on rain fall and irrigation

practices (Jenkins 1994). Insecticide leaching also depends on the formulation

21

components (Gupta et al. 2002) and pH of aqueous solution (Sarkar et al. 2001).
Local soil compactness and water flow may extend leaching beyond the root
zone (upper 30 cm) and could lead to a reduction in residual activity of these
products (Gupta et al. 2002).

The effect of water flow in soil on neonicotinoid performance has been
demonstrated by the differential perforrnanoe of imidacloprid depending on the
specific irrigation system. Enhanced whitefly control in poinsettias was reported
for imidacloprid when used in sub-irrigated pots (van lersel et al. 2001). A
combination of application method (in irrigation water) and irrigation method
(water layer on bench) improved plant insecticide uptake, providing uniform
control throughout the growing season and improving whitefly control in
poinsettias compared to other irrigation systems.

Application method

For at-planting applications with imidacloprid and thiamethoxam in
potatoes, two methods are used: seed treatment and in-furrow application.
Potato seed pieces can be treated with insecticide prior to placement in the
furrow, this method is recommended for some soil pests. Genesis ®
(imidacloprid, Bayer CropScience) and Cruiser ® (thiamethoxam, Syngenta Crop
Protection Inc.) are the commercial formulations designed for this method of
application. These compounds can also be used for in-furrow application as
Admire ® 2F (imidacloprid, Bayer CropScience) or Platinum ® (thiamethoxam,
Syngenta Crop Protection Inc.). For this method no extra step is required prior to

planting.

22

Each of the two methods uses low amounts of pesticide and places the
compound in the required site, close to the plant. However these formulations
might interact with the other factors related to the application. When insecticides
are applied at-planting, as neonicotinoids are recommended for Colorado potato
beetle control, the effects of application method on product performance are still
unclear. Although seed-treatment is reported to promote more insecticide uptake
by plant tissue (Rouchaud et al. 1994), because the insecticide is applied in close
proximity to the developing plant, the advantages or lack of advantages of this
method in potatoes are unknown and it is important to compare this method with

in-furrow application method.

Insecticide uptake

The physiology of neonicotinoid uptake has been studied extensively for
imidacloprid - the first released and the best known of these insecticides. Fewer
reports are available for thiamethoxam. Once applied to the soil, imidacloprid
may have several routes of metabolism. Among its soil metabolites imidacloprid
urea is the most abundant hydrolytic metabolite.

Water solubility is a determining factor for insecticide movement in soil,
and ultimately insecticide availability to the plant. lmidacloprid is transported
acropetally in the xylem. After application, the rate of insecticide concentration in
the plant declines as a function of time. The decline of imidacloprid concentration
in sugar beet leaves after seed treatment, decreases from 15.2 uglg (fresh

weight), on day 21, to 0.5 pglg on day 97, and by day 97, 4.5% of the parent

23

compound remains and 44.5% is in the form of absorbed metabolites (Westvvood
et al. 1998). By the end of the season, imidacloprid was no longer present in
sugar beet (Rouchaud 1994) and only traces, considered safe for human
consumption, were found in eggplant, cabbage and mustard at harvest
(Mukherjee and Gopal 2000).

Movement of imidacloprid and thiamethoxam in the soil is a concern,
especially given their high levels of water solubility. Although several studies
report no leaching of imidacloprid under normal crop situations (Rouchaud et al.
1994, Rouchaud et al. 1996, Leib and Jarret 2003), there is potential for leaching
especially under high rainfall shortly after the application, or through formation of
stable, soluble organic fraction-pesticide interactions in solution (Gonzalez-
Pradas et al. 2002). Further studies should address whether this potential
leaching, related to excessive water, produces reduction in insecticide
performance, especially for high soluble compounds.

It is also unknown how the combined effects of insecticide and application
method affect beetle mortality when plants are grown in mineral vs. organic soils.
A potential reduction in insecticide uptake will significantly affect mortality of
susceptible beetles. If beetles are tolerant to imidacloprid, mortality from
imidacloprid is expected to decline according to the degree of tolerance. Due to
the chemical similarities between imidacloprid and thiamethoxam (Nauen et al.
2003) and the recent reports of resistance in Colorado potato beetles to
thiamethoxam (Grafius et al. 2004), cross resistance of imidacloprid resistant

strains to thiamethoxam, is likely.

24

The combined effects of application method and environmental factors
could result in shorter half-lives of the compounds, higher degradation rates, and
leaching or run-off losses. These changes could, in turn, lead to lower insecticide
uptake by the plant tissue. A reduction of the insecticides’ residual activity after
at-planting application could select for beetles emerging late from ovenrvintering
or moderately resistant beetles, making control no longer effective and eventually
requiring additional insecticide applications.

Insecticide efficacy may be impaired by: 1) environmental factors such as
rainfall and soil type; 2) agronomic factors such as application method, and 3)
ecological factors, such as insecticide resistance level of the pest populations.
Consequently, it is necessary to determine whether and to what extent those
environmental and agronomic factors affect insecticide performance and
eventually modify application techniques accordingly to maintain insecticide

performance and minimize potential for resistance build up.

Research Objectives

General objective

Determine whether and to what extent environmental (rainfall, soil type),
ecological (pest population susceptibility to insecticide), and agronomic
(application method) factors affect the efficacy and residual activity of the
neonicotinoids, imidacloprid and thiamethoxam, when applied at planting for

Colorado potato beetle control. I hypothesize that those factors affect the

25

insecticides’ performance for pest control in this system and that there will be

significant interaction between factors.

1)

2)

3)

4)

5)

Specific objectives for field studies

Compare the efficacy of the neonicotinoids imidacloprid and
thiamethoxam, applied in the field at two rate levels, for Colorado potato
beetles control.

Determine whether in-furrow application vs. seed treatment influences the
efficacy and residual activity of these neonicotinoids

Determine the response of two strains of Colorado potato beetle, differing
in susceptibility to imidacloprid, to the combined effects of insecticide,
application method, and application rate. Since the two insecticides are
from the same chemical class, cross-resistance is expected.

Identify any interactions and /or additive effects between the above.
Compare the responses in (3) at two sites that differ mainly in soil type
(mineral vs. organic). The very high organic matter content of organic
soils may hamper the efficacy and residual activity of these insecticides,

compared with activity in mineral soils.

Specific objectives for greenhouse studies

1)

Determine the effect of soil type (mineral soil versus organic soil) on beetle

mortality from the two insecticides.

26

2)

3)

4)

Determine under greenhouse conditions whether in-furrow application or
seed treatment affects the efficacy and residual activity of these
neonicotinoids.

Determine the effect of excessive irrigation at planting on the residual
activity of imidacloprid and thiamethoxam.

Examine possible interactions between: soil type, application method, and

insecticide, and their effect on beetle mortality.

27

CHAPTER 2:

THE EFFECT OF APPLICATION METHOD AND RATE ON THE RESIDUAL
ACTIVITY OF THIAMETHOXAM AND IMIDACLOPRID FOR THE CONTROL
OF THE COLORADO POTATO BEETLE, Leptinotarsa decemlineata (SAY) IN
THE FIELD

INTRODUCTION

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is the most
important insect pest of potatoes in the midwestern United States (Foster and
Flood 1995) and is the most devastating defoliator of potatoes worldwide (Hare
1990, Casagrande 1987). Larvae and adults can cause complete defoliation and
total yield loss may occur if control measures are not taken. Potato plants in the
bloom stage or at early tuber formation are especially vulnerable to beetle attack
(Hare 1990). Consequently, control programs for this pest are aimed to minimize
the effect of insect damage during early and mid season and rely extensively on
synthetic insecticide use.

Currently, the two most widely used insecticides to control this pest are
the neonicotinoids imidacloprid and thiamethoxam. These insecticides constitute
the best control tools available where Colorado potato beetle shows resistance to
other major insecticide groups. lmidacloprid was registered for use in Colorado
potato beetle control in the United States in 1995 and thiamethoxam was

registered in 2001.

28

These two systemic insecticides belong to the neonicotinoid chemical
class: a relatively new group of compounds that act as nicotinic acetylcholine
receptor agonists (Bi et al. 2002). Neonicotinoids can be applied by different
methods, they have systemic residual action, require low application rates
(Maienfisch et al. 2001), and are highly effective on many pests (Bi et al. 2002).
Insecticide residual activity refers to the biological activity of the insecticide over
a period of time after a single treatment application. It is governed by factors
related to the nature of the product used, the sensitivity of the target, and the
application conditions.

In the northeastern U.S., applications of thiamethoxam or imidacloprid are
recommended at planting for control of Colorado potato beetle. A single
application per season is desired because this can 1) lower the costs of control
and 2) reduce the potential for insecticide resistance. However, the residual
activity of these two chemicals when applied at planting depends on factors such
as: soil characteristics, beetle resistance, application method and rate used.
Understanding the effect of these factors on imidacloprid and thiamethoxam
performance (in terms of residual activity and efficacy), and how they interact is
crucial for optimal pest control.

Of the two insecticides, more research has been done on the use and fate
of application of imidacloprid than thiamethoxam. The biological activity of
imidacloprid varies depending on the soil's agronomic conditions and the
physical and chemical characteristics of the soil. Most field dissipation studies on

crops show half-lives of imidacloprid of around 80 to 110 d (Krohn and

29

Hellpointner 2002) and up to more than 1 yr (Cox 2001). Longer half lives are
reported in laboratory studies (Krohn and Hellpointner 2002). Vegetation is a
major variable responsible for determining the product's half-life; longer
persistence is observed on bare soils than when vegetation is present (Scholz
and Spiteller 1992). No leaching of imidacloprid has been reported in terrestrial
dissipation studies (Rouchaud et al. 1994, Rouchaud et al. 1996, Krohn and
Hellpointner 2002, Leib and Jarret 2003) and the product has not been found to
accumulate in the soil (Krohn and Hellpointner 2002). The persistence of
imidacloprid in water solutions is affected by pH and formulation type (Sarkar et
al.1999). The organic content of the soil, which may enhance compound binding,
reduces the insecticide’s activity (Krohn and Hellpointner 2002).

Thiamethoxam degrades at moderate to low rates in laboratory soils, with
a half life of 34 to 75 d under favorable cropping conditions and longer under
unfavorable cropping conditions (Maienfisch et al. 2001). Anon. (2003d) reported
half life in soils in the laboratory of 101 d.

lmidacloprid and thiamethoxam for Colorado potato beetle control can be
applied as a seed treatment or by application in furrow (Bird et al. 2005). For
both application methods the insecticide is in proximity to the plant, but different
insecticide placement with respect to the developing roots may result in
insecticide uptake variation. The active ingredient placement and the effect of
the insecticide formulation components may alter the efficacy of these

insecticides.

30

Seed treatment is reported to promote more insecticide uptake by plant
tissue because the insecticide is said to be applied in intimate proximity to the
developing plant (Rouchaud et al. 1994). The advantages of this method in
potatoes are however unknown and the effect of application method, on control
of Colorado potato beetle, is still unclear. It is important to compare the effect of
seed treatment methods with the more convenient in-furrow application methods.
It is also important to know the effect that organic matter may have on insecticide
performance in mineral vs. organic soils.

lmidacloprid and thiamethoxam currently provide acceptable control levels
for Colorado potato beetle; however, resistance to both insecticides has been
reported in the eastern United States. Resistance to imidacloprid in field
populations of Colorado potato beetle has been reported in the United States in:
Long Island, NY (Olson et al 2000, Zhao et al 2000), Massachusetts and
Delaware (Bishop et al. 2003). In Michigan, a strain of Colorado potato beetle,
selected in the laboratory is more than 100-fold resistant to imidacloprid
indicating a high potential for resistance to develop in that state (Bishop et al.
2003). Similarly, increased tolerance to thiamethoxam already occurs in Long
Island, NY, Delaware (Bishop et al. 2003), and Massachusetts (Grafius et al.
2004) apparently due to cross resistance with imidacloprid. Cross resistance is
expected because imidacloprid and thiamethoxam belong to the same chemical
class. Although cross-resistance between the two insecticides is not 1:1, it is

high enough to suggest that alternating between the two insecticides may not be

31

an effective resistance management tactic (Nauen et al. 2003, Grafius et al.
2004).

As indicated above, many factors may play a roll in determining the
availability of the insecticide for plant uptake from application time throughout the
time protection is expected to occur. The combined effect of these factors could
result in faster decrease of the insecticide concentration in the soil. Insecticide
uptake by the plant could be reduced and consequently, protection from the pest.
Therefore, when the neonicotinoids thiamethoxam and imidacloprid are applied
at planting in the field for control of Colorado potato beetle, resistance,
application method, soil site conditions, and insecticide rate can affect the
efficacy and the residual activity of the compound either directly or through

interactions between those factors.

Objectives
The objectives of this research were to:

1. Determine the residual activity of the neonicotinoids imidacloprid and
thiamethoxam, applied in the field at two rates, for control of Colorado
potato beetle adults.

2. Determine whether in-furrow application vs. seed treatment influences
the efficacy and residual activity of these neonicotinoids.

3. Determine the response of two strains of Colorado potato beetle,
differing in susceptibility to imidacloprid, to the combined effects of

insecticide, application method and rate.

32

4. Identify any interactions and lor additive effects between the above.

5. Compare the responses in (3) at two sites that differing mainly in soil
type (mineral vs. organic). The very high organic matter content of
organic soils may hamper the efficacy and residual activity of these

insecticides, as compared with activity in mineral soils.

MATERIALS AND METHODS
The efficacy and residual activity of neonicotinoid insecticides on Colorado
potato beetle adults were assessed in 2002 and 2003 through no-choice feeding
bioassays conducted in the laboratory, using potato foliage collected from field

plots at two experimental sites.

Field planting

Field research was conducted at the Michigan State University (MSU)
Montcalm Potato Research Farm, Montcalm Co, MI, and the MSU Muck Soils
Research Farm, Clinton Co, MI. These two sites were chosen to compare
insecticide performance in mineral soil (Montcalm Farm) vs. organic soils (Muck
Soils Farm). Soil at the Montcalm Farm is a McBride Sandy Loam, with < 5%
organic matter and pH of 6.9. Soil at the Muck Soils Farm is a Houghton Muck
containing 75 % organic matter with a pH between 6.2 and 6.9.

Two imidacloprid insecticide formulations were used: Admire® 2F (Bayer

CropScience), and Genesis® (Gustafson) (Table 1). Two thiamethoxam

33

insecticide formulations were used: Platinum® ZSC (Syngenta Crop Protection)
and Cruiser 5FS (Syngenta Crop Protection). Admire and Platinum were applied
in furrow and Genesis and Cruiser were applied as seed treatments. For seed
treatment, the insecticide was applied to the potato seed pieces just before
planting, spraying them with a mist spray bottle at the proper treatment rate
(Table 1 and 2). ln-furrow applications were made immediately after planting, but
before covering the seed pieces with soil, using a single nozzle, hand held boom
(30 gpa, 35 psi).

The four insecticides were applied at a single rate in 2002 (Table 1). In
2003, two insecticide rates were applied (Table 2): The maximum labeled rate
(Bird et al. 2005) and a lower rate. In 2002 plots were planted on May 14 at the
Montcalm Farm and on May 21 at the Muck Soils Farm using cv. Snowden at
both sites. In 2003, planting was done on May 15 at the Montcalm Farm and on
May 23 on the Muck Soils Farm, using cv. Atlantic at both sites. Rainfall was
supplemented with irrigation as needed to provide adequate soil moisture
conditions. Plots were 8 m long by 3 rows wide, with 0.8 m between rows and 30
cm between plants, and were arranged in a randomized complete block design
with four replications. The crops were grown using standard commercial

practices for irrigation, fertilization, herbicide and fungicides treatments.

Bioassays
Three weeks after planting (about 1 wk after plant emergence), foliage

collection and bioassays were initiated. Four leaves from the youngest, fully

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36

expanded leaves were collected, each one from a randomly selected plant in the
center row of each plot. Leaves were transported to the laboratory with ice packs
and kept in the refrigerator overnight. Bioassays were started the next day. In
2002, the leaves from all four plots of a given treatment were mixed and five
leaves per treatment were randomly selected to use in the bioassays. In 2003,
the foliage from each plot was kept separately. Leaves were placed in
polystyrene Petri dishes (150mm diam. X 15mm) with the petiole inserted in a
vial containing water; five beetles were then placed in the dish. The dishes were
randomly arranged and the bioassays conducted at 22 — 25 °C. Food was
replaced with freshly collected foliage on day 3. Treatments were evaluated on
day seven by examining each beetle and classifying it as walking, poisoned or
dead.

“Walking” beetles showed normal, coordinated, forward walking; “poisoned”
beetles were unable to walk fonlvard one body length; and "dead" beetles were
non-responsive when probed with forceps and often exhibited dark coloration,
shrunken abdomens or signs of decomposition. A combined category of dead
and poisoned beetles was counted as “affected beetles” and used for analysis
purposes later on. During treatment evaluation, Petri dish labels were covered to
avoid bias in assessment. The bioassays were run every 2 wk in 2002 (June 4
through July 30) and weekly in 2003 (June 11 through July 25), until insecticide
effectiveness had declined and no significant mortality occurred in most of the

treatments

37

Beetle strains

Susceptible and imidacloprid-tolerant strains of Colorado potato beetle
were used in the feeding bioassays. The susceptible beetles were either field-
collected or reared in the laboratory. For bioassays conducted in 2002, beetles
were collected from the Michigan State University (MSU) Montcalm Research
Farm, Montcalm County, MI. For the 2003 bioassays, susceptible beetles were
reared in the laboratory from egg masses purchased from the Phillip Alampi
Beneficial Insect Rearing Laboratory, New Jersey Department of Agriculture.

The imidacloprid-tolerant strain was used in 2003 only and it was originally
collected near Jamesport, Long Island, NY in 1999. This strain had been reared
for over 12 generations in the laboratory without selection. At the time of the
experiments, these beetles had an LD50 of 0.340 pglbeetle, ca. 9.4 fold tolerant
to imidacloprid compared with New Jersey beetles, and also ca. 2.9 fold more
tolerant to thiamethoxam than susceptible beetles with an LD50 of 0.096
uglbeetle (Grafius et al. 2004). Adults of both strains were kept at 22 - 25°C in
the laboratory and fed daily with untreated, greenhouse potato foliage for 5 d
after emergence from pupation. Afterwards they were transferred to growth
chambers at 11°C, fed on untreated foliage and used in bioassays within 1 - 2

wks.

Data analysis

The efficacy of the treatments was the response variable and was

evaluated based on the proportion of affected (dead and poisoned) beetles.

38

In 2002, the experimental design was a completely randomized design
with five replications per treatment. In 2003, the experimental design was a
randomized 4complete block design. Since the data did not have a normal
distribution and the response variable was categorical, a generalized linear
model was used to compare factors. Proc Genmod in SAS’ml (software for
Windows“, Release 8.02 TS Level 02M0) was used. Then a two-way, type 3
analysis was run using the Poisson distribution. This analysis was run for each
bioassay of the experiment. An analysis was also done with all dates combined
to find the significance of the factors studied over the season. In these analyses
the significance of main effects (“days after planting’, ‘active ingredient” and
‘application method’), and the interactions between them were tested. Each
individual bioassay had non treated controls and affected beetle numbers in
these controls were very low or zero, so the conditions of bioassays were
assumed to be the same throughout the season so an overall analysis can be
conducted.

To analyze the treatment residual activity, three treatment groups were
considered here: 1) treatments with percentage affected > 50% throughout the
experiment, 2) treatments with percentages affected always < 50% and, 3) those
treatments with initial percentages affected > 50% and final percentages < 50%.
Comparisons of residual activity in the latest group, was done by using Probit
Analyses (Proc Probit, in the same SAS ® software mentioned above). For this
purpose data were corrected for the proportion of affected beetles found in the

untreated controls using Abott's formula (Abbott 1925). The time after planting

39

during which percentage affected was > 50% (median lethal time, LTso) was
determined. Also the times for efficacy of 95% (LT95) and 5% (LT5) were
calculated and used to plot probability curves using the software Origin ®. From
the percentage of affected beetles obtained at each evaluation time, the curves
of time vs. affected beetles were constructed. After Proc Probit was run, the

general analysis was discussed including the first two groups.

In 2003, the experimental design was a randomized complete block
design with four replications (=blocks). The same categorical data analysis
conducted for 2002 data was conducted for 2003 and then the Genmod
procedure was used. The model aimed to find the effect of the tested factors
(‘days after planting’, ‘active ingredient’, ‘rate’, ‘application method’ and ‘strain’),

and the interactions among them.

The analysis was done without correcting for control mortality, because
the Poisson distribution in the model does not accept fractional numbers. Special
caution is given in the discussion to those cases where mortality of untreated
beetles was > 15%. Probit analyses were conducted to explore the residual

activity of the treatments as was done for year 2002 data.

For the graphic representation of the results I used the proportion of
affected beetles corrected for the affected beetles in the untreated controls using
Abbott’s formula (Abbott 1925). The graphic representation of results for year

2003, were grouped by beetle strain and by rate. Comparisons of a “best case

40

scenario ” of full dose and susceptible strain, to the combination of cases of low

dose and resistant strain, are made (Figure 4, as example).

RESULTS AND DISCUSSION

Mineral soils site, year 2002

Weekly results. On day 28 after planting, only one treatment (imidacloprid
as a seed treatment) affected less than 90% of the beetles, and affected
significantly fewer beetles (LSMeans P< 0.05) than the other treatments. In
general, thiamethoxam treatments affected significantly more beetles than
imidacloprid ones and the in-furrow treatments resulted in more affected beetles
than the seed treatments (Figure 2a, Table 3 for significant effect of the factor
‘Application method’). lmidacloprid applied in-furrow affected significantly more
beetles than imidacloprid applied as a seed treatment; however, both
thiamethoxam treatments resulted in the same percentage of affected beetles
(See significant interaction ‘Application method (seed treatment vs. in-furrow) *
active ingredient’ (imidacloprid vs. thiamethoxam) in Table 3).

On day 42 after planting, two weeks later, thiamethoxam treatments
affected significantly more beetles than imidacloprid (Figure 2a, Table 4). A
higher percentage of affected beetles by the in-furrow treatments compared with
the seed treatments may have been of biological importance since the P value
was very close to significance for this date (P=0.0507). No significant effect for
the interaction ‘active ingredient * application method’ was observed on this date.

Thiamethoxam in-furrow (Platinum) continued to show high efficacy (ca. 90%

41

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42

 

 

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affected beetles). This was significantly higher than percentage affected beetles
in the other treatments, which declined from the previous date and were around
20 - 50% (Figure.2a)

On day 56 after planting, again ‘Active ingredient’ and ‘application method’
significantly affected efficacy (Table 5). Thiamethoxam was significantly more
effective than thiamethoxam and the in-furrow treatments resulted in significantly
more affected beetles than the seed treatments. The interaction between these
two factors was significant (Table 5) showing a differential effect of application
method on active ingredient efficacy: imidacloprid as a seed treatment resulted in
no affected beetles on this date while thiamethoxam applied in furrow affected
90% of the beetles (Figure 2a) significantly more than were affected by the two
imidacloprid formulations.

On day 63, active ingredient and application methods factor were not
significant (Table 6). All treatments affected less than 50% of beetles.
Thiamethoxam in furrow affected significantly more beetles than thiamethoxam
as a seed treatment (sz = 5.48, P = 0.02). lmidacloprid as a seed treatment
affected more beetles than during the previous week for the same treatment, but
efficacy was < 30%. For the rest of the treatments a general decline in affected
beetles was observed (Figure 2a).

Treatment residual activity. In addition to the weekly analyses,
treatment comparisons overall in the season showed the following:
thiamethoxam applied in furrow exhibited the longest residual activity (Figure 3).

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45

 

 

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48

for this treatment was the longest of the four (Table 8), and significantly longer
than for thiamethoxam applied as seed treatment or imidacloprid applied in
furrow (Table 7). The percentage of affected beetles in of this treatment was
also the highest for most individual dates and > 40% affected beetles on the last
date of evaluation. The treatment with the lowest residual activity was
imidacloprid applied as seed treatment. In addition, the projected probability
curve in Figure 3 places this product far behind the residual activity of the other
three. A final increment in the efficacy for this treatment (Figure 2a) raises
questions about what could caused the plant to resume uptake. The higher
efficacy of in-furrow applied treatments could be explained by the fact that more
insecticide was readily available for uptake by potato roots.

With this application method, the insecticide is more evenly located along
the furrow where many roots can come in contact with the product. For in-furrow
imidacloprid residual activity experiments Boiteau et al. (1997) found protection
to the crop from adults and larvae attack for about 50 - 54 days at similar rates, in
contrast with the finding here that protection by imidacloprid against adults may
not be that long.

Overall season results at the mineral soils site in 2002.
Thiamethoxam products affected more beetles than imidacloprid products (Table
8), although 56 d after planting both active ingredients performed equally well.

" lmidacloprid seemed to have less residual activity than thiamethoxam as shown
by the lower proportion of affected beetles throughout the season, and by the

significant interaction “active ingredient * time after application’ (‘days after

49

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51

planting’, Table 3). This interaction showed an earlier reduction in efficacy (lower
residual activity) for imidacloprid over time with the initial high levels of affected
beetles (ca. 90%) declining rapidly for imidacloprid.

The difference in percentage of affected beetles for the two application
methods were evident after day 30 for thiamethoxam, while for imidacloprid that
difference was evident from day 30. This suggested that the lower initial efficacy
shown by imidacloprid as a seed treatment could be due to the application
method interacting with active ingredient. The two application methods also
affected residual activity as shown by the significant interaction ‘application
method * days after planting’ (Table 3); the seed treatments resulted in shorter
residual activity than did the in-furrow treatments. This effect could be explained
by the allocation of insecticide in a larger area on the ground (in-furrow
treatments) when the insecticide is applied along the furrow vs. the localized.
seed treatment. As the plant grows and roots expand, more treated area is
found and more insecticide uptake may occur. The best treatment, for a longer
median lethal time was thiamethoxam applied in furrow (Table 7), with almost 15

d more effect for affected beetles.

Organic soils site, year 2002
For the organic soils site, on day 28 after planting, imidacloprid and
thiamethoxam affected only 40%-50% of the beetles and no-significant
differences between the insecticides were found (Table 9). Seed treatments

affected significantly more beetles than in-furrow treatments (Figure 2b,

52

 

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53

Table 9). By day 43, all the treatments had no affected beetles.

Because all percentage affected beetles were < 50%. No median lethal
time analyses were conducted. There was a markedly low residual activity of
insecticides under organic soil conditions with zero efficacies by day 43 after
planting. Even on day 28 < 50% of the beetles were affected. This effect could
be associated with the very high content of organic matter in the soil (75 -80%),
supporting previous research showing a shorter half life of these products when
organic matter is present (Krohn and Hellpointner 2002).

The effect of the organic soil in depressing insecticide efficacy was apparent both
in the initial number of affected beetles and in the residual activity of the product.
The negative effect on insecticide performance by the high organic matter could
be reduced by the application as seed treatments because the insecticide is
more in contact with the tuber than with the soil phase compared with the in-
furrow treatment. This is shown by the higher efficacies of the seed applied
products over the in-furrow treatments. This also corroborates the advantages of
seed treatments in organic soils reported by other authors (Rouchaud et al.

1 994).
Mineral soils site, Year 2003

Weekly results. On day 26 after planting, percentage affected beetles in
the susceptible strain (80 - 100%) was generally higher than in the resistant
strain (40 -90%) (Figure 4, Table 10). Seed treatments were generally more
effective (Table 10, for significance of ‘Application method’) than in-furrow

applications. There was a significant difference in percentage of affected

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56

beetles between the two thiamethoxam treatments for the susceptible strain at
the low rate (Figure 4—d). This difference could be associated to potato plant
development: at the time of evaluation, the plants were very small and the
insecticide in seed treatments may have been more readily available for uptake.
Neither ‘rate’ nor the ‘active ingredient’ alone was a significant factor. Both active
ingredients showed high efficacy on this date for the susceptible strain (Figure 4
a,c), but efficacy for the resistant strain was variable (Figure 4 b,c). This
interaction ‘active ingredient * beetle strain’ was close to the significance level of
P = 0.05 (P = 0.064, Table 10). Also on this date, a significant effect of the three-
factor interaction: ‘active ingredient by application method by rate’ was observed.
The percentage of affected beetles in the untreated control for the susceptible
beetle strain was 21% on this date and the reason of this high mortality in the
control is unknown. However, the percentage of beetles affected by the
treatments in this strain was very high (most cases more than 90%, only one
case with 70%). A significant difference between the two application methods of
imidacloprid at the high rate on the resistant beetle strain (X2=11.39, P value
<0.01) (Figure 4 c) was observed, the seed treatment affected 90 % of the
beetles compared with <30% affected in the in-furrow imidacloprid treatments.
On day 33 after planting, both active ingredients at either application method had
high levels of efficacy when used at the high rate on the susceptible beetle strain.
Under other conditions (low rate, resistant strain or both) thiamethoxam was
more efficient than imidacloprid (Figure 4) as indicated by the significance of the

factor ‘active ingredient’ (Table 11). The two strains responded differently

57

 

 

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58

(‘beetle strain’ significant, Table 11); there were high numbers of affected
beetles (85% or more, Figure 4 a,c) in the susceptible straincompared with a
variable response (20 and 100%) in the resistant strain (Figure 4 b,d). The
difference between the two active ingredients in the susceptible strain (Figure 4
a,c) was not significant (P = 0.5998), but it was significant (P < .0001) in the
resistant strain; thiamethoxam treatments were more efficient than imidacloprid
treatments at both rates (Figure 4 a,c). This variable response is confirmed by
the significant interaction ‘beetle strain * active ingredient’ (Table 11) and could
be due to direct effect of the imidacloprid-resistant strain that reduced the
efficacy of that insecticide.

The treatment ‘in-furrow thiamethoxam’ applied at lower rate affected
more beetles on day 33 (LSMeans P = 0.0457) than the week before (Figure 4
c,d). This could be due to increase in insecticide uptake of the young growing
plant (plants were 15 to 20 cm tall on this date), which could be enough to
counteract an early lower insecticide availability due to the low rate.
On day 40 after planting, there were significant differences between
thiamethoxam and imidacloprid (‘active ingredient’ significance, Table 12). At the
high rate and with the susceptible strain, all insecticides showed high efficacy
levels (80% or more). For other conditions (low rate and/or resistant strain) most
thiamethoxam treatments showed higher efficacy than imidacloprid treatments
(Figure 4). On this date also, a decline in number of affected beetles was
observed for imidacloprid when used at the low rate for the susceptible beetle

strain (Figure 4 c) and for all thiamethoxam treatments for the resistant strain.

59

 

 

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60

The factor ‘application method’ was significant (Table 12): the seed treatments
probably did not affect as many beetles as the in-furrow treatments. The factor
‘beetle strain’ was also significant (Table 12). The interaction ‘rate * beetle strain’
was also significant (Table 12); at the high rates, more beetles were affected in
the susceptible strain than in the resistant strain; but at the low rate, the range in
response was similar for both beetle strains.

On day 47 after planting, thiamethoxam applied in-furrow still had 100%
affected beetles at the high rate in the susceptible strain. Most of the other
thiamethoxam treatments had a steep decline in the percentage of affected
beetles, especially with low rate and resistant strain. Most imidacloprid
treatments resulted in a lower percentage of affected beetles on this date than on
the previous evaluation date. The decline in thiamethoxam efficacy led to the
two active ingredients to perform equally overall on this date. The resistant
beetle strain had lower percentage of affected beetles than the susceptible strain.
Only the main factor, ‘beetle strain’ showed a significant effect (Table 13). In the
resistant strain no difference was observed between application methods, while
in the susceptible strain the in furrow applications produced more affected
beetles than the seed treatments (interaction ‘application method * beetle strain’,
Table 13). The interaction ‘rate * beetle strain’ was also significant (Table 13)
meaning that while the high rates produced more affected beetles compared to
the low rate in the susceptible strain; there was no difference in affected beetles
between rates for the resistant strain, mainly because all treatment effects had

lowered to near zero at this date for the resistant strain.

61

 

 

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62

On day 54 after planting, significant differences were observed only
between the two beetle strains (Table 14). More affected beetles were observed
in the susceptible strain (20 to 80% affected) compared to the resistant strain
(<20% affected (Figure 4).

Season Results. At the rates used in this experiment from the mineral
soils, generally more beetles were affected by thiamethoxam than imidacloprid.
This tendency was more consistent for the resistant strain (Figure 4). This
showed that the specific active ingredient significantly determined efficacy in
mineral soils (P <0.0001 in Table15). As expected, compared with susceptible
beetles, imidacloprid-resistant beetles were generally less affected by
imidacloprid (significance of ‘beetle strain’ P <0.0001, Table 10). The two factors
(‘beetle strain’ and ‘active ingredient’) interacted in the following fashion: while
most of the season both active ingredients had similar efficacy for the susceptible
strain, the efficacy of imidacloprid was significantly lower than efficacy of
thiamethoxam in most cases for the resistant strain. The numbers of affected
beetles declined during the season, showed by ‘Days after planting’ significantly
influencing efficacy. These results suggest little cross resistance of the
imidacloprid-resistant strain towards thiamethoxam (as indicated by resistance
ratios of 9.4 fold for imidacloprid vs. 2.9 fold for thiamethoxam. However the day-
by-day analysis showed significant differences at day 47 between the
thiamethoxam treatments on susceptible and resistant strains (from 100%

affected beetles to ca. 5%). The active ingredient overall efficacy varied

63

 

 

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66

depending on the beetle strain and in the case of imidacloprid applied in-furrow
some efficacy may be lost due to the resistance strain.

No significant interactions between ‘active ingredient * days after planting’
were observed and both active ingredients exhibited the same declining pattern
in efficacy throughout the experiment (Figure 4). A significant interaction “rate *
beetle strain’ was also observed and this interaction may have been expressed
especially beyond 33 days after planting: the high rates resulted in more affected
susceptible than resistant beetles, while there was no difference between beetle
strains at the lower rates. Since the level of resistance is directly related to the
amount of insecticide the insect encounters, this interaction may be a direct
response of the different resistant level of the beetle strains used. These first-
order (two factors) interactions varied through the duration of the experiment,
resulting in second-order interactions with ‘days after planting’. Other
interactions of second and third order were also observed (Table 15).
Treatment residual activity. Three treatment combinations (‘imidacloprid - in-
furrow - high rate - susceptible strain”, ‘thiamethoxam — in-furrow - high rate -
susceptible strain’ and “thiamethoxam - in furrow - low rate — susceptible strain’)
affected more than 50% of the beetles throughout the season (Table 16). These
treatments are assumed to be at the top of treatment performances when
comparing the treatment residual activities, since their efficacy was higher than
the rest of the treatments throughout the season. However, because percentage
affected was always >50%, they were not included in the median lethal time

analysis. Three other treatments (“imidacloprid-in furrow—high rate—resistant

67

 

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69

strain’, “imidacloprid-in furrow—low rate—resistant strain’ and ‘imidacloprid—seed
treatment—low rate-resistant strain’) affected less than 50% of the beetles
throughout the season. These treatments were assumed to be the poorest
treatments (in terms of treatment performance) for the season because they
affected beetles in a lower percentage than the rest of the treatments throughout
the season. Again, because percentage affected was always < 50%, they were
not included in the median lethal time analysis.

Treatments applied to the seed usually had a steep decline in percentage
affected beetles, especially notable for the thiamethoxam treatments even
though they had an initial high percentage of affected beetles (Figure 5).
Treatments with the susceptible beetle strain ranked as ‘top treatments’ or
resulted in significantly higher (LTso) median residual activity than treatments
using the resistant beetle strain. Even though the significance of factor ‘rate’ was
variable, the high rate had one treatment combination with imidacloprid as one of
the most effective treatments. There was no effect on residual activity of the
lower rate and there was no clear effect for the application method.

The LT50 levels for imidacloprid products are within the reported ranges for
half life of the product (Figure 5, Table 15). Our results gave LTso values of
about 30 to 50 d and the dissipation studies show a range from 7 to 150d (Krohn
and Hellpointner 2002). While some of the factors discussed above my influence
the insecticide presence in the soil, the biological activity investigated is also

dependant of the insecticide uptake by the plants.

70

 

 

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71

Overall season results at the mineral soils site in 2003. At about day
40 after planting, the imidacloprid products declined in the ‘high rate —
susceptible strain” and were not as effective as thiamethoxam (Figure 4). At
about the same date, thiamethoxam, when applied either at a low rate or to the
resistant strain, declined to an efficacy level similar to imidacloprid.

In the mineral soils of this site, there were no differences in performance
by the two active ingredients at a high rate and for the susceptible strain. When
other factors are included (resistant strain and lower rate), there was higher
efficacy for thiamethoxam than imidacloprid at about 40 days after planting and
after day 47 both active ingredients had similar effects. No overall significance
between rates used in this experiment was found for the mineral soils site.
These results agree with results shown by Boiteau et al. (1997) who tested
different rates of Admire ® (imidacloprid, in-furrow) for control of Colorado potato
beetle with no major significance between 222 and 333 g A.l./ha. Rates lower
than that may reduce the efficacy of the application, according to their results.

The decline in the residual activity for imidacloprid in furrow found in this
study, along with the effect of resistant strains on such decline, confirmed
previous findings about the residual activity of imidacloprid. Bishop et al. (2001)
found a decline in affected beetles from 49 to 70 days after planting from 78% to
70% of affected beetles. The response in intermediate and resistant beetle
strains was not higher than 40% in efficacy with a declining pattern. Boiteau et
al. (1997) reported crop protection (from adults and larvae of the Colorado potato

beetle) with in-furrow applied imidacloprid of 36-46 d and 71 d in two different

72

seasons for susceptible field populations. Although results in this study showed
efficacy of both neonicotinoids against adults until around day 40 after
application, projecting these results to the field the treatments under mineral soils
could show longer residual activity for field protection than the residual activity
observed for affected adults because the crop is also attacked by the more
insecticide-sensitive larvae.

The low rate did not have a major effect on the overall efficacy and
residual activity of the insecticides, thus the lower rates could provide appropriate
protection to the crop. However, some impact of the lower rate may have
occurred in: 1) reducing the residual activity of imidacloprid when applied at the
low rate; 2) hindering the efficacy of imidacloprid as a seed treatment on resistant
beetles on day 26 after planting (Figure 4b) and, 3) reducing the residual activity
of in-furrow thiamethoxam on susceptible beetles (Figure 4 a,c).

The apparent greater sensitivity of imidacloprid to the adverse effects of
lower rate and insecticide resistance complies with the nature of the resistance of
the strain I used (imidacloprid resistant) and with the fraction of the rate used for
each compound (imidacloprid’s low rate.= 0.64 high rate; thiamethoxam’s low
rate= 0.86 high rate). Then, reductions in efficacy, about day 40 after planting, or
early drop in residual activity would be expected if low rates are used or
imidacloprid resistance is present.

Seed treatments may have higher efficacy than in-furrow applications

early in the season but that difference shifted 1 - 2 wks later or disappeared.

73

Low levels of resistance to imidacloprid on beetles (as the 9.4-fold for the
strain treated here) severely hampered the performance of imidacloprid
formulations and also could cause reduction of thiamethoxam products by day 40
— 47 after planting (Figure 4 b). The reduced efficacy due to the resistant strain
was significant throughout the season for imidacloprid and important for
thiamethoxam on day 40 after planting. The effect of some level of cross
resistance made the efficacy of thiamethoxam decline by day 47 to levels lower
than 20% affected beetles.

Despite of the random selection of plants in the field from the untreated
check, I could have obtained selected foliage from these plots because over time
the remaining foliage was only that not consumed by beetles. However the low
mortality levels observed in the control made me assume that any change in that
foliage was of no weight when comparing the effect of the insecticide treatments.

Organic soils site, year 2003

Weekly results. On day 26 after planting, the factor ‘active ingredient’ and
its interaction with ‘beetle strain’ (Table 17) was significant. The susceptible
beetles were significantly more affected (80 - 100% affected), than resistant

beetles (2 - 80% affected, Figure 6) (‘beetle strain’ Table 17). Thiamethoxam
treatments had significantly (LSMeans, X"m=13.53, P < 0.01) more affected

beetles than imidacloprid treatments for the resistant strain, while both active
ingredients showed equally high percentages (> 80% affected beetles) in the

susceptible strain. Seed treatments resulted in higher percentage of affected

beetles than the in-furrow applications (LSMeans )6 (1)=6.44, P = 0.01).

74

 

 

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75

For beetles fed with imidacloprid-treated foliage, strain was not a
significant factor (Figure 6, Table 17). However the two beetle strains responded
differently to thiamethoxam (more affected beetles in the susceptible strain than

in the resistant strain, LSMeans X2 (1)= 6.63, P =0.01). The interaction ‘active

ingredient * beetle strain’ was significant on day 26 (Table 17).

For the resistant beetle strain only, seed treatments performed better than
in-furrow applications (LSMeans X2 (1)= 7.26, P = 0.0071 ). There was no
difference in efficacy between application methods for susceptible beetles. The
interaction ‘application method * beetle strain’ was significant (Table 17). Seed
treatments may reduce the effect of resistance early in the season by promoting
more rapid insecticide uptake. The advantages of seed treatment due to the
more availability of insecticide close to the emerging plant have been reported
(Rouchaud et al. 1994) and may be playing a key role here in providing the roots
with more available insecticide. Also, the insecticide is concentrated on the seed
and there is less contact with the soil and less potential for binding. On day 33
after planting, ‘active ingredient’ and ‘beetle strain’ were the only factors that
significantly affected efficacy (Table 18). Thiamethoxam at the rates used
resulted in significantly more affected beetles than did imidacloprid (X2(1)= 5.12,
P = 0.02). Fewer affected beetles were found for the resistant beetle strain

compared with the susceptible beetles (Figure 6).

By day 40 after planting, a steep decline in treatments efficacy for ‘high

dose - susceptible strain' conditions had occurred and all percentages of

76

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78

affected beetles were then at ca. 15% (Figure 6). The factor ‘active ingredient’
was significant (Table 19), significantly more beetles were affected by
thiamethoxam than by imidacloprid (X2(1)= 5.99, P = 0.02). “Beetle strain’ was a
significant factor as well (Table 19).

On day 47 after planting, the imidacloprid treatments were not included
because <10% of the beetles had been affected the previous week. Only the
factors: ‘application method’, ‘rate’ and ‘beetle strain’ were tested. Significantly
more susceptible beetles were affected than resistant beetles (Table 20), but the
individual treatment combinations did not show significant differences.

Season results. All the variables studied had significant effects during
the season at the organic soils site (Table 21). For ‘active ingredient’,
thiamethoxam showed significantly higher numbers of affected beetles than did
imidacloprid (Figure 6). “Beetle strain’ also significantly affected the efficacy of
the treatments and was probably the most important factor influencing
differences among treatments (Table 21). Seed treatments generally resulted in
significantly higher numbers of affected beetles than in-furrow treatments.

Finally, the two rates used influenced also produced significant differences
(Table 21); the reduced rate resulted in fewer affected beetles than the high rate.
This significance must be due to an additive effect date after date since
significant differences were not seen for any individual date. The factor ‘days
after planting’ was significant; all treatments had a declining efficacy through time
as expected. The interaction of ‘days after planting * beetle strain’ was significant

(Table 21). The main effect of this interaction was observed in the lower initial

79

 

 

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83

efficacy of several treatments against resistant beetles, especially those with
imidacloprid. The residual activity was also influenced by the interaction ‘active
ingredient * beetle strain’ as shown by the significant second-order interaction of
“days after planting * active ingredient’ with ‘beetle strain’. The interaction
“application method * beetle strain’ varied through time, interacting significantly
with ‘days after application’ and affecting the residual activity (Table 21).

The conditions of lower rate and resistant strain (or both) induced a
decrease in efficacy of the products compared with the efficacy levels observed
for high rate and susceptible strain.

High percentages of affected beetles were found on some treatments with
the susceptible beetle strain, but those percentages declined quickly, principally
at the lower rate for imidacloprid. With the resistant beetle strain only
thiamethoxam as seed treatment (Cruiser) resulted in >60% affected beetles and
then only on the first sample date (Figure 6).

The interaction ‘active ingredient * beetle strain’ for the whole season was
not significant (X2(1)=3.1, P = 0.08), suggesting that both beetle strains
responded similarly to the two insecticides and a that certain level of cross
resistance in the resistant strain to thiamethoxam may be present. However on
the first date, this interaction is significant, and then almost all efficacies decline
rapidly. The similar performance of thiamethoxam and imidacloprid on the
resistant strain could be due to overall ineffectiveness of the insecticides, rather

than cross resistance to thiamethoxam in the imidacloprid-resistant strain.

The effective control afforded by imidacloprid and thiamethoxam in
organic soils declined sooner than most of the reported half life ranges reported
for the two active ingredients in soils. While the half life of imidacloprid is reported
as 80 - 100 d, (Krohn and Hellpointner 2002), thiamethoxam half life values are
reported as 34 to 75 d (Maienfisch et al. 2001) or 101 d (Anon. 2003d). The half
life of imidacloprid is usually longer than the biological activity in plants, but the
drastic declines in residual activity and efficacy of the different treatments in
organic soils in this study suggest that the half life of these insecticides may be
reduced as well.

Treatment residual activity Five treatment combinations (all with the
resistant beetle strain) were not included in the Probit analysis because
percentage of affected beetles was always < 50% (Figure 6). These treatment
combinations were the least effective treatments where residual activity was
severely depressed. From those included in the analysis, four Probit analyses
gave no fiducial limits of comparison because of high variability (Table 22). Two
of these treatment combinations were: ‘thiamethoxam — in furrow — high rate -
susceptible strain’ and ‘thiamethoxam - susceptible - high rate — seed treatment
had a final increase in efficacy, thus increasing variability in the statistical
analysis. For ’imidacloprid - in furrow — susceptible strain — low rate’ and
‘imidacloprid - seed treatment - low rate - susceptible strain' a steep decline
was seen just at the second date of study, increasing variability as well. LT5O for
the treatment combination ‘thiamethoxam — in furrow - high rate -susceptible

strain’ was the highest of all, but no fiducial limits were given, again because of a

85

 

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87

large increase in percentage affected beetles on the last sample date. Most
treatments were similar and the only significant difference detected was between
‘thiamethoxam-seed treatment-low rate-susceptible strain’ and ‘imidacloprid-in
furrow-high rate-susceptible strain’ (Table 22, Figure 7). No differences can be
detected between the thiamethoxam treatments of the susceptible strain at the
low rate and the resistant strain at the high rate. No signs of cross resistance

were observed.

Discussion Organic soils site 2003. For the first two evaluation dates
(26 and 33 days after planting), consistently more beetles were affected for the
susceptible strain than for the resistant strain (Figure 6). Differences in
performance between imidacloprid and thiamethoxam have been found for other
situations (Nault et al 2004). There were relatively few beetles and only minor
defoliation in the field, so it is unlikely that beetles fed differentially on foliage with

low toxin levels and left only foliage of unusually high toxicity for my samples

Overall analysis 2002 and 2003

For the two seasons, there was a decline of total affected susceptible
beetles through time for all treatment combinations at both locations and this
decline occurred much sooner on the muck soils site than in the mineral soils site
(Figure 2, and 4, 6). This observation supports previous work that observed that
organic soils reduce efficacy and residual activity of neonicotinoids due to binding

of the insecticide to organic fraction in the soil (Liu et al. 2002, Krohn and

88

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89

seuaaq papaya to tuemad

Hellpointner 2002).

A decline in efficacy from highly, medium, low or non-existent to levels
near zero is expected to occur within 25 to 40 days after planting in organic soils
similar to those in the Muck Soils Farm. The organic soils also may emphasize
the expression of cross resistance late in the season. This may be the result of
reduced effective rate and less amount of insecticide taken up by the plant.

The effect of application method on efficacy and residual activity varied
between the two sites. More beetles were affected by the in-furrow application in
the mineral soils site than in the organic soils site in 2002, low effect of the
application method in efficacy was observed for those soils in 2003. In the
organic soils there was a more marked effect of application method, with the
seed treatment affecting more beetles than in-furrow applications. This relation
was found on the first date of assessment during both years. Seed treatment
applications may improve insecticide performance on organic soils; probably by
reducing the direct contact of more applied insecticide with the organic fraction of
the soil, compared with the in-furrow application.

The insecticide performance with on resistant beetles or at the lower rate
was relatively poorer for imidacloprid than for thiamethoxam, showing that
imidacloprid products were less efficient under the detrimental factors tested in
this study. It should also be noted that the resistant beetles had been selected
for resistance to imidacloprid and showed only a low cross resistance to

thiamethoxam. The percentage of affected beetles was initially high in most

90

cases but declined more rapidly as the season progressed in the adverse
conditions.

Thiamethoxam applied in-furrow, in 2002, and thiamethoxam whith either
application method, in 2003, had the higher percentage of affected beetles with
susceptible beetles in the mineral soils.

For the type of evaluation that was done here, laboratory bioassays, the
products based on thiamethoxam produced more affected beetles. This result
could be the effect of the retarded effect of thiamethoxam, which has to be
converted into clothianidin to be active (Nauen et al. 2003). This delay may allow
in the insect to consume more treated foliage before becoming intoxicated,
ultimately producing more ingestion of the toxin and probably more insecticide in
the insect’s body than when foliage is treated with imidacloprid. This does not
mean necessarily that thiamethoxam is more effective for control than
imidacloprid, especially because in the field more factors interact to produce
protection. Also, much of the crop protection the field involves control of larvae,
which are more sensitive to imidacloprid than adults (Zhao et al.2000)

The median lethal time and effective control afforded by imidacloprid in
organic soils was shorter than previous reports of residual activity of imidacloprid
(Bishop et al. 2001, Boiteau et al. 1997). In mineral soils imidacloprid gave
residual control comparable to those reported in that study. The efficacy of
imidacloprid and thiamethoxam lasted longer in mineral soils than in organic soils
probably because the insecticides bind to the organic matter of muck soils (Krohn

and Hellpointner 2002).

91

The differences in the insecticide rates tested in 2003 did not have major
effects on the efficacy of the insecticides or their residual activity. Both rates
used were within labeled rates. In general efficacy was not affected by the lower
rate but residual activity was reduced in mineral soils for susceptible beetles. In
a two-year study, Boiteau et al (1997), found some differences in efficacy
between rates of imidacloprid applied in furrow. In 1993, they assessed the
residual activity of the insecticide by rating defoliation produced by adults and
larvae of the Colorado potato beetle. The rates of 111 and 222 g [A.|.]Iha
provided protection for 44 - 47 days after planting. Longer protection was
observed at 333 g [A.|.]Iha (62 - 65 days) in 1994, less residual activity was
observed at 111 g [A.|.]Iha compared with 222 g [A.|.]Iha or 333 g [A.|.]Iha. In
the present study, differences in rates tested in 2003 did not have major effects
on length of control. Only for imidacloprid as seed treatment on susceptible
beetles around day 40 -47, and on resistant beetles at the beginning of the
season, were differences in efficacy due to the different rates observed.

The differences in efficacy between resistant and susceptible beetles were
in general statistically significant; with the resistant strain the efficacy of
imidacloprid was generally lower than the efficacy of thiamethoxam. This is
surprising because the resistance level to imidacloprid in the resistant beetle
strain was ca. 9.4-fold which is considered a low level of resistance or tolerance.
This study shows that even that low level can make a difference in beetle
response to imidacloprid at field rates. This response may be magnified if more

factors (insect plant interactions, environmental factors, etc) are involved. Low

92

levels of resistance may be enough to produce a lower performance of
insecticides especially if organic content of the soil is high.

At the beginning of the 2003 season, there were ca. 50% affected beetles
for thiamethoxam in furrow in mineral soil at a low rate, and higher efficacy was
attained the next week. This suggests that slower insecticide uptake could occur
early during plant growth. One factor could contribute to this: the in-furrow
application may delay insecticide uptake until the young plants have attained
appropriate root development to reach the treated area in the soil. This in turn
may result in low initial concentrations in the foliage that the insect consume.
Those concentrations may not be enough to achieve high levels of control. Why
this happened with thiamethoxam applied in furrow but not to imidacloprid

remains unclear.

Conclusions

lmidacloprid and thiamethoxam at the rates used provided good levels of control
of Colorado potato beetle adults when tested on the susceptible strain in mineral
soil, beyond 30 days after planting; high efficacy continued until day 40 in mineral

soils, for both products.

Colorado potato beetle control provided by each insecticide was much higher in

mineral soil site than organic soil site.

The detrimental effect of organic soils on efficacy and residual activity of
neonicotinoid insecticides may further result in lower efficacies if resistance to

imidacloprid, is encountered in Colorado potato beetles.

93

Thiamethoxam applied in-furrow at a full rate on mineral soils was the most
consistent treatment in efficacy and had the longest residual effect. High
efficacies (more than 95% affected beetles) were observed up to at least day 47
after planting. This is very important for controlling beetles arriving late due to
late emergence from diapause or migration from distant ovenNintering sites. It
provided high control levels of control in 2003 under the optimal conditions and

even at the lower rate.

Low levels of resistance (<10 fold) in Colorado potato beetle affect control by
neonicotinoids and even lower levels of cross resistance of imidacloprid-resistant
beetles to thiamethoxam (<3 fold) may influence the residual activity of the

product.

CHAPTER 3:
THE EFFECT OF SOIL TYPE, EXCESS WATER, AND APPLICATION
METHOD ON THE RESIDUAL ACTIVITY OF NEONICOTINOIDS FOR THE
CONTROL OF COLORADO POTATO BEETLE LEPTINOTARSA

DECEMLINEA TA (SAY) UNDER GREENHOUSE CONDITIONS

INTRODUCTION

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is the most
important insect pest of potatoes in the northeastern and midwestern United
States (Mahr et al. 1995, Ferro and Boiteau 1993). Worldwide, it is the most
devastating potato defoliator (Hare 1990, Capinera 2001). Larvae and adults can
cause complete defoliation and total yield loss may occur if control measures are
not taken. Potato plants in the critical stages of bloom and early tuber formation
are especially vulnerable to beetle attacks (Hare 1990). Control programs for
this pest aim to minimize the effect of the insect during the early and mid season.
Historically control has relied extensively on synthetic insecticides (Casagrande
1987)

lmidacloprid and thiamethoxam are currently the two most commonly used
insecticides to control this pest in the northeastern and midwestern U.S.
lmidacloprid was first registered in 1995 and thiamethoxam was registered in
2001 for Colorado potato beetle control in the United States. These insecticides
are the products of choice especially in areas where there is Colorado potato

beetle resistance to other major insecticide groups. Both insecticides are

95

neonicotinoids, a relatively new group of compounds that act as nicotinic
acetylcholine receptor agonists (Bi et al. 2002). Neonicotinoids can be applied
through different methods, have systemic action, require low use rates, and are
highly effective (Maienfisch et al. 2001). Thiamethoxam and imidacloprid
applications are recommended at planting for Colorado potato beetle control
(Bird et al. 2005). For both products a single application is desirable during the
season because it can 1) lower the costs of control and 2) reduce the pest
population potential for resistance build up. However, the efficacy and residual
effect of these two chemicals when applied at planting may vary depending on
factors like application method, rainfall or irrigation, and soil type.

For instance, the availability of soil-applied imidacloprid may vary through
the season depending on cropping conditions, soil characteristics, and water
flow. Under diverse cropping conditions imidacloprid half-life ranges differs
greatly, average values are ca. 80 to 110 d, and can be up to 174 d in bare soils
(Krohn and Hellpointner 2002). Scholz and Spiteller (1992) demonstrated the
effect of vegetation in half life of imidacloprid: longer half lives are found on bare
soil whereas shorter half lives are found in soil covered with grass. lmidacloprid
remains mainly in the upper 30 cm of the soil during its lifetime and is only
partially translocated into deeper soil (Krohn and Hellpointner 2002). In aqueous
solution, acidic pH and liquid formulations produce a shorter half-life compared to
alkaline pH and powder formulations of imidacloprid (Sarkar et al. 1999). Organic
soils may additionally reduce insecticidal activity because the compound binds to

the organic fraction of the soil (Krohn and Hellpointner 2002); humic acids have

96

higher sorption capacity than Ca-clay minerals (Liu et al. 2002). Ultimately,
imidacloprid does not accumulate in the soil (Krohn and Hellpointner 2002)
although some studies have reported persistence of the compound in the soil for
more than 365 d (Anon. 2001).

Water solubility is a determining factor for insecticide movement in the soil
and also for insecticide availability to the plant. The activity of imidacloprid and
thiamethoxam may be reduced by excessive rainfall or irrigation especially
because potatoes are generally grown on sandy loam or loamy sand soils.
lmidacloprid is a hydrophilic substance with water solubility values of 0.5 gll at 20
°C at pH 7 (Krohn and Hellpointner 2002). Thiamethoxam is a hydrophilic
substance as well, its water solubility is 4.1 gll (Anon. 2003d).

Both lmidacloprid and thiamethoxam could easily move through sensitive
soil types such as porous, gravelly, or cobble soils, depending on irrigation
practices and rainfall (Jenkins 1994). Insecticide leaching depends also on the
formulation components (Gupta et al. 2002) and pH of aqueous solution (Sarkar
et al. 2001). Local soil compactness and water flow may extend compound
leaching beyond the root zone (upper 30 cm) and could lead to a reduction in
residual activity of these products (Gupta et al. 2002).

lmidacloprid is transported acropetally in the xylem of the plant and
degrades almost completely depending on plant species and time after
application (Buchholz and Nauen 2002). The rate of insecticide uptake by the
plant declines after application as a function of time. For example, in sugar beet,

concentration in foliage declined from 15.2 ug/g (fresh weight) on day 21 to 0.5

97

pg/g on day 97 (Westwood et al. 1998). By the end of the season little or no
residues was present in sugar beet (Rouchaud 1994), eggplant, cabbage, or
mustard (Mukherjee and Gopal 2000).

Thiamethoxam degrades at moderate to slow rates in laboratory soils, and
its half-life ranges from 34 to 75 d under favorable crop conditions, but may
increase by a factor of three under unfavorable crop conditions (Maienfisch, et al.
2001). pH sensitivity studies show that thiamethoxam is very stable at pH 5 and
pH 7, but labile at pH 9 (Maienfisch et al. 2001).

The commercial formulations of imidacloprid and thiamethoxam available
for Colorado potato beetle control are recommended as a range of rates and
usually the highest rates are recommended for soil containing high levels of
organic matter (Bird et al. 2005).

For at—planting applications, two methods are used: 1) seed treatment, or
2) in-furrow application. For seed treatment, potato seed pieces are treated with
an insecticide solution prior to in-furrow placement. Genesis ® (imidacloprid) at a
rate of 6.2-9.4 9 active ingredient [a.i.]l100 kg seed, and Cruiser ® 5FS
(thiamethoxam) at 3- 6 g a.i.l100 kg seed are the recommended seed-treatment
formulations (Bird et al. 2005). Seed treatments are advantageous in that the
pesticide is placed in the required site and treatment is done prior to planting and
can be combined with fungicides treatment.

For in-furrow application, the seeds are placed in the furrow and the
insecticide is sprayed into the furrow before closing. For this application method

Admire® 2F (imidacloprid) is recommended at application rates of 269 - 389 g

98

[a.i.]lha in furrow and Platinum ® (thiamethoxam) is recommended at a rate of
84 — 134 g [a.i.]l ha (Bird et al. 2005). In-furrow application is convenient and
does not require an extra step in the planting process.

Active ingredient placement, related to the plant site and root
development, and probable effect of insecticide formulation components may
produce a varying response in the efficacy of the insecticides. Although seed-
treatment may promote more insecticide uptake by the plant, because the
insecticide is applied in intimate proximity to the developing plant (Rouchaud et
al. 1994), the advantages of this method in potatoes are unknown, especially
since potato “seed” is actually whole or cut tubers. The effect of application
method on the efficacy of Colorado potato beetle control is still unclear. It is
important to compare seed treatment with the more convenient in-furrow
application method, and to also study the effect that organic matter may have on
performance in mineral vs. organic soils.

Movement of insecticides in the soil is a constant concern, especially with
the relatively high water solubility values of imidacloprid and thiamethoxam
compared to other insecticides. Although several studies report no leaching of
imidacloprid under normal crop situations (Rouchaud et al. 1994, Rouchaud et al.
1996, Leib and Jarret 2003), there may be a potential for leaching especially
under high rainfall events shortly after application, or through formation of stable,
soluble organic fraction-pesticide interactions in solution (Gonzalez-Pradas et al.
2002). Water solubility of thiamethoxam (4.1 gll at 25 °C) is higher than solubility

of imidacloprid (0.5 gll at 20 °C), thus increasing the leaching potential. Studies

99

should address whether the leaching potential, related to excessive rainfall or
irrigation, results in reduction of insecticide performance, especially for soluble
compounds like imidacloprid and thiamethoxam.

It is also unknown how the combined effects of insecticide and application
method affect beetle mortality when plants are grown in mineral vs. organic soils.
A potential reduction in insecticide availability could significantly impact the
mortality of Colorado potato beetle. If imidacloprid tolerant beetles are tested,
then the mortality caused by imidacloprid treatments is expected to have a
corresponding decline according to the degree of tolerance. Since imidacloprid
and thiamethoxam are chemically related, the chance for cross-resistance is
high. The thiamethoxam’s metabolite, clothianidin, acts with high affinity on
imidacloprid sites like other neonicotinoids, making thiamethoxam a poor option
for insecticide rotation and resistance management (Nauen et al. 2003).

The effect of application method on the compound’s length of activity is
unclear, as is the effect of high rainfall or irrigation levels. How this response
interacts with mineral vs. organic soils and against susceptible and resistant
beetles is also unknown. Therefore, knowing the effect these factors have on
insecticide performance and how they interact is crucial for chemical control
optimization.

The objectives of this research were to:

Determine the effect of soil type (mineral soils versus organic soils) on beetle

mortality from the two insecticides under greenhouse conditions.

100

Determine under greenhouse conditions whether in-furrow application or seed
treatment increases the efficacy and residual activity of these neonicotinoids.
Determine the effect of excessive rainfall or irrigation at planting on the residual
activity of imidacloprid and thiamethoxam.

Examine possible interactions between: soil type, application method, and

insecticide and their effect on beetle mortality.

MATERIALS AND METHODS
The efficacy and residual activity of imidacloprid and thiamethoxam-based
insecticides on Colorado potato beetle adults were assessed through no-choice
feeding bioassays conducted in the laboratory, using treated foliage from

greenhouse plants that had received an at-planting insecticide treatment.

Beetle strain

A susceptible strain of Colorado potato beetle was used in the study.
Adults were purchased from the Phillip Alampi Beneficial Insect Rearing
Laboratory of the New Jersey Department of Agriculture. Upon arrival, beetles
were 3-5 d post emergence from pupation. They were kept for 5 d at room
temperature (ca. 17°C) and fed daily on untreated greenhouse potato foliage,
then transferred to growth chamber at 10°C, with only one initial food supply, and
used in the study within 1 - 2 wks of arrival. The day prior to the bioassay, the
beetles were taken out of the 10° C growth chamber and kept in the laboratory

with no food to ensure that all were active and hungry.

101

Plants

Potato plants cv. Atlantic were grown under greenhouse conditions. Seed
was manually cut and seed piece weight was 84.7 g :1: 1.4 (SE). Two soil types
were used: mineral soil from the Michigan State University (MSU) Montcalm
Potato Research Farm, Montcalm Co, MI; and organic soil from the MSU Muck
Soils Research Farm, Clinton Co, MI.

The Montcalm Farm has a typical mineral soil, used for potato production
in Mid-Michigan, a McBride Sandy Loam, with pH of 6.7 < 5% organic matter.
Fraction components for this soil are: sand 80%, silt 9% and clay 11% (Cottrell,
2003)

The MSU Muck Farm has a highly organic soil commonly used for
vegetable crops, a Houghton Muck containing > 75% organic matter and a pH
between 6.2 and 6.9 (Cottrell 2003).

Both soil types were collected from a depth of 0 - 25 cm and were
transported in pots to the greenhouse. The pots used were cylindrical plastic pots

22 cm tall and 23 cm diameter, drilled with additional holes for drainage.

Watering

Two water levels were tested: 1) normal watering throughout plant growth
and 2) excessive watering immediately after planting, followed by normal
watering thereafter. Normal water corresponded to the irrigation provided with a

sprinkler attached to a hose. Applied water amounts are shown in Figure 8. For

102

 

cm of watering

 

38 45

 

 

Days after planting

 

a Mineral soil I Organic soil jI

 

Figure 8. Accumulative weekly amounts of water applied on two soil types
for potato development for the evaluation of neonicotinoid insecticides in
greenhouse

the excessive watering, excess water was applied once, the day after planting,
simulating the occurrence of a 7 cm, 24 h-Iong rainfall event that occurs in
Michigan about every 5 yrs according to regional historic records (Andresen,
MSU Department of Geography, Pers. Comm). The corresponding amount of
water for each pot was calculated for the hypothetical rainfall event. This amount
of water was applied with a sprinkler in five waterings throughout the day.
Preliminary tests indicated that the potted soil did not have appropriate
drainage conditions for excessive watering, especially with mineral soil.
Therefore, the bottoms of all pots were lined with a 2.5 cm-thick bed of pea
gravel and a plastic mesh (1 mm) for gravel retention. Additionally, the mineral
soil was mixed with three parts volume: volume of sand (Play Sand ®). All pots
were filled with soil to within 3 cm from the top. To achieve uniform water
conditions in all the pots, they were saturated at field capacity 2 wks prior to
planting. After the excess water treatment was applied, measurements of water
leach-out were taken. An aluminum pan was placed under each pot and the
water in the pans was measured. Average leach-out was 646.2 ml (134.7 SE)
for organic soil pots and 777.1 ml (21:33.2 SE) for mineral soil pots in the
excessive water treatments. No leach occurred in the normal water treatments.
For the remainder of the experiment water was applied every day or every other
day, cumulative weekly amount are reported in Figure 8. This water was applied
with the sprinkler attached to a hose and the flow was constant and the time of

watering controlled to ensure equal amounts each time. The differences in

104

amount of water in some weeks are related to greenhouse temperatures and soil

type requirement.

Insecticides

The insecticides used were Admire 2F®, and Genesis® (imidacloprid),
Platinum ZSC® and Cruiser 5FS® (thiamethoxam). Insecticide application
methods were: seed treatment (Admire and Platinum) and in-furrow (Genesis

and Cruiser). For the seed treatments, the insecticide was applied to the cut seed

 

at the recommended rate just before planting; and the in-furrow application was I
made immediately after planting and before seed furrow covering.

Treatments resulted from the combination of: active ingredient, application
method, water level, and soil type (Table 23). Untreated controls for each soil
type were also planted. Pots were arranged in the greenhouse in a randomized
complete block design with five replications for each treatment and each
experimental unit consisted of three plants to provide enough foliage for 2 x
weekly collections. Blocking was set to account for gradients in physical
conditions in the greenhouse (natural light, air flow, location of greenhouse
entrance).

The potatoes were watered daily or every 2 d and fertilized weekly with
20-20—20 fertilizer (0.33 g/pot). The photo period was set at 15:9 h light: darkness

and temperatures were 25 - 30°C.

105

Bioassays

For laboratory bioassays, starting the third week after planting, one leaf
from the most developed and robust plant from each experimental unit (3 plants)
was collected. In subsequent weeks plants were selected randomly from each
experimental unit and the newest fully developed leaf was taken. After collection, 1!.-
the leaves were transported to the laboratory in a cooler and were used the same
day in the bioassays.

Each leaf was placed in a Petri dish with the petiole in a vial with water;

 

five beetles were placed in the dish. Food was replaced on day 3 with freshly
collected foliage from the same experimental unit. The dishes were labeled with
the corresponding treatment and replication, randomly arranged on a table in the
laboratory, and kept at 20 - 25 °C. The effect of the treatments on the beetles
was registered after 7 d by recording walking, poisoned, and dead beetles.
“Walking” were those active beetles that showed coordinated, fonNard walking;
“poisoned” were those beetles unable to walk fonNard one body length; and
"dead" beetles were unresponsive to probing with forceps and exhibited dark
coloration, shrunken abdomens or signs of decomposition. Foliage consumption
was estimated by a visual scale from 0 to 10 (from no consumption to totally
consumed leaf). During treatment evaluation, Petri dish labels were covered to
avoid bias. The bioassays were run weekly from 24 d after planting until 80 d

after planting, when plants began to senesce.

106

Table 23. Greenhouse treatments with neonicotinoid insecticides, for

control of the Colorado potato beetle.

 

Application
Product Active ingredient Soil Type Water level
method
Admire imidacloprid in-furrow mineral normal
Admire imidacloprid in-furrow mineral excessive
Admire imidacloprid in-furrow organic normal
Admire imidacloprid in-furrow organic excessive
Genesis imidacloprid seed treatment mineral normal
Genesis imidacloprid seed treatment mineral excessive
Genesis imidacloprid seed treatment organic normal
Genesis imidacloprid seed treatment organic excessive
Platinum thiamethoxam in-furrow mineral normal
Platinum thiamethoxam in-furrow mineral excessive
Platinum thiamethoxam in-furrow organic normal
Platinum thiamethoxam in-furrow organic excessive
Cruiser thiamethoxam seed treatment mineral normal
Cruiser thiamethoxam seed treatment mineral excessive
Cruiser thiamethoxam seed treatment organic normal
Cruiser thiamethoxam seed treatment organic excessive
Untreated - - mineral normal
Untreated - - organic normal

 

107

 

Recovery from intoxication

Each week, after evaluation on day 7, the treated foliage and dead beetles
were removed and the walking and poisoned beetles were kept with untreated
foliage under laboratory conditions to observe recovery of the poisoned beetles,
3 and 7 d later. Recovery from intoxication after lengthy periods sometimes
occurs in the laboratory with neonicotinoids (Zhao et al. 2000) but its significance
in the field is unknown. Harsh environmental conditions and predators or
scavengers may not allow long periods of intoxication in the field. Recovery was
recorded on day 10 and day 14 (from the initial the bioassay set up) as a new

assessment in the categories walking, poisoned or dead beetles as used before.
Data processing and statistical analysis

The collected data of the experiments was computed as percentage of
beetles from the total used per Petri dish. Two response variables were taken
directly from data recorded in the experiment: the percentage of dead beetles
and the percentage affected beetles (dead plus poisoned beetles). Mortality and
affected beetles in the controls was zero or near zero on all dates so the
treatment data were not corrected for control mortality.

For either response variable the following data analysis was done:

Since the data exhibited a Binomial distribution and because the response
variables were categorical, the data were analyzed as categorical. Proc Genmod
in the SAS System® (SAS Release 8.02 Level 02M0) and the Poisson
distribution was used to describe the data distribution. Type 3 analyses were

conducted to detect the significance of the main factors and interactions using

108

the X2 statistic. An analysis for the whole study was conducted to test the total
effect of the variables studied. Then, individual analyses by each date were
conducted. Least Square Means were also calculated to analyze differences
between treatments.

Due to the many interactions this study involves, the main comparisons
were grouped, contrasting a “best case scenario” of mineral soil and normal
water level, compared to the treatment combinations with excessive water and/or
organic soil. Graphic results are shown in split plots separating them by soil type
and water level (Figure 9 -13).

The analysis of the treatment residual activity was assisted by Probit
analyses, which were conducted when mortality of affected beetles included a
range above and below 50% as time progressed. Parameters for this analysis
(Median lethal time LT50 with confidential limits and slope) were estimated to
compare the overall efficacy of the treatments.

Data for beetle recovery were grouped by treatment and are preliminarily
presented here in a plot of treatments recovery vs. days after application. This
intends to show general tendencies of this variable. Because initial numbers of
poisoned beetles varied so widely (0 to 25 per treatment), statistical comparison
of percent recovery would be of questionable value. For example 100% recovery

could be 1 out of 1 initially poisoned or 25 out of 25 initially poisoned.

RESULTS AND DISCUSSION

Evaluation for total affected beetles

109

Weekly results. On day 24 after planting, all the treatments resulted in 60
-100% affected beetles (Figure 9). Efficacies in the mineral soils were in the
range 90 -100% affected beetles while efficacies in the organic soils were more
variable within a range 60 — 100% affected beetles. The mineral soil treatments
resulted in significantly higher percentage of affected beetles than organic soil
treatments (Table 24). ‘Water level’ also showed statistical differences (Table 24)
in affected beetles with higher percentages for the ‘normal water’ vs. ‘excess
water’. This shows an initial reduction in percentage of affected beetles due to
‘organic soil’ and ‘excess water’. On day 31 after planting, ‘soil type’ was the
only main factor that caused significant effects in percentage affected beetles
(Table 25); the mineral soil resulted in higher percentages of affected beetles
than did the organic soil. Seed treatment affected more beetles than in-furrow
applications for thiamethoxam, while imidacloprid affected more beetles with in-
furrow applications than it did as seed treatments. This was evident in the
significant interaction ‘active ingredient * application method’ (Table 25). And this
interaction was more evident in the organic soil than in mineral soil, resulting in a
significant second-order interaction ‘application method * active ingredient * soil
type’ (Table 25).

There was no effect of ‘water level’ on day 31. Significant differences
(LSMeans test P<5%) were only observed between the most effective treatments
(near 100%) in the mineral soil and the two treatments with lowest efficacy,

thiamethoxam in-furrow and imidacloprid, seed-treatment at the excessive water

level in organic soils (Figure 9).

110

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113

On day 38 after planting the main factor effects of ‘soil type’ and ‘active
ingredient’ significantly affected percentage affected beetles and ‘water level’ did
not (Table 26). The percentage of affected beetles was consistently high for the
mineral soil; more variability and fewer affected beetles were observed in the
organic soil treatments (Figure 9). lmidacloprid products performed better than
thiamethoxam. The interaction ‘soil type * active ingredient' was significant as
well (Table 26), this interaction shows that imidacloprid affected more beetles in
organic soil than thiamethoxam did, but in the mineral soil, both active
ingredients performed equally well (80 - 100% affected beetles). In the organic
soils, imidacloprid applied in furrow affected high percentages of beetles at both
water levels that significantly higher than results for other treatments (LSMeans P
< 0.05) in the excess water conditions (Figure 9).

On day 45 after planting, the factors ‘soil type’ and ‘active ingredient’
continued to be statistically significant (Table 27.). lmidacloprid products affected
more beetles than did thiamethoxam products. Most treatments in mineral soils
were close to the 100% efficacy while the organic soils induced much variability
and the percentage of affected beetles was reduced in several treatments. The
interaction ‘active ingredient * application method’ was also significant: while the
imidacloprid in-furrow application resulted in more affected beetles than the seed
treatment; the thiamethoxam seed treatment performed better than in-furrow
application. All the treatments in the optimal soil and water conditions continued
to affect > 75% of beetles. In the organic soil high variability between treatments

was observed, but imidacloprid in-furrow continued to affect significantly more

114

 

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116

beetles (statistical significance for LSMeans, P < 0.05) than other treatments in
that soil.

On day 52 after planting, the factors ‘soil type’ and “active ingredient’ were
again significant (Table 28). More beetles were affected in the mineral soils than
in the organic soils and generally more beetles were affected in treatments with
imidacloprid compared to treatments with thiamethoxam. ‘Water level’ was
significant, after 4 wks of not showing significant effects; this may be the result of
reduced amount of insecticide shortening residual activity. The interaction
‘insecticide * application method’ was also significant; again, imidacloprid
generally was more effective when applied in-furrow, and thiamethoxam
performed better as a seed treatment. Within the organic soils the factor ‘water
level’ was not significant statistically but some biological effect may still exist
(X2(1) = 3.76, P = 0.0525). The best-case scenario treatments (mineral soil and
normal water) remained at the top with no differences between them. The effect
of thiamethoxam in furrow continued to decline and its difference efficacy
compared with the top treatments was significantly lower under excess water
treatments (LSMeans P < 0.05). Under excessive water conditions on mineral
soils thiamethoxam in-furrow was significantly less effective (X2(1) = 4.41, P
=0.0358) than the other treatments. In the organic soils all treatments but
imidacloprid in-furrow, affected less than 60% of beetles and percentage affected
beetles continued a slight, steady decline especially in the excessive water

treatments. Thiamethoxam in-furrow was statistically significant less effective

117

 

 

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118

(X2(1) = 8.99, P = 0.0027) than thiamethoxam as a seed treatment. In both soil
types this active ingredient was affected by excessive water. (Figure 9 c,d )
On day 59 after planting, only the factor ‘soil type’ was significant with more
beetles were affected in mineral soil treatments than in organic soil treatments
(Table 29). Also significant was the interaction ‘application method * active
ingredient’. lmidacloprid was more effective when applied in furrow than when
applied as a seed treatment, and thiamethoxam was more effective as seed
treatment. This interaction varied with ‘soil type’ when a second-order interaction
‘soil type * active ingredient * application method’ was also significant. The
decline of affected beetles for thiamethoxam in-furrow in mineral soils stabilized
at ca. 70%. In-furrow imidacloprid continued to be the most effective insecticide
at both water levels in the organic soil. For the optimal conditions a decline in
percentage affected beetles for imidacloprid as seed treatment was observed.
On day 66 after planting, the main factors ‘insecticide’ and ‘soil type’ had
significantly affected percentage affected beetles (Table 30). lmidacloprid
treatments generally affected more beetles than thiamethoxam treatments. The
interaction ‘insecticide * application method’ continued to be significant (Table
30); imidacloprid affected more beetles when applied in furrow and
thiamethoxam affected more beetles when applied as seed treatment. ‘Water
level’ was not significant. In-furrow imidacloprid continued to be the most
effective in both soil types, and resulted in > 80% of affected beetles. However,
for first time in the study, this percentage was not close to the 100% and a slight

decline was observed. The interaction ‘soil type * water level * active ingredient’

119

 

 

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121

was also significant.

On day 80, ‘soil type’ continued to have a significant effect, with generally
more affected beetles on mineral soils (Table 31). ‘Active ingredient’ was highly
significant in favor of imidacloprid treatments over thiamethoxam. Also significant
was the interaction ‘active ingredient * application method’. Again imidacloprid
was more effective with the in-furrow application and thiamethoxam was more
effective with the seed treatment, but this relation was more evident in the
organic soil than in the mineral soil. ln-furrow imidacloprid continued to affect >
80%or the beetles for both soil types and both water levels. Thiamethoxam in

furrow affected < 40% of the beetles in all the conditions.

Overall results The factor ‘soil type’ was significant throughout the
experiment (Table 32), showing the differential effect of the tested soil types on
the insecticides’ effectiveness (Figure 9). Under normal water conditions
percentages of affected beetles were generally 70 —- 100% throughout the
experiment in mineral soils and 40 — 90% in organic soils (Figure 9). The factor
“active ingredient’ was significant; imidacloprid generally affected more beetles
than thiamethoxam at the application rates used. Overall the excessive water
resulted in significantly reduced percentages of affected beetles (Table 32).
‘Days after planting’ was significant, showing the expected decline in percentage
affected beetles, however the decline was not steep as expected based on the
results from the field chapter 2. No overall effect of “application method’ on

percentage affected beetles was detected.

122

 

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125

The significant effect of the interaction “days after planting * active
ingredient’ (Table 32) showed that the dynamics for both active ingredients did
not follow the same trend. A significant interaction ‘soil type * active ingredient’
(Table 32) showed that the soil affected each active ingredient to a different
extent: while in the mineral soil the imidacloprid products generally caused higher
percentages of affected beetles; in the organic soils there was no consistent
difference between the two active ingredients

The interaction ‘active ingredient * application method’ was significant
(Table 32) showing a differential effect of the application methods tested in this
work for each active ingredient. lmidacloprid affected more beetles when applied
in furrow compared with the imidacloprid seed treatment, but thiamethoxam
affected more beetles when applied as a seed treatment. This last interaction
varied during the experiment resulting in a second order interaction ‘days after
planting * active ingredient * application method’. The interaction ‘soil type *
active ingredient * application method’ was also significant (Table 32)

Some treatments were less sensitive than others to ‘water level’ or ‘soil
type’: imidacloprid applied in furrow appeared to be unaffected by the excess
water or by the organic soil (Figure 9). The persistence of imidacloprid, as has
been reported in leaching studies (and probably enhanced by the application
method or the formulation of this commercial product) may be an important factor
for this effect. In contrast, thiamethoxam applied to seed or in furrow seemed to

be more responsive to water and organic soil.

126

In the mineral soil treatments, a high initial level of affected beetles was
observed in all treatments. After 3 wks effectiveness of thiamethoxam in furrow
started to decline, and effects were accentuated by the excess water (Figure 9).
The imidacloprid products were apparently non responsive to the water excess.

In the organic soil treatments, imidacloprid, applied in furrow caused the
highest percentage of affected beetles. Thiamethoxam applied in furrow for both
soils and water levels showed a steady decline all along the experimental period,

this decline was also followed by thiamethoxam as seed treatment.

Foliage consumption

Overall, beetles in all treatments consumed little foliage compared to
beetles that were fed untreated foliage (Figure 10) and consumption was
inversely proportional to percentage of affected beetles, as expected. For the
normal water level treatments consumption was less than 20% throughout the
experiment, with a slight increase on the last date. For the excess water,
consumption in the imidacloprid seed treatment appeared to be consistently
around 30%. Although the percentage of affected beetles remained high for the
same treatments (Figure 9, c). Higher foliage consumption was observed in the
organic soil which agrees with the lower percentage affected beetles from
treatments in this soil. High foliage consumption for thiamethoxam in furrow
agrees with the low efficacy of that treatment under those conditions. In general
excessive water may have had some effect in increasing foliage consumption for

both soil types, for imidacloprid as seed treatment and in-furrow thiamethoxam.

127

 

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128

This response agrees with the percentage affected beetles for these treatments

(Figure 9).

Recovery of poisoned beetles

Recovery was observed for poisoned beetles in treatments in organic soils
than in mineral soils throughout the study. High levels of recovery (75 - 100%)
were registered for imidacloprid as seed treatment in the organic soil throughout
the experiment (Figure 11). The organic soil seemed to have increases in
recovery sooner than the other conditions. Most of the changes from low to high
recovery were seen after day 38 after planting in nearly all cases.

These observations suggest that the residual activity that results in beetle
poisoning induces weaker symptoms as time passes and makes the beetles
recover sooner. Also, under greenhouse conditions, even though plants seem to
maintain insecticide uptake throughout the season, the efficacy of the insecticide
is lower as time passes. Zhao et al. (2000) reported more recovery of Colorado
potato beetles from topical doses that caused intermediate mortality than from
high doses.

Recovery from intoxication has also been observed in beetles treated
topically with neonicotinoids (Zhao et al. 2000). However, much more recovery
was observed in my study than in Zhao et al. (up to 100% of poisoned beetles
recovered in this study). The higher levels of recovery are probably the result of
the feeding type assay. Beetles did not receive a measured topical dose but ate

only until they became intoxicated and likely did not receive high doses of

129

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130

insecticide except in the most potent treatments. The significance for field
situations of this recovery 7 d or more after intoxication is not known. It is
questionable whether poisoned beetles would survive adverse environmental
conditions in the field, predators, etc. for 7 to 14 d in the field and recover to

become active pests.

Discussion of ‘total affected beetles’ variable

‘Water level’ probably had a delayed effect that was only
statistically significant on day 52 and more easily seen for thiamethoxam
treatments. After that date, that effect stabilized. Thiamethoxam applied in-furrow
may have been more sensitive to the excessive water due to components in the
formulation adding to the more water solubility of the active ingredient compared
with imidacloprid. In addition, due to differences in product allocation in the in-
furrow treatment (along a band as opposed to the more concentrated seed
treatment), insecticide exposure to the water flow could have been higher.

For the potted plants in the greenhouse, the organic soil did not have the
clear effect that was seen in field studies (Chapter 2). These results however,
may
be contrasted with recovery information where the lasting effect of poisoning is
tested. Although the recovery information gathered here is preliminary, some
general tendencies probably occurred. The medium efficacy levels of
imidacloprid seed treatments in soils were followed by high levels of recovery

(ca. 90%) in almost the entire study. In contrast, in-furrow treatments, that had

131

high percentage of affected beetles, did not show high recovery levels at the
beginning of the season and they reached high recovery values only after day 45
after planting (Figure 11 a, b, c).

lmidacloprid in-furrow had a high efficacy in organic soil and was not
affected by the water level at either soil type, as opposed to imidacloprid as seed
treatment that was affected in the organic soils but not in the mineral soil by the
water level. This was due probably to the lower solubility level of imidacloprid.
Surprisingly the organic soil did not affect imidacloprid in furrow, as it did with the
other treatments.

For the mineral soils the recovery values are low at the beginning of the
study for all treatments (Figure 11 a, c). This corresponds to the high efficacy
levels obtained at that time.

At the beginning of the season ‘soil type’ seems to be a main source of
variation with higher recovery in organic soil treatments but that pattern fades in
the last weeks of study (Figure 11). The product performances tend to stabilize
for this response variable. Except that imidacloprid in-furrow and both
thiamethoxam treatments in mineral soil shoed low recovery during most of the

experiment.

Dead beetles (no movement, darkened color, sunken abdomen)

Mortality from neonicotinoids is slower than with other insecticides, despite
intoxication occurring rapidly. Zhao el al. (2000) reported initial knock down in 2.4
h but no beetles died in the first day after treatment. In my study even on day 7,

fewer beetles were dead than were rated as affected (poisoned or dead) in most

132

treatments (Figure 9 vs. Figure 12). Dead plus affected beetles is probably the
more accurate category for measuring overall treatment impact early in the
course of this study. However, the high percentages of recovery late in the
study, 7 - 14 days after treatment, in some treatments, confuses the pictures.

As mentioned in the recovery section, whether recovery 7 — 14 d after treatments
occurs under field conditions is unknown.

Weekly results. On day 24 after planting, the factor ‘soil type’ was the only main
factor significantly affecting percentage mortality (Table 33). ‘Water level’ did not
have a significant effect and this could be associated with small plants and short
roots in the young plants that were tested on this date because all of them could
reach the insecticide. Neither “active ingredient’ nor ‘application method’ had a
significant effect on this date. Within the mineral soil, only ‘active ingredient’ was
highly significant (X2(1)= 17.76, P <0.0001); thiamethoxam products caused
higher mortality than imidacloprid, in all cases. Only thiamethoxam as seed
treatment on mineral soil at both water levels was above 60% mortality. These
insecticide treatments caused significantly higher mortality (LSMeans P < 0.05)
than the other treatments on mineral soils or organic soil treatments. The other
treatments in mineral soils reached low to medium mortality levels. In the organic

soil no mortality was higher than 10%.

On day 31 after planting, ‘soil type’ was the only factor significantly
affecting percentage mortality (Table 34). Within the mineral soils only, ‘active

ingredient’ was highly significant (X2(1)= 51.22, p <.0001). ‘Water level’ was not

133

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significant within the mineral soils. Thiamethoxam as a seed treatment continued
to be the top treatment in mineral soils at both water levels (Figure 12 a, b). At
normal water and in mineral soils, thiamethoxam caused the same mortality with
either application method. The impact of water excess on the percentage
mortality was not significant (x2(1)= 3.20, P = 0.0736). On this date three
treatments caused > 70% mortality. lmidacloprid products fell below 20%
mortality. In the organic soil, mortality in all treatments was near zero.

On day 38 after planting ‘active ingredient’ was the only factor that
significantly affected percentage mortality, as was also shown by the LR Analysis
(Table 35). However soil type was also significant using LSMeans (X2(1)= 5.71, p
0.02). The same three treatments continued to cause > 70% mortality.
Thiamethoxam treatments caused similar levels of mortality to results of the
previous weeks and were again statistically significantly higher than imidacloprid
insecticides, all on mineral soil treatments.

On day 45 after planting soil type significantly affected percentage
mortality (Table 36). There was also a ‘block’ effect observed on this date.
Although ‘active ingredient’ did not significantly affect mortality in this analysis,
the differences between the two active ingredients within the mineral soil was
significant (X2(1)=7.39 p 0.0066) and thiamethoxam products continued to show
better performance than imidacloprid Thiamethoxam products on the mineral soil
started to reduce the obtained mortality levels. All of the thiamethoxam
treatments decreased in mortality. Thiamethoxam as seed treatments was

now around 40% at both water levels and thiamethoxam as in-furrow (Platinum)

137

 

 

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139

is 40% at normal water and less than 10°/o at excessive water and this
differences are statistically significant (X2(1) = 9.48, P = 0.0021). Thiamethoxam
treatments on medium level of efficacy kept significant differences (LSMeans P <
0.05) with the bottom-controlling treatments. None of the treatments in the
organic soil show more than 10°/o mortality.

On day 52 after planting no significant effects were observed on this date
(Table 37). All treatments dropped to around < 20% mortality or less. No
significant treatments effects were detected for this date.

On day 59 after planting no significant treatment effects were observed
(Table 38) and mortality was < 10% in all treatments but thiamethoxam as seed
treatment in the mineral soil. The higher level of mortality of this treatment was
significant.

On day 66 and day 80 all treatments showed mortality levels near or equal

to zero. No analysis was conducted.

Overall analysis for the ‘dead beetles’ variable

Overall, the soil type significantly affected percentage mortality (Table 39).
The detrimental effect of the organic soil produced marked differences in
mortality in contrast to the lower (but still significant) impact on percentage
affected beetles. The factor “active ingredient’ was also significant; thiamethoxam
products caused more mortality during the time span the insecticides were active
for mortality. In contrast, imidacloprid often caused more “affected beetles"

(poisoned plus dead) no other main factor effect or interactions were significant

140

 

 

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144

for the total analysis.

Highly significant differences occurred in percentage mortality between the
two soil types. For the mineral soils, mortality in all insecticides declined to less
than 20% by day 52. For the organic coil mortality was never > 20%The effect of
soil tends to diminish after day 38 and then all treatments produced low mortality
after that.

In thiamethoxam treatments mortality was significantly higher than in
imidacloprid treatments until about day 52 as well. The excessive water may
have a slight positive effect on these products on organic soil, perhaps
compensating slightly for binding with the organic soil fraction. Due to the low
mortality in the organic soils no LT50 could be calculated for this soil type. In the
mineral soil LT50 only could be calculated for the thiamethoxam treatments that
resulted in some mortality. Under these conditions the expected declining curve

of mortality is only shown by the thiamethoxam treatments (Figure 13)

Treatment residual activity

The percentage of dead beetles in most treatments was < 50% and probit
analysis could not be run. Only three treatments were included in this analysis,
all of them for mineral soils with thiamethoxam as active ingredient. The
treatments were: seed treatment and in-furrow application for the normal water
and the seed treatment for the excessive water. The three treatments analyzed

had similar Median Lethal Time LT50 values of 32.6 to 45.4 d (Table 40).

145

 

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147

General discussion

lmidacloprid as seed treatment showed medium to high percentages of
affected beetles in treatment combinations, but its mortality levels were very low.
This difference however, agrees with the higher foliage consumption by the end
of the study period (compared with the rest of the treatments, Figure 10) and high
recovery (Figure11).

lmidacloprid applied in furrow, also showed a high percentage of affected
beetles (Figure 9) and resulted in low foliage consumption (Figure 10). Recovery
of the beetles poisoned in this treatment was especially common in the second
half of the study (Figure 11), although that recovery was not associated with
foliage consumption. Even though in-furrow imidacloprid showed high recovery
and low beetle mortality, the low foliage consumption is a positive effect of those
treatments, which would give this insecticide eligibility as plant protection tool.

The effect of the excessive water used in this study was generally low,
contrary to what I had expected given the high solubility values imidacloprid and
thiamethoxam. These results however, support the findings that no effect of
water-saturated soil on the persistence of imidacloprid has been reported
(lndumathi et al. 2001). An initial small difference to the effect of water is rapidly
balanced and only a late effect was observed in the in-furrow thiamethoxam
treatments. The watering technique I used probably created conditions of water
saturation but not the water flow dynamics that may occur in the soil when low
and constant irrigations are used. Improvement of imidacloprid performance by

means of irrigation system manipulation has been demonstrated for potted plants

148

(van lersel et al. 2001). However this benefit seems to be possible only under
certain range of water content, way below the field capacity where more
interaction insecticide-water flow is allowed improving insecticide uptake. This
was not the case in this study due to the high requirements of potato plants
under these greenhouse conditions (Figure 7) and the poor drainage conditions
observed. Still water level may have some effect in reducing the numbers of
affected beetles and shortening residual activity in sub-lethal (total affected
beetles) but not for lethal effects (dead beetles).

Organic soils not only induced a lower performance on the insecticides,
also makes the interactions with the soil factors more variable and less
predictive.

The number of total affected beetles by imidacloprid in-furrow was not
affected by the soil type in the greenhouse. This is contrary to what was
observed in the field (Chapter 2). No clear explanation for this finding can be
reported at this time; the rapid plant growth, the irrigation regime, and higher
desorption of the active ingredient, may all could be involved in this response.
For the potted plants the root system has more chance to use the applied
insecticide, perhaps explaining this phenomenon. By the end of the experiment
plant roots had entirely filled the pots, as can be seen in Figure 14.

The noticeable difference between the variables poisoned and dead
beetles in the greenhouse experiments could be explained in part by the

differences in symptoms that sub lethal and lethal neonicotinoid poisoning have.

149

Thus, for the experimental arrangement used, the pot probably generated

favorable conditions for

 

Figure 14. Root growth of potted potatoes grown under greenhouse conditions to
study residual activity of neonicotinoid insecticides in mineral soil (A) and organic
soil (B).

150

sustained insecticide availability at low concentration, producing sub-lethal
effects. The insecticide that is taken by the potted plants could be not enough to
kill or severally poisoned the beetles, but just to produce a knock-down effect,
that did not last more than 7 d and recovery occurred during that time.

This difference between percentage affected and percentage dead beetles
appeared to be larger for imidacloprid treatments, suggesting that imidacloprid
has a broader range of concentration for the production of sub-lethal effects, than
thiamethoxam.

The effect of application method could have been masked by the set up
conditions used in the experiment. After planting, and with the continuous
watering and confinement in the pot, the insecticides distribution in the pots could
be equalized for the two application methods, this is supported by the fact that
movement of neonicotinoids is reported to be active only in 20 — 30 cm upper
levels of the soil (van lersel et al. 2001, Krohn and Hellpointner 2002).
Additionally the appropriate greenhouse conditions for the plants make them
grow fast and rapidly place their roots in the available space.

All responses in affected beetles may have been influenced by the
arrangement in which the plants were set up. More effect was seen in
imidacloprid products than in thiamethoxam. The pots | used were 22 cm tall but
the root system of this potato variety may go far beyond that. After removing
plants form the pots it was observed that the root system had grown to much of
the available spaces in the pot. As the plant grows, and due to the persistence of

the neonicotinoids in the first 30 cm of the soil, the root system may be taken

151

 

more insecticide that is desorbed through time in the pots. These conditions did
not promote insecticide uptake to levels enough to cause more mortality on
beetles.

Uptake and biological effect of neonicotinoids in potato seem to be very
related with plant physiology and the environmental conditions that govern plant
growth and development. An optimal uptake could be obtained if the root system
is always in the zone were the insecticides are present in the soil.

Conclusions Greenhouse studies

A regime of excessive water in the way was applied here, did not have a
detrimental effect for the residual activity of neonicotinoids.

Confined conditions for plant development, like potted plants provide them
with a modified environment that affect neonicotinoid residual activity,
presumably by affecting insecticide uptake.

Beetle true mortality is a variable that define better a differential effect of
the two neonicotinoids studied.

A complement of the analysis for insecticide effect is the recovery that

some treatments may be followed by.

152

 

 

 

 

CHAPTER 4:

OVERALL CONCLUSIONS

lmidacloprid and thiamethoxam are good options for control of Colorado
potato beetle adults. Both products show 2 80% affected beetles or more under
field conditions with loam sandy soil, which is the most common type of soil for
potato production. I;

Although soil- or seed-applied neonicotinoid insecticides are a good
strategy for control of the Colorado potato beetle, reduced activity may occur if

factors like insecticide tolerance or high organic matter content are present.

 

Organic matter content is detrimental to the residual activity of
neonicotinoid insecticides when used as at planting applications for Colorado
potato beetle control. Seed treatments may reduce this detrimental effect
compared to in-furrow applications.

When applied in organic soils such as the muck soils studied here,
efficacy and residual activity of neonicotinoids was severely hampered. Under
greenhouse experiments this detrimental effect was even more apparent on
percentage beetle mortality than on percentage affected beetles.

Planting conditions, like those imposed under greenhouse, may alter the
expected detrimental effect of organic soils on residual activity of neonicotinoid
insecticides. Different patter of water flow, root growth would be associated with
these changes. However, when lethal effects (dead beetles) were recorded the
depressing effect of organic soils on these neonicotinoid insecticides efficacies

was clearly observed.

153

Water regime at the levels tested in this study may have an impact in
reduce the activity of thiamethoxam soil applied formulations. Excessive water
regimes may reduce thiamethoxam efficacy. In this case seed treatments may
be an alternative to limit that effect.

Both experiments showed that there is potential for a reduced efficacy in
the middle of the season (about day 50 after application) for thiamethoxam
applied in furrow (Platinum) and the water excess may be associated with that
potential.

The low rate used in this work did not reduce product efficacy; it provided
good performance and could be used under commercial conditions to reduce the
amount of product applied thus reducing application costs.

No general conclusion about the effect of excess water can be drawn from
this study; however these results indicated that excess water may be associated
with reduced biological activity.

A steep decline in percent affected beetles under field conditions is
expected by day 40-45 after application.

lmidacloprid products are more sensitive than thiamethoxam to adverse
factors studied in this research.

Seed treatment applications may help circumvent the detrimental effect of
organic soils. Effect of application method is variable in mineral soil, but in furrow

applications tended to have longer residual activity.

154

Greenhouse conditions such as potted plants, have strong effect in the
residual activity of neonicotinoid insecticides, making it difficult to apply results to

field situations. Changes in the plant physiology may be involved in such effect.

 

 

155

 

CHAPTER 5:

RESEARCH PROJECTION

The hypothesis about the negative influence of organic soils on
neonicotinoid activity was confirmed by this research. Soil type, related to
organic matter content, strongly determines neonicotinoid residual activity when
applied at planting for control of Colorado potato beetle. This phenomenon was
observed between the two sites of the field studies and confirmed in the
greenhouse studies.

Once the association of organic soil with a drastic reduction in insecticide
efficacy is determined, the next step is to determine what level of organic matter
is critical in reducing the efficacy of the products. Also important it is to determine
if certain types of soil (from those found in commercial potato areas) are more
sensitive to changes in this soil feature.

Feeding bioassays are a useful tool for determination of insecticide
biological activity and residual activity of neonicotinoid insecticides on Colorado
potato beetle under controlled conditions. This technique could be used in future
studies that address similar questions to those approached here. Particularly,
keeping turgid leaves, during the time each bioassay was conducted, was
important for consistent results.

The set up used here, worked well when plants from the field were taken.
However, for the plants kept in greenhouse, it may need some adjustments to
more effectively reflect the conditions that plants encounter under field

conditions. Some of the adjustments to the set ups use here could include:

156

bigger pots, non disturbed soil and a watering regime that better simulates the
rainfall situation that a potato crop has in the field.

Confirming that the reduced application rate does not have important
effect on affected beetles or dead beetles is important in terms of optimizing the
management program. Reduced rates are expected to results in shorter residual
activity. However, under the production conditions of mineral soil and susceptible
beetle strains, the reduced rates may give results as good as the full rate.

The excessive irrigation simulated here was chosen as one of the extreme
situations that can occur under field conditions. However this is not the only
variable that can be encountered in the field and further evaluations are required
where other regimes may be tested for their effect on neonicotinoid efficacy and
potential leaching. More detailed research is recommended on the effects that
excessive rainfall seasons may be causing for neonicotinoid programs of
Colorado potato beetle control, testing a more precise level and frequency of
rainfall event that may be detrimental for insecticide performance.

Future research on effects of irrigation and rainfall should be conducted in
the field with undisturbed soil structure. Watering regimes that affects potato root
growth in the soil profile should be considered as a variable.

Complementarily to what was seen here, overall length of effective control
in the field is likely to be longer than reported here because larvae, which
commonly occur early in the season, are much more susceptible to these

products than adults.

157

 

The neonicotinoid insecticides evaluated here give effective control under
the common conditions for potato growth. However detrimental factors like
organic soil or a low level of resistance to imidacloprid may induce a reduction in
efficacy. Longer residual activity is in general expected for soils with a sand
fraction and mineral composition than in organic soils. Duration of protection
found here and elsewhere shows that periods of 40 -50 days are acceptable as
good treatments. lmidacloprid however has been found to have long lasting
protection against aphids (3-4 months). This difference may be because the
Colorado potato beetle consumes the entire foliage, with a lower concentration of
insecticide than the phloem sap ingested by sucking insects.

Thiamethoxam had a more lethal effect in my experiments perhaps
because this insecticide is metabolized to clothianidin before lethal action is
reached. This could allow the insect to ingest more treated foliage before
becoming intoxicated and consequently to reach lethal amounts of the
insecticide.

Although these experiments depicted an approximate idea of the
performance of the insecticide, when used in control, caution must be taken if the
results are intended to be extrapolated to field conditions. This is true especially
because of the artificial conditions that the feeding bioassays impose. Two
factors are related to the conditions of the experiment: (1) the protection the
beetles have inside the Petri dish with no other factor inflicting mortality, and (2)
the continuous feeding stops that beetles experimented as a response to

poisoning.

158

 

Conducting this research will give more sustained technology for use of
these products with less chance for insecticide resistance occurrence. A
judicious use of the insecticides in the appropriate rates according to the varying
factors should be followed to avoid the occurrence of resistance.

To study the processes that govern water distribution in the soil, their
relation with insecticide movement and their effect on insecticide uptake is crucial
to better understand the effect of heavy rainfall regimes in potato crops.

The inferences obtained here may be better supported by absorption
studies in plant tissue. Contrasting absorption curves, given by such studies,
and the residual activity curves will explain better whether the factors we assume
affect residual activity are really producing changes in insecticide content in plant
tissue and then protecting plants from insect attack.

An approach to the effect in the field will also be supported by
physiological studies on plants under field conditions to know how the insecticide
is distributed in the parts of the plant. In this way the projection for the effect on
larvae of the Colorado potato beetle will be better proposed. Rapid growth of the
potato plant makes very important to find a rapid and consistent absorption of

insecticide for the crop protection

159

 

LITERATURE CITED

Abbott, W. S. 1925. A method of computing the effectiveness of an
insecticide: J. Econ. Entomol. 18 265-267

Acosta,N.M. and R.J. O'Neil. 1999. Life history characteristics of three
populations of Edovum putt/en' Grissell (Hymenoptera: Eulophidae) at
three temperatures. Biol. Control: Academic Press. Orlando, FL. 16: 81-
87.

Andresen J 2003 Personal communication.

Alyokhin, AV. and D.N. Ferro. 1999. Electrophoretic Confirmation of Sperm
Mixing in Mated Colorado Potato Beetles (Coleoptera: Chrysomelidae).
Annals Entomol. Soc. Am. 92: 230-235

Anonymous 2006 http://www.beekeeping.com/articles/usfimidacloprid.htm
lmidacloprid From EXTOXNET - Extension Toxicology Network
Pesticide Information Profiles

Anonymous 2004 a www.ers.usda.gov/publications/vgs/tableslworld.xls
“World table 1--Se|ected vegetable production in leading countries and
the world, 1993-2002 1/”

Anonymous 2004 b Major constrains in World potato Atlas - Iran
http://www.cipotatoorgNVPA/asia/irgn.htm#Prodiction%20constr_aints

Anonymous 2004 c Resistant Pest Management: Arthropod Database

h_ttg:l/www.gesticideresistance.org/DB/sgecies profile.php?arthrogodid=
a Michigan State University-Center for Integrated Plant Systems

 

Anonymous 2004 d Michigan Agricultural Experimental Station- Montcalm
Research Farm http://www.maes.msu.edu/montcalml

Anonymous 2003 a. Economic research service. USDA, 2003 Briefing room.
Potatoes: background.

h_ttp://www.ers.usda.gov/briefinglgotatoes/background.htm
Anonymous 2003 b NASS http://www.nass.usda.gov/mi/presslpr0198.txt

 

Anonymous 2003 c [NASS] MEI/wwwnass.usda.gov:81lipedb/ consulted on
12/11/03

160

 

Anonymous 2003d Material safety data sheet. Product name: Platinum.
Syngenta Crop Protection.
mpzllwwwchiletaskforce.org/agronomv/oest/pesticides/ZOOQesticiclLy
insecticides/glatinum msdspdf

Anomymous 2001 lmidacloprid. Insecticide Factsheet. . Northwest Coalition
for Alternatives to Pesticides/NCAPJournal of Pesticide Reform Vol. 21:
15 -21

Bi, J.L., N.C. Toscano and GR. Ballmer. 2002. Field evaluations of novel
chloronicotinyls and insect growth regulators against the greenhouse
whitefiy on strawberry. HortScience. 37: 914-918.

Biever,K..D., R.l.Chauvin, 1990 Prolonged dormancy in a Pacific Northwest
population of the Colorado potato beetle, Leptinotarsa decemlineata
(SAY) (Coleoptera: Chrysomelidae). Can-Entomol. 122: 175-177

Bishop, B. A. and, E.J. Grafius. 1996. Insecticide resistance in the Colorado
potato beetle, pp. 355-377. In P.H.A. Lolivet and ML. Cox (eds.),
Chrysomelidae biology, vol.1: the classification, phylogeny and genetics.
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