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ECOLOGY OF THE COLORADO POTATO BEETLE IN MEXICO

by
David Lawrence Cappaert

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

 

MASTER OF SCIENCE

Department of Entomology

1988

-7 4. 2..

[T’IZF'T
dd”!

ABSTRACT

ECOLOGY OF THE COLORADO POTATO BEETLE IN MEXICO

by
David Lawrence Cappaert

The ecology and distribution of the Colorado potato beetle (CPB) on the native
host Solanum angusty’oltum were studied in central Mexico. CPB population
dynamics and natural enemy abundance were monitored on small plots. Predators
including asopine petatomids. carabids. and coccinellids were the primary factor
accounting for up to 99.8% CPB mortality. Tachinid larval parasitoids. and a eulophid
egg parasitoid were also observed. The potential of the Mexican CPB to adapt to the
potato was evaluated. Field experiments demonstrate that the potato is not a preferred
host but was colonized at low frequency by Mexican beetles. Larval survival of Mexican
CPB on potato was not significantly different than on the native host. Recent changes
in potato production appear to increase the possibility of a host switch by Mexican CPB.
CPB elevational distribution was investigated in a regional survey. Beetles emerged

later at higher elevation and were absent above 2000 m.

Acknowledgements

Thanks to Dr. Dean Haynes for the initial idea of this project. Dr. Gary Bernon
offered useful advice and a contagious enthusiasm for Leptinotarsa. Drs. Ellie Groden
and Frank Drummond were my greatest resources at Michigan State and remain my
favorite entomologists.

In Mexico. I acknowledge the unusual generosity of Dr. Cesar Garcia Montalvo.
who provided facilities at the UAEM. I also give thanks to Rambn Herrera L6pez. Maria
Leyva Silva. Telésforo Hernandez. Eduardo Aranda. and Jose Manuel Perez Coronel.
who worked on this project and offered their friendship.

I am especiallly grateful for the support and inspiration of my partner. Gina

Amalfitano (she's a doctor too).

iii

Table of Contents

Cdaadopaaiobeetle ..................................... .

Colaacbpaatoboetle .....................................

firiisDevequnentasaPest? ...............................

iv

12
12
l3
14

2388833353

3383983838

8.738383

91

Appendix 1 Distribution of Chrysomeline Beetles and Host Plants in
Macias. Memo ........................................ 109
Appendix 2 Weather Data for Zacatepec Ebcperiment Station ........... l 14

fist of Tables

Table l. 1 Chrysomeline associates of the CPB at 20 survey sites
in Morelos. Mexico. 1987-1988. .....

Table 2.1 Visual count of predators at Xoxocotla. Morelos. .....
Table 2.2 CPB feeding trials .....
Table 2.3 CPB egg consumption rates .....

Table 2.4 Parasitism of CPB egg masses by Edovum puttlerl in
Morelos. Mexico. .....

Table 2.5 Parasitism of CPB fourth instar larvae by Myiophams spp.
in Morelos. Mexico. .....

Table 2.6 Mortality of CPB adults caused by Strongyaster spp. and
unidentified fungal pathogen in Morelos. Mexico. .....

Table 2.7 Incidence of Chrysomelobia labtcomerae on adult CPB
in Morelos. Mexico .....

Table 2.8 Natural enemies of the CPB: key species observed in
Morelos. Mexico. .....

Table 3. 1 Stage specific mortality of the CPB at Xoxocotla.
Morelos. 1987. .....

Table 3.2 Emergence of CPB adults at Xoxocotla. Morelos. .....

Table 3.3 Stage specific mortality of the CPB at Xoxocotla.
Morelos. 1988. .....

Table 3.4 Time series cross correlations between the CPB and its predators
at Xoxocotla. Morelos. 1987. .....

Table 3.5 Regression relationships between the CPB and its predators.
Xoxocotla. Morelos. 1987. .....

Table 4. 1 Survival of CPB egg cohorts at Toluca. Mexico. .....
Table 4.2 UAEM host choice experiments. .....

vi

8&3

8’.

8

List of Figures

Figure 1. 1 CPB and host plant survey locations in Morelos. Mexico .....
Figure 1 .2 Annual temperature and precipitation regime at three
representative elevations in Morelos. Mexico. .....

Figure 1.3 Distribution of the CPB and its host plant at 45 survey sites in
Morelos. Mexico. 1987. .....

Figure 1.4 Relative abundance of the CPB and its host plant at 45 survey sites
in Morelos. Mexico. 1987-1988. .....

Figure 2.1 Parasite sample collection locations in Morelos. Mexico. .....

Figure 2.2 Phenolog of pentatomid species at Xoxocotla. Morelos.
1987-1988. .....

Figure 2.3 Tachinid parasitism in relation to CPB density at

Xoxocotla. Morelos. 1987 .....
Figure 3. 1 CPB Incidence at Xoxocotla. Morelos. Early site. 1987 .....
Figure 3.2 CPB Incidence at Xoxocotla. Morelos. Late site. 1987. .....
Figure 3.3 CPB Incidence at Xoxocotla. Morelos. 1988. .....

Figure 3.4 Relative abundance of key CPB predators.
Xoxocotla. Morelos. .....

Figure 4. 1 Colonization of host choice plots by CPB. field study.
Xoxocotla. Morelos. .....

Figure 4.2 Survival of CPB larvae on two hosts.

UAEM experiments. 1987. ' .....
Figure 4.3 Survival of CPB larvae on S. tuberosum and S. angustifolium

trap plant.. UAEM experiments. 1987. .....

Figure 4.4 Development rate of the CPB on two hosts.
UAEM experiments. 1987. .....

Figure 4.5 Survival of CPB larvae on two hosts. field study.
Xoxocotla. Morelos. 1988. .....

Figure 4.6 Development rate of the CPB on two hosts. field study.
Xoxocotla. Morelos. 1988. .....

vii

21

22

24

47

7O
71

72

95

97

97

INTRODUCTION AND LITERATURE REVIEW

The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is the only pest
among some 40 species in the chrysomeline genus Leptinotarsa (Jacques 1988) In the
1800's, the potato beetle, like the cotton boll weevil, Anthonomus grandis Boheman,
switched from its native hosts in Mexico and rapidly became one of the most infamous
pests in modern agriculture. The progress of these host shifts has been documented through
historical records both for the CPB (Tower 1906; Casagrande 1985) and the boll weevil
(Burke, et al. 1986). In this study, I examine the CPB in the area of its presumed origin in
central Mexico, and describe the ecology of this species in circumstances similiar to those

which preceeded its development as a pest.

CPB Life History. In temperate climates, the CPB overwinters as an adult in soil
beneath its host plant. Beetles in tropical climates undergo a similiar diapause period
during the dry season (Anaya 1988). In temperate areas, emergence occurs in spring in
response to accumulated heat (Lashomb et al. 1984); beetles then feed and mate on the host.
In Mexico, emergence is cued at least partially by changes in soil moisture with the onset of
the rainy season (Tower 1918). Females begin oviposition within a week, depositing egg
masses on host foliage during a four to five week life span. Fecundity of CPB females has
exceeded 4000 eggs in the laboratory (Brown et a1. 1980). Larvae complete four instars on
the host plant, and then pupate in the soil. In the U.S., there are one to three generations per
year, depending on latitude (Gauthier 1981). Adults of the last generation enter diapause
after burrowing into the soil, in response to decreasing photoperiod (in temperate

populations) and declining host plant quality (deWilde and Hsiao 1981).

2

Host Range and Distribution. The CPB probably evolved in central Mexico, the
likely center of origin of the genus Leptinotarsa (Tower 1906). In Mexico, the CPB feeds
on several closely related endemic Solanum species in the section Androceras, which also
evolved in Mexico (Whalen 1979). Common hosts of the CPB include Solanum
angustifolium Mill, S. rostratum Dunal, and S. elaeagnifolium L. (Hsiao 1985).
Solanum angustifolium is the principal host of the CPB south of the Valley of Mexico.
Although the range of S. angustifolium extends to Honduras, the CPB has not been
recorded south of the Isthmus of Tehuantepec (Jacques 1988). North of the Valley of Mexico,
the CPB is found on S. elaeagnifolium in the Mexican central plateau and the U.S.
southwest. Solanum rostratum is the most abundant of the native CPB hosts. With an
original distribution throughout northern Mexico and into the U.S. central plains
(Whalen 1979), S. rostratum has spread in historical times to every continent except
Antarctica (Whalen 1979).

Hsiao (1986) has provided evidence that the host specificity of the CPB in Mexico
closely follows the phylogenetic origin of its hosts. In a test of nine species of section
Androceras, all of which overlap the range of the CPB, only species of the series
Androceras supported adequate growth the CPB, while species of Androceras series
Violaceiflorum and Pacificum did not. This close association with Androceras suggests
that the CPB is innately stenophagous. However, several factors about CPB biology suggest
a propensity for adaptation and host race formation. These include broad environmental
tolerance, high intrinsic rate of increase (Harcourt 1971), and high genetic variability
(Jacobsen and Hsiao 1983). Such characteristics are predictable from the apparent
evolutionary history of the CPB. The primary CPB hosts are patchily distributed
successions] plants with high rates of local extinction (Whalen 1979). The Androceras
species are also characterized by rapid speciation (Whalen 1979). The CPB has thus
evolved under conditions favoring extensive within population variation and effective

dispersal mechanisms to cope with frequent changes in food and habitat.

3

The adaptive potential of the CPB is seen in a study of the pest resistant potato
species Solanum berthaultii. Resistance in S. berthaultii is related to the mechanical
effects of glandular trichomes. In initial trials, Groden and Casagrande (1986) found a
dramatic reduction in CPB oviposition rate and larval survival on S. berthaultii relative
to potato. However, after three generations, significant differences in CPB performance
disappeared. That the CPB overcame S. berthaultii resistance so quickly demonstrates

that factors regulating CPB host affinity are highly plastic.

Evolution of Pest Populations. The CPB has greatly expanded its geographic and
host plant range in historical times. The adaptation of the CPB to the potato, S. tuberosum,
first reported in Nebraska in 1859 (Walsh 1865) provided the impetus for the rapid spread of
a pest population of the CPB. Moving eastward, the beetle reached the Atlantic coast in 1874
and had disseminated throughout the eastern U.S. before 1900 (Riley 1877; Tower 1906).
Movement to the west was more gradual, although the CPB had reached the western slope of
I the Rocky Mountains by 1905 (Haegele and Wakeland 1932). The CPB became established
in France in 1918, and its range now includes most of continental Europe and extends into
Asia (deWilde and Hsiao 1981).

The expansion of the range of the CPB has been accompanied by its adaptation to
several wild and cultivated hosts. Cultivated hosts besides the potato include the tomato,
Lypersicon esculentum, and the eggplant, S. melongena. The CPB also successfully
colonizes at least 10 wild Solanum species in different parts of its range (Hsiao 1982). In
the western U.S., hairy nightshade, S. sarachoides Sendt., is an important secondary host
(Brown et al. 1980; Horton and Capinera 1987). Bittersweet nightshade, S. dulcamara L.,
and the horsenettle, S. carolinense L. , are common alternate hosts in the eastern U.S.

(Hare 1883; Hare and Kennedy 1987).

4

Geographic Races of the CPB. Several studies have demonstrated that geographic
populations of the CPB differ in host affinity and other biological traits. Hsiao (1978)
studied host plant adaptation of four geographic populations of the CPB: two populations
from S. rostratum (New Mexico and Texas), and beetles collected from S. elaeagnifolium
(Arizona) and potato (Utah). Measurements of survival, pupal weight, and larval
development time on seven solanaceous hosts showed significant differences in host
suitability between populations. The clearest result was for Arizona beetles, which
exhibited greater survival and faster development on S. elaeagnifolium , than the other
populations (Hsiao 1978). Host plant adaptation was shown to have both a genetic and an
environmental component. Reciprocal crosses between Arizona and Utah beetles yielded
F-l progeny with a host tolerance that was intermediate to that of the parental generation.
An environmental effect was indicated by an experiment that showed greater fecundity for
Arizona beetles raised both as larva and an adult on a single host (potato or S.
elaeagnifolium) than for beetles that were changed from one host to the other following
adult emergence, i.e., adult host-specific performance was conditioned by larval
experience (Hsiao 1978).

Later studies by Hsiao (1982) and deWilde and Hsiao (1982) investigated the larval
rearing success of 12 CPB populations tested against 11 solanaceous plants. They found
that the potato, bittersweet nightshade, and S. rostratum were suitable hosts for all
geographic populations. Differences in rearing success were found for Mexican beetles
(which did not develop on bittersweet or hairy nightshade) and Arizona CPB (which were
uniquely adapted to S. elaeagnifolium).

Hare and Kennedy (1987) compared host adaptation of CPB from Connecticut and
North Carolina. Both populations were collected from commercial potato fields; however,
S. carolinense was known to be a secondary host only of the North Carolina population.
Rearing experiments demonstrated greater survival of North Carolina beetles on S.

carolinense ;but no differences on potato. Genetic studies established that population

5

differences in survival were heritable (Hare and Kennedy 1987). As in Hsiao's work, this
study demonstrates the rapid evolution of host adapted biotypes of the CPB as the beetle has
encountered new hosts.

Genetically distinct subpopulations of the CPB are also likely to occur within
Mexico. Tower (1906; 1918) investigated the the genus Leptinotarsa in Mexico and
identified nine varieties of L. decemlineata and four species that have since been
determined to be synonyms of L. decemlineata (Jacques 1972 1988). Tower's varieties
differ in distribution and morphological features, primarily coloration and size. How
these varieties differ in host use and the extent to which they define genetically distinct
populations has not been investigated. However, the existence of substantial geographic
barriers within Mexico (Halftter 1987), and the patchy distribution of CPB hosts makes it
likely that Mexican CPB populations are highly variable.

One fact appears to distinguish the CPB in Mero'co from beetles in most of the U.S.
and Europe: Mexican CPB rarely colonize the potato in areas where the range of the two
species overlap (Anaya 1988; T. Hsiao, pers. comm.; A. Marin, pers. comm.). The basis
for this difference in host use has not been investigated. Several studies have, however,
compared the CPB from Mexico to pest biotypes. Jacobsen and Hsiao (1983) used
electrophoresis to compare CPB from the Mexican central plateau to U.S. pest populations:
They concluded that Mexican beetles may constitute a subspecies. Hsiao (1985) used
cytogenetic analysis to provide another clue to the relationship of the Mexican CPB to other
geographic populations. He found evidence, based on a chromosomal inversion, of three
chromosomal races of the CPB. Combining analysis of the distribution of the three
chromosomal races of the CPB with the historical record, Hsiao theorizes that Mexican and
Arizona populations (metacentric race) are the original karyotype of the species. He
suggests that the acrocentric karyotype evolved in Texas and produced the race which

spread across the U.S. and ultimately to Europe on potatoes. The implication that Mexican

6

CPB are genetically distinct from pest biotypes of the CPB may help explain apparent

differences in the host affinity of Mexican beetles.

CPB Natural Enemies. Many endemic natural enemies have been reported to
attack the CPB. Investigating natural enemies associated with the CPB in potatoes, Riley
(1871) listed 26 natural enemies, and Bethune (1872) identified 22 species. A later study by
Wegorek (1959) reports 59 predators and parasitoids of the CPB. Logan et al. (1987) studied
CPB natural enemies on native hosts in Mexico. Eighteen predators and parasitoids were
found attacking seven species of Leptinotarsa.

The most abundant parasitoid in the U.S. appears to be Myiopharus doryphorae
(Riley). Riley (1869) reported that it kills 10% of the second larval generation, and 50 % of
the third. The significance of this parasitoid has also been studied by Kelleher (1966),
Harcourt (1971), Tamaki et al., (1983), and Groden (1988). Although Kelleher found a
significant correlation between CPB pupal mortality and parasitism, there was no
correlation between CPB larval density and percent parasitism. Harcourt found pupal
parasitism to be inversely density dependent, suggesting that M. doryphorae does not
respond to changes in host density. However, Kelleher suggests that the main limitation of
M. doryphorae is its lack of synchrony with its host. In Manitoba he found that adult flies
are most abundant at the end of the second larval generation; the first larval generation
and early part of the second generation escape attack. Tamaki et al. (1983) also reported
the impact of M. doryphorae to be limited by its low abundance during the first generation
of CPB in Washington, although maximum parasitism of the second generation reached
75%. Horton and Capinera (1987) found, however, higher levels of parasitism in the first
generation CPB in both potatoes and the wild host S. sarachoides. The overwintering stage
of M. doryphorae is unknown. M. doryphorae has been reared from the CPB and other

Leptinotarsa species in Mexico (Logan et al. 1987).

7

The endemic biological control agent Beauveria bassiana (Bals.) Vuill. has been
intensively studied in recent years. B. bassiana is a fungal pathogen that infects more
than 200 hosts (Lips 1967). All stages of the CPB, except eggs, are susceptible to infection.
B. bassiana has been mass produced for control of the CPB in Europe, and spray
formulations have been used successfully in the Soviet Union (Ferron 1981 ). The natural
incidence of B. bassiana has been studied by Clark (1980), who found that significant
mortality occurred late in the season. The pathogen caused negligible mortality in the
first larval generation, but 58% mortality in the second generation.

The two spotted stink bug, Perillus bioculatus F., is an important predator of the
CPB in the U.S. and Canada (Riley 1869; Knight 1923; Tamaki and Butt 1978; Franz 1957;
Harcourt 1971). The range of P. bioculatus includes most of the U.S., southern Canada,
and Mexico (McPherson 1982). P. bioculatus completes two generations per year in the
northern U.S.. Diapause was reported by Shagov (1977) to be induced by photoperiods of less
than 15 hours . Greater than 90% overwintering mortality has been reported in Minnesota
(Knight 1923) and Europe (Jermy 1980).

The CPB is the predominant prey of P. bioculatus , but it also attacks

the asparagus beetle, cotton leaf beetle, spinach flea beetle, cabbage looper, and other crop
pests (McPherson 1982).
Both nymphs and adults of P. bioculatus prey on the CPB. Adult P. bioculatus feed on eggs,
larvae, and adults, while nymphs prefer eggs and young larvae (Knight 1923). P.
bioculatus may effectively reduce the impact of low density CPB populations, but not
control the pest at high density (Tamaki and Butt 1978).

The spined soldier bug, Podisus maculiventris (Say) is found in all potato growing
regions of North America and has a very broad host range. McPherson (1982) lists over 100-
prey species, primarily larvae of Lepidoptera and Coleoptera. Landis (1937) and
Drummond et al. (1985) found prey species to strongly influence development rate and

survival of P. bioculatus nymphs. In their study of nymphs] development, Drummond et

8

al. (1985) reported the CPB to be an inferior host in comparison with the Mexican bean
beetle, the greater wax moth, and the eastern tent caterpillar.

The seasonal dynamics of P. maculiventris have not been studied. McPherson
suggests that the predator probably completes two generations per year in the southern
portion of its range, and one generation per year in northern areas. Drummond et al.
(1985) simulated development of P. maculiventris and the CPB under average Rhode
Island temperatures and found the predator to have a considerable developmental
disadvantage early in the season. Several unsuccessful attempts have been made to
introduce P. maculiventris and P. bioculatus into Europe (Jermy 1980).

The foliar searching carabid, Lebia grandis Hentz, was first reported as a predator
of the CPB by Riley in 1872. Chamboussou (1938) described L. grandis biology and
succeeded in mass rearing the predator on the CPB. He also suggested that L. grandis
could be effective in biological control of the CPB. The life history of the genus Lebia was
described by Madge (1967). The adults are predaceous on the eggs and larvae of
chrysomelid beetles, and the larvae are solitary ectoparasitoids on pupae of the same host.
L. grandis appears to be host specific to the CPB.

Groden (1988) suggests that L. grandis may be the most significant predator of the
CPB in Rhode Island and Michigan. A key advantage appears to be the early season
activity of this predator. Groden found that L. grandis population dynamics were well
synchronized with the CPB, and that predation was not significantly inhibited by cool
early season temperatures. She also found in field cage studies that predation on the CPB
by L. grandis was strongly density dependent. Parasitism of CPB pupae by L. grandis
was between 13 and 50% of CPB pupae in unsprayed potatoes in Michigan.

Surveys of ground dwelling carabid species have been conducted by pitfall trapping
in potato fields in the U.S. (Evans, unpublished data), and Europe (Scherney 1959; Sorokin
1981). The relationship between carabid populations and CPB mortality was studied by

Thiele (1977) and Sorokin (1981). Scherney (1959) confined two species of Carabus

9

common in potato fields in Germany with CPB eggs in 4 m2 enclosures, and found up to
95% reduction in CPB larvae and pupae after 33-40 days. However, carabid populations in
the enclosures exceeded natural incidence levels by several times (Thiele 1977). Jacques
(1988) found four species of Calasoma attacking the CPB congener L. linealata on native
host plants in Arizona.

Several species of Coccinellidae that are common in potato fields probably prey on
the CPB. In Michigan, both adults and larvae of Coleomegillla maculata DeGeer pey on
CPB eggs and small larvae. In field studies, Groden (1988) found that C. maculata
reduces CPB populations. C. maculata is a polyphagous predator that is generally
associated with crops (including potato) supporting aphid populations (Wright and Laing
1978). Groden (1988) found that C. maculata populations were influenced by aphid infested
crops interplanted in blocks with potatoes. Hippodamia convergens Guer. is another
coccinellid abundant in Michigan potato fields that preys on the CPB (Groden 1988).
Anaya (1988) reported H. convergens as the most frequently observed predator of the CPB
on the native host Solanum rostratum in Mexico.

Natural enemies of the CPB and other Leptinotarsa species were surveyed during
three collecting expeditions to Mexico undertaken by Logan et al. (1987). The most
abundant predators were asopine pentatomids. The most abundant species, Oplomus
dichrous (H.S.) was evaluated as a biological control agent of the CPB by Drummond et
al. (1987), who found it was unable to overwinter in the northeastern U.S.. Drummond
(1986) also investigated a parasitic mite collected from CPB adults in Mexico. The mite,
Chrysomelobia labidomerae Eickwort, was found to be insignificant as a direct cause of
CPB mortality, although beetle flight behavior was inhibited at high mite densities
(Drummond 1986).

A eulophid egg parasitoid, Edovum puttleri Grissel, was discovered in Colombia in
1980 by Puttler, who reared it from Leptinotarsa undecemlineata (Stal) (Grissell 1981).

The biological control potential of E. puttleri has been studied by Lashomb et al. (1987),

10

Obrycki et al. (1985), and Schroeder and Athanas (1985). To date, most studies using E.
puttleri in mass release against the CPB have utilized the progeny of the Columbian biotype
(G. Bernon, pers. comm.). A recent study by Ruberson et al. (1987) demonstrated that a
Mexican biotype of E. puttleri differs in characteristics that may affect the efficacy of E.

puttleri in biological control.

Management of the CPB. The CPB is now a key pest of potatoes in the United States,
Canada, and Europe. Since the late 1800's, efforts at control of the CPB have emphasized
insecticides. Arsenic based compounds provided erratic control of the beetle until they
were replaced by DDT in the 1940's (Gauthier 1981). The development of resistance to DDT
was first noted in 1952 (Gauthier 1981). Resistance of local CPB populations to a succession
of insecticides followed, and the CPB is now virtually immune to all the major classes of
insecticides in some areas (Forgash 1985). Several methods of non-chemical
management of the CPB were well understood in the 1800's, including the use of early
maturing varieties and crop rotation (see review by Casagrande 1987). However, these
were abandoned for the short term production advantages of chemical insecticides
(Casagrande 1987). The problems associated with traditional chemical management -
including aquifer contamination, costs, and pesticide resistance - have re-stimulated
interest in alternative methods of control. Recent efforts have emphasized host plant
resistance (Dimock and Tingey 1985), cultural controls (Lashomb and Ng 1984), and
natural enemies (Jermy 1980; Tamaki 1981; Roberts et al. 1981; Drummond et al. 1987;
Groden 1988; Groden et al., in press).

Research efforts on CPB biology and ecology have focussed almost entirely on pest
populations in cultivated crops. However, the development of biologically rational control
strategies will require a better understanding of insect populations in complex
agroecosystems. Basic studies of pests such as the CPB in natural systems should provide

insight into host plant relations, population dynamics, and patterns of mortality. In this

11

study, I examine the ecology of the CPB on its native host in the state of Morelos, Mexico,
focussing particularly on the role of natural enemies in mortality. The possibilities for
biological control using endemic natural enemies encountered in this study is also
discussed. This thesis also addresses the potential importance of the CPB to the growing
potato industry in Mexico. I will discuss current circumstances in Mexico that may
facilitate the adaptation of Mern'can beetles to the potato, and present experimental results
that suggest the mechanisms governing host specificity of a CPB population in Morelos,

Mexico.

Chapter I
WOLOGY AND ELEVATIONAL DISTRIBUTION

01" THE W POTATO m IN NORM. _CO

INTRODUCTION

The Colorado potato beetle (CPB) Leptinotarsa decemlineata (Say). is a
persistent and economically significant pest of potato. tomato. and eggplant. Most field
studies describing CPB biology have been conducted in these cultivated Solanaceae. the
principal host plants of the CPB in the U.S. and Europe. Many features of CPB life
history and ecolog are host plant dependent. Diapause induction and larval growth
rates may vary between host species as a result of differences in secondary chemistry
and nutrients (Hare 1983). The distribution of host plants has been shown to afl'ect CPB
density (Horton and Capinera 1987) and colonization rates (Cort 1982). Thus. the crop
system bias of most studies of the CPB may obscure an understanding of CPB biologY on
the native hosts upon which it evolved.

The CPB probably evolved in central and southern Mexico. and has spread into
temperate areas as a pest only since the mid 1800's (Power 1906). In Mexico the CPB is
oligophagous on several closely related weedy species of Solanum (Hsiao 1981). Since a
study by Tower in 1906. very little has been reported on CPB ecology on natives in
Mexico. Recent studies in Mexico have focussed on CPB natural enemies (Drummond et
al. 1984b; Logan et al. 1987). In this study. I investigate the seasonal phenology and

elevational distribution of the CPB in the state of Morelos. Mexico.

12

13

MATERIALS AND METHODS

The distribution of the CPB. insect associates. and host plants were surveyed at
45 sites located on highway transects in the state of Morelos. Mexico (999W 19°N).
’h’anscct routes were chosen to sample an elevational and climatic gradient. from 950
m at Tequesquitengo (mean annual temperature 25’ C: mean annual precipitation 960
mm) to 2700 m at Tres Marias (mean annual temperature 12’ C; mean annual
precipitation 1400 mm). Graphs representing precipitation and temperatures typical of
low. medium. and high elevations in the study area are displayed in Fig. 1. 1 (data from
Ponce de Leon 1979). The locations of sample sites in Morelos are displayed in Fig. 1.2.

Selection of census sites were based on three criteria:

1) presence of "typical" CPB habitat--i.e.. disturbed areas and soils with high
moisture such as creek beds. irrigation canals. etc..

2) diversity of habitats--cr0ps in different stages of succession. fallow fields. tree
cover. etc..

3) adequate replication of elevation zones.

In 1987. the censuswas taken during the last week ofJune. lastweek ofJuly. and
first week of September. In 1988. a single census was taken during the third week of
August. At each roadside site. an area of ca. 100 m diameter was searched for 15-30
minutes. and habitat types noted. Due to variability in the time and intensity of search
at different sites. categorical estimates of CPB numbers and host density were made.
The CPB count at each site was recorded as '0' (no CPB present) . '+' (1-20 beetles). and '++'
(>20 beetles). Maximum height of CPB host plants was classed as '0' (hosts absent). '+'

(<40cm) or '++' (>40cm). Height classes for host plants were established to discriminate

14

well established sites from newly colonized sites. where small plants have a high
probability of mortality due to competition. The presence of other Lepttnotarsa
species. other Chrysomeline beetle genera. and their host plants was also recorded.
Beetles were identified using keys by Wilcox (1972) and Jacques (1972); host plants were

identified by Dr. Michael Nee of the New York Botanical Garden. Bronx. N.Y..

RESULTS AND DISCUSSION

CPB and Host Plant Phenolsgy. Survey results are displayed in Appendix 1.
The climate of Morelos. as in most of the range of the CPB in Mexico. is strongly

seasonal (Fig. 1.2). During the "dry season" (from November to June). CPB host plants
are absent and adult CPB are in diapause (Anaya 1988). Emergence of CPB adults in
Morelos in 1987 was first observed on June 12. synchronous with the first emergence of
host plants that followed the onset of the rainy season during the first week of June.
Only one CPB host plant species. S. angustifolium Mill. was commonly
observed during this survey. (A second host. 8. elaeagny'olium Cav. was present at very
low density but was never observed with CPB). On the first census in the third week of
June. 24% of sites had been colonized by S. angustifolium . The number of colonized
sites nearly doubled by the second census in late July. and increased again marginally
by the third census in early September (Fig.3a). The seasonal increase in host resource
is also indicated by an increasing percentage of large plants-sites with plants >40cm
constituted 27. 65. and 86% of colonized sites in June. July. and September.
respectively. Plants in favorable sites had attained up to 1m in height and were
flowering by the second observation in July. Emergence of host plants appears to be

regulated in part by soil moisture levels. Within sites. the earliest plant stands were

15

located in wetter habitats such as irrigated fields and along stream beds. Plants
appeared later on higher slopes and drier soils as the rainy season progressed.

Following emergence of S. angustifolium at a given site. plant abundance
increased within the censused area of ca. 100m diameter (on only two occasions did
plants noticeably decrease in abundance from one observation period to the next).
However. within sites. plants frequently experienced mortality in wetter habitats as
drier habitats were colonized. This plant mortality was frequently caused by
competition from Tithonia tubiformes (Jacq.). a rapidly growing composite that
reaches four meters in height. Another important cause of plant mortality was the
cultivation of fallow fields. which tended to support the largest host plant stands. Thus.
on a small spatial scale. S. angustifolium appears to be a variable resource both in
terms of emergence date and stand longevity.

The colonization of host plant stands by CPB adults occurred with some delay
after initial plant emergence. Beetles were present at less than half of sites with host
plants in June. although by September 86% of these sites had been colonized (Fig. 1.3a).
Recruitment of CPB to new host plant stands may be accounted for by newly emergent or
immigrating beetles. Both groups probably contribute to colonization. although
relatively high early season survival of CPB observed at Xoxocotla (Chapter III) suggests
that large numbers of beetles produced on relatively scarce host resources early in the
season are an important source of colonists in late-emerging host stands.

Emergence time and abundance of S. angustifolium appears to be influenced by
elevation in Morelos. No host plants were located in any of 12 census sites located at or
above 2000 m. corresponding to the transition between semi-trepical and temperate
climatic zones. Below 2000 m. lower elevation plants appeared both earlier and at a
greater proportion of sites in three arbitrary elevation range classes (Fig. 1.3b). The
decrease in S. angusttfolium abundance with elevation has been observed previously in

Morelos (Ponce de Leon 1979). and is consistent with the known distribution of this

l6

plant. which is generally tropical in its preferences (Whalen 1979). The apparent delay
of ca. one month in S. angustifolium emergence at higher elevations is probably not
due to lack of moisture. as rainfall is greater and earlier. and evapotranspiration lower.
at higher elevations in Morelos (e.g.. Fig. 1.1). A more likely explanation is lower
temperatures that retard seed germination and growth. The significance of delayed
host emergence at higher elevation would be a shorter growing season for CPB. which
also appeared later in the season at higher altitudes (Fig. 2.3c). Assuming that host
plants are available from mid-June to November at an elevation of 950 m. and from
mid-July to November at 1900 m. and given the greater heat accumulation available for
development at the lower elevation. beetles at the higher elevation would have less than
one half of the developmental time of beetles at the lower elevation (1240 vs 2820 degree
days. calculated for Fig. 1.2 temperature curves and developmental threshold
temperature of 10’ C--Logan et al. 1985). This development time difference between
neighboring populations may help explain the asynchronous appearance of beetles and
host plants. Because the period of host plant emergence is protracted. beetles emerging
early (cued by onset of the rainy season) will have less probability of encountering host
plants; thus delayed emergence would appear to be favored. 0n the other hand. beetle
populations at the cooler high elevations are limited by a shorter growing season and
are under greater selective pressure to emerge early. Such an extreme difi'erence in
selective pressures on CPB populations 30 km apart provides one clue for the existence
of extreme developmental plasticity that has been observed in pest populations of the

CPB (Hsiao 1985).

Between Year Trends. survey sites below 2000 m that had S. angustifolium in
1987 were re-visited in mid-August 1988. Both CPB and S. angusty'olium appeared to be
considerably less abundant than in the previous year. The relative abundance of

beetles and host plants between years is compared in Fig. 1.4. The distribution of plant

17

size classes were comparable between years (X2=0.077; p=0.96). for a 1988 survey date
three weeks later than in 1987. The distribution of CPB abundance classes in 1988
suggests a more marked reduction in beetle populations in 1988. Fewer sites were
colonized. and fewer colonized sites had >20 beetles: distribution of abundance appears
different between years (X2=4.82; p=0.09). particularly given the later 1988 census date.
Causes of decreased abundance in 1988 may be partially explained by difi'erences in the
distribution of rainfall-~lower May precipitation in 1988 (4.8 vs. 65 mm. data for
Zacatepec. see Appendix 2) may have retarded the emergence of host plants in June.
However. rainy season precipitation (May through August total) varied little between
years (615 mm in 1987; 608 mm in 1988. appendix 2). Other causes for the 1988 decline

of CPB populations are not evident.

Association ofthe cm with Related Chrysomeline Beetles. L deoemiineata was
observed at a total of 20 of 45 sites (all observation dates combined). With one
exception. these sites were shared with at least one other species of the Leptinotarsa
tribe Doryphorini. CPB associates and host plants are displayed in Table 1. l. The most
common associate. Zygogramma signatipennis (Stal) shared 75% of sites with the CPB.
Seventy percent of sites were shared with one of three Calligrapha species: C.
multipunctata (Say) C. multiguttaia (Stal). and C. sylvia (Stal). Four species of
Leptinotarsa. sharing a total of 60% of the sites with the CPB included L. tlascalana
Stal. L. undecemlineata Stal. L. haidemani (Rogers). and L. dilecta Stal. Although
some species. such as L. dilecta and L. haldemani are uncommon. at least two of the
chrysomeline associates (2. signatlpennis and L. tlascalana) frequently occur at
population densities that equal or exceed CPB densities. For example. in July 1987. L.
tlascalana was sampled with CPB in a 2 ha fallow corn field at Xoxocotla. Morelos (D.

Cappaert. unpublished data). In the field. which was representative of typical CPB

18

habitat. L. tlascalana occurred at a density of 0.46 beetles/m2. compared to 0.45
beetles/m2 for the CPB.

The frequency of sympatry of closely related chrysomeline species with the CPB
may be significant in explaining the efi'ects of CPB natural enemies under natural
conditions. As alternative prey. CPB relatives may augment natural enemy activity by
maintaining enemy populations at higher levels. increasing mortality rates and
stabilizing enemy population cycles. Alternative prey have been shown. for example. to
be important contributors to biological control of the grape leafhopper. Erythroneura
elegantula Osborn. by the egg parasitoid Anagrus epos Girault (Doutt and Nakata 1965).
Species of Doryphorini that are sympatric with the CPB may also host biotypes of some
natural enemies that will prove to be more effective biological control agents than CPB
enemies per se. This may occur because of the tendency of homeostasis to evolve
between some hosts and their natural enemies (Pimentel and Al-Hafidh 1965). Enemies
that have not coevolved with the CPB may thus more successfully overcome its
defenses.

CPB relatives are known to serve as prey for generalist predators including
several Asopine Pentatomids. Coccinellids such as Hippodamia convergens (Guer).
Reduviidae. and Aranae (Logan et al. 1987). Congeneric associates also share some
natural enemies known to specialize on the CPB. The Carabid predator/parasitoid
Lebia spp. have been observed feeding on L. undecemlineata. the tachinid parasitoid
Myiopharus sp. has been reared from L. tlascalana (D. Cappaert. unpublished data).
and a mite parasite. Chrysomelobia labidomerae is shared by numerous Leptinotarsa
species (Drummond et al. 1984). Edovum puttleri Grissel, an egg parasitoid that has
received much attention recently has been reared from L. undecemlmeata and L.
mpographica Jacoby (Logan et al. 1987)). Based on 1987 observation of L. tlascalana
abundance. an attempt was made to study natural enemy activity in L. tlascalana

populations in 1988. However. the study was abandoned because of the extremely low

19

density of L. tlascalana populations in 1988. Further field investigations of the CPB

associates found in Morelos are necessary to clarify their role in natural enemy

population dynamics.

Summary. In the state of Morelos. the principal host of the CPB. S.
angusty'olium. first emerges with the onset of the rainy season in early June. New host
plant stands continue to emerge until September. appearing later at higher elevations
and in drier habitats. S. angustifolium is absent above ca. 2000 m elevation. The CPB
first emerged within one week of the beginning of the rainy season. although beetles
continued to colonize new sites until September. when the CPB was present at 86% of 22
sites colonized by S. angustifolium. The CPB in Morelos is sympatric with several
species of closely related chrysomeline beetles. including at least four species of
Leptinotarsa. These species share CPB natural enemies and host predators and

parasites with potential for biological control.

Table 1.1.

20

Chrysomeline associates of the CPB at 20 survey sites in
Morelos, Mexico, 1987-1988.

 

Sites shared

 

Total sites with
Taxon colonized CPB Host Plant

chtinotarsa. tlascalana 12 8 Kallstroemia spp.
L. undccemlineata 6 5 Solanum spp.
L. haldemani 6 5 S. nigrum S. angustifolium
L. dilecta 1 l ??
Total Leptinotarsa 20 12
Zygogramma signatipennis 19 15 Tithonia tubiformes
Calligrapha spp. 19 14 Malvaceae. Compositae
Ictalexnhntini -- 19

 

21

 

“:f..s..o 7. ‘0. 0..
.' ..o II E X I C O «"0
.0 ‘. 00" ....IO.. ' ....O. .1 '
t, u o n E L o s

 

filikt-‘K‘xfflvoééy&?<M:;‘-

 

 

 

 

CPB and host plant survey locations in Morelos,

in Appendix 1.

Figure 1.1.
described

Mexico. Numbered sites

22

 

 

 

 

 

 

 

 

 

 

 

3 3° B
EE':mxi- P 8?
Z
O.
2 2..
<
L:
9.. 100
O
UJ
‘1 l
D. o
30
’ o—‘b‘s ’
300- .II'
’I
. 3”“
200-
‘ —'— Precipitation
1co- ""fl-- Temperature
0.1 I I I 1 I’

 

 

Figure 1.2. Annual
three representative elevations
in b) Cuernavaca,

in Morelos,

Mexico.

1900 m c) Tequesquitengo, 950 m

 

8)

 

Huizilac,

temperature and precipitation regime at

2500

 

23

 

  
   
 
  
 
 
 
   

 

 

 

 

 

 

 

 

UV? so . A
I: - - S. angustifolium
E 40~ - CPB . .7):
Lu .
g 30-
9 2° .
O
O
U) 10-
LLI
':
(D 0 7
.,\° JUNE JULY SEPT
O 100
g . ELEVATION , B
a. so: I 1600-2000 a, 'E
0 . fl 1100—1600 3 c
E - I 900—1100 .
D so -
El .0:
O ‘ ..»:x' '
§ 20 - if: .
(I) j}?!
g 0 v1. , ............ ,
‘3 JUNE SEPT
0

so

ELEVATION
60 _ (meters)

I 1 600-2000
E 1 1 00-1 600
E 900-1 1 oo

   

.
40"

 

 

% SITES COLONIZED BY CPB

Figure 1.3. Distribution of the CPB and its host plant at 45
survey sites in Morelos, Mexico, 1987. a) all sites. b) host
distribution by elevation. c) CPB distribution by elevation

24

 

 
   
  

 

 

 

 

  

 

 

A

(D
g - July 1987
(I) August 1988
LL
0 10 -
CI:
LlJ
CD
2
I)
Z

0 —

ABSENT Plants<40cm Plants>40cm
S. angustifolium abundance
30
B
(D .
g I July1987
a) 20 - I August 1988
LL.
0
E
m 10 -
2
a .......
ABSENT <20 Beetles >20 Beetles
CPB Abundance

Figure 1.4. Relative abundance of the CPB and Its host plant at
45 survey sites in Morelos Mexico, 1987-1988. a) S. angustifolium.b)
CPB.

 

Chaptern

NATURALMIESOFTHEOOLORADOPOTATOWINMOMMEXICO

INTRODUCTION

The Colorado potato beetle (CPB). Leptinotarsa decemuneata (Say). is well
known as a pest of the potato and other solanaceous crops. The CPB evolved in
association with several closely related species of Solanum. and was originally
restricted to disturbed habitats of the southwestern U.S. and Mexico (Hsiao 1978). The
potato was introduced to the range of the CPB in the mid 18005. permitting the
expansion of the geographic range of the CPB (Casagrande 1987). The range of the CPB
now extends acrom the U.S.. southern Canada. and much of Europe. and its host plants
include at least 10 wild and cultivated species of Solarium (Hsiao 1978).

The CPB has been the focus of intensive and often futile efforts at control
(Casagrande 1987). Research supporting these efi‘orts has emphasized insecticides.
although CPB populations are increasingly resistant to chemical control (Ferro 1985).
Early investigations revealed a complex of natural enemies with potential to control
CPB populations (Riley. 1871; Bethune. 1872). These include the predators Pertllus
bioculatus (Fab) and Lebia grandis Hentz, the tachinid parasitoid. Myiopharus
doryphorae Riley. and the fungal pathogen Beauveria basstana (Bals.) Vuill..
Relatively little effort has been made to search for additional biological control agents.
although results of several collecting trips made in Mexico during 1980-1985 appear
promising (Logan et al. 1987).

The original range of the CPB appears likely to include effective natural enemies
for at least two reasons. First. the Mexican range of the CPB is geographically.
climatically and biologically diverse. and is likely to contain genetically diverse

2 5

26

natural enemy populations which may have characteristics suited to biological
control. Secondly. because Mexico is the center of origin of the genus Lepttnotarsa
(Tower 1906; Jacques 1972). numerous species of this genus (and related genera of the
tribe Doryphorini) are sympatric with the CPB. Any of these species may share natural
enemies with the CPB. or host parasites with potential virulence against the CPB in
crop systems. Edomun puttleri Grissel is one such parasitoid that has been collected in
Mexico (Logan et a1. 1987).

In this study. native host plant stands in the central Mexican state of Morelos
were investigated to determine the incidence of CPB natural enemies and their
potential as biological control agents is discussed. The effects of these natural enemies
on population dynamics of the CPB on its native host in Mexico are discussed in

Chapter 3.

METHODS

Predator Incidence. Predators associated with the CPB were counted on study
plots at Xoxocotla. Morelos every 30-50 degree days (base 109C) from June 25 to October
30 1987. Observations were made in 1988 from August 16 to October 6. Sampling was
conducted concurrently with the CPB population census. as described in Chapter 3.
Entire plants were examined. All spiders and predatory insects encountered were
identified at least to family and any predatory acts were noted. Ground-dwelling

arthropods and nocturnally active predators were not represented in the count.

Predator Feeding Trials. Suspected CPB predators were collected in July and
August 1987 from S. angustifolium infested with CPB. Collection sites were the CPB
sample plots described in Chapter III. Predators were transferred to an outdoor

laboratory at Cuernavaca. Morelos and held individually in 100 by 15 mm diameter

27

petri dishes overnight without food. CPB egg masses or larvae collected in the field were
then introduced to the petri dish and left for ca. 48 hours at ambient conditions (12:12
LzD). The number of prey (eggs or larvae) introduced exceeded estimated 48 hr.
consumption. e.g.. ca. 100 eggs or 2-3 fourth instar larvae. Larvae were provided with
fresh S. angustifolium foliage. Laboratory temperature was monitored with a
thermograph. and varied between 15 and 22’C. Numbers of undamaged CPB prey (alive
or dead) were counted and subtracted from the initial number to determine
consumption. Prey killed. but not apparently damaged by predators were thus not
included in consumption. Predators were first offered CPB eggs; if they ate. they were
maintained on eggs for seven or more days to determine consumption rate. Predators
that did not consume eggs were given CPB larvae for a second 48 hour trial. Some
predators were identified by comparing our specimens with examples in the insect
collection of the Colegio de Postgraduados at Chapingo. Mexico. Others were sent to
GA. Ball. Dept. of Entomology. University of Alberta. Edmonton. Alberta. (carabids) or
D. Ryder. Dept. of Entomology. LSU. Baton Rouge. Louisiana (pentatomids).
Parasitism. CPB egg masses. larvae. and adults were collected from S.
angustifolium and held for emergence of parasitoids. Collections were made in six
locations in Morelos, to allow sampling from several CPB populations (Fig. 2. 1).
Whenever possible. at least 30 individuals of each life stage were collected. and up to 300
individuals were collected from high density populations (see tables 2.4-2.6).
Collections were made each week during the period from the end of June to late October
1987. and at irregular intervals from mid-July to October 1988. CPB samples were
transferred to an outdoor laboratory at Cuernavaca. Morelos and placed in covered 100
by 15 mm petri dishes. Larvae and adults were provided with S. angustifolium foliage.
A dampened cotton ball in each dish provided moisture. Samples were maintained at
ambient conditions (see above) and checked every two days for parasitoid emergence.

Samples were kept until the parasitoids emerged or eggs hatched. larvae pupated or the

28

insect died. Surviving adults were discarded after 7- 10 days. larvae collected were
mature fourth instars (except where noted) within one or two days of leaving the host
foliage for pupation; because collected larvae had less exposure to parasitoids. observed
levels of larval parasitism underestimate actual levels. Parasitism of CPB by the mite
Chrysomelobia labidomerae Eickwort was determined by counting mites visible

beneath the parted elytra of beetles.

RESULTS AND DISCUSSION

Incidence of Natural Enemies. In 1987. predators were counted at an
"early site" until July 25. when defoliation of the sample plot occurred. A second
location ("late site") approximately 50 m away was chosen and sampling
continued until October 30. Predators were counted at a single location in 1988.
from August 16 to October 6. Mean numbers of predators observed are shown in
Table 2. 1.

In 1987. the predators were dominated numerically by spiders and an
omnivorous ant species. Solenopsis geminata Fab.; these predators account for 76% of
observations in the early site. and 62% in the late site. In 1988. these predators were
even more prevalent. comprising 86% of observations. These predators may be
significant in interactions between predators. For example. Risch and Carrol (1982)
showed that S. geminata can reduce the species richness and abundance of predator
species in com-bean-squash culture in Mexico. Certain spider species may also
influence predator densities by preying preferentially on predatory species (Stowe
1986). However. these predators are probably not important as CPB predators. S.
geminata is an agressive omnivore with a broad range of prey. including eggs of the
chrysomeline beetle genera Acalymma and Diabrotica (Risch 1981). Nevertheless. in

hundreds of observations, S. geminata was never observed attacking any CPB life stage.

29

(S. geminata was not evaluated in feeding trials. Normal foraging behavior could not
be expressed in ants isolated from the colony). Defensive mechanisms of the CPB
including integumentary secretions and reflex bleeding have been shown to be repellent
to ants (DeRoc and Pasteels 1977). and may confer some immunity to predation by S.
geminata . Spiders were predominantly of the orb-weaving family Araneidae. whom
prey is generally small and airborne. although CPB larvae were captured (Table 2. 1). Of
the actively hunting spiders. crab spiders (l‘homisidae) were particularly abundant.
especially in 1988. However as ambush predators. crab spiders tend to capture highly
mobile prey such as flies (Gertsch 1949). and they would infrequently encounter CPB.
Mobile arthropods were in fact common prey of Thomisidae (Table 2. 1). Other taxa of
hunting spiders may have been poorly represented in the count because of the relative
importance of nocturnal species among spiders (although hourly nocturnal
observations of larvae and eggs on 30 plants on July 4 1987 did not reveal hunting
spiders).

Nabidae. Reduviidae. and Dermaptera were common in the early site in 1987.
and Reduviidae were also abundant in 1988 (Table 2. 1). These generalist predators
attack CPB at least occasionally (Table 2.1). but consumption rates appear to be low (see
feeding trials below).

In 1987 Pentatomidae. Carabidae. and Coccinellidae together comprised 4% of
observations on the early site. and 23% on the late site. In 1988. these predators totaled
15% of predators counted. Pentatomids were particularly abundant later in the season.
and coccinellids in the early season. Carabidae were observed frequently only at the
late site in 1987. These predator groups have been recorded as key predators of the CPB
in crop systems. and are frequently associated with CPB populations in Mexico (Logan
et al. 1987).

Four pentatomid species were observed feeding on CPB. Three species have been

previously identified as CPB predators in Mexico (Logan. et al. 1987): Sttretrus

30

anchorage F.. Oplomus dichrous (H.S.). and Apatetkrus sp.. The fourth species observed
in this study. Pen-abides confluens (H.S.) has not been previously identified as a CPB
predator. although it has been observed in Morelos attacking Epilachna corrupta Mule.
(Plummer and Landis 1932). The relative abundance of the common pentatomid species
is shown in Fig. 2.2.. S. anchorage was the most frequently observed species overall.
In 1987 P. confluens was observed more frequently early in the season. 0. dichrous
numbers peaked at the end of the season following the disappearence of S. anchorago.
In 1988. P. confluens was continuously present. and O. dichrous again increased later
in the season. Predator observations were discontinued before the decline of the CPB
population in 1988. so that end-of—season trends in pentatomid phenoloy could not be
observed. Apateticus sp. (not shown in Fig. 2.2) was observed only once. in early July
1987. These difi'erencw in the phenolog of the pentatomid species may be significant
for the impact of pentatornids on CPB mortality. For example. in 1987 CPB numbers
declined markedly late in the season. increasing the predator-prey ratio of the late
season pentatomids (i.e.. O. dictumrs).

The potential of Pentatomids as CPB biological control agents has been
evaluated by several investigators (Tamaki and Butt 1978. Drummond et al. 1984a
1987). Perillus bioculatus (Say). a widely distributed species that I observed at low
densities in Morelos. is common in potatoes in some parts of the U.S. and contributes to
CPB mortality (Harcourt 1972): however. Tamaki and Butt. (1978) found this predator
ineffective when CPB densities were high. The control potential of O. dichrous
imported from Mexico. was evaluated by Drummond et al. (1987). who found it unable to
overwinter in the northeastern U.S.. The most commonly observed pentatomid in this
study. S. anchorago. is a common predator of several lepidopterous and coleopterous
pests in the U.S. ( McPherson 1982). S. anchorage occurs as far north as Ontario
(McPherson 1982), but has not been reported as a significant predator of the CPB in

potatoes (although it has been observed on S. dulcamara. a wild CPB host in Rhode

31

Island (F. Drummond. pers. comm.)). Research to date suggests that pentatomids
are inefl'ective as biological control agents of the CPB in potato crops. However. they
may be useful as components of a predator complex in a diversified potato
agroecosysten. For example. as a known predator of the alfalfa weevil. the mexican
bean beetle. and the gypsy moth (Richman 1977; Whitcomb 1974; Baker 1972). S.
anchorage populations might be maintained at high levels in a multi-crop system. A
similiarly broad host range has been reported for P. bioculatus and Apateticus species
(McPherson 1982). Further research on pentatomids in biological control might also
consider P. caUluens which appears to emerge early in the season (Fig. 2a) when most
natural enemy populations are low (Groden 1988. see also chapter III).

Visual count totals given in Table 2.1 include at least seven species of carabids.
all in the tribe Lebiini. These include four species of Lebia (L. cyane. L. yqfosutura. L.
guatemalena. and L. 4-notata ). two species of Onypterygia (O. fulgens and O. tricolor).
and Callida decora. Lebia species are of particular interest because of their unusual life
history--their larvae are ectoparasitoids of Chrysomelid pupae. and adults are obligate
predators of the same host species (Madge 1967). Parasitic habits are also known in
other Lebiini genera (White 1983).

The Lebiini species encountered in association with the CPB in Mexico may
exhibit characteristics that will be important to future biological control of the CPB.
Experiments have demonstrated that Mexican Lebia spp. from Morelos do parasitize
CPB. and that the adults are voracious egg predators (P. Logan. pers. comm.). Mexican
Lebia do exhibit one characteristic that appears to difl'erentiate them from L. grandis:
the species observed at Xoxocotla were diumally active. whereas L. grandis is distinctly
nocturnal (Groden 1988). Future field work should elaborate the phenology and
population dynamics of the various Lebia species and their relative contribution to

CPB mortality. If any of these Lebia species survive temperate winters. they might then

32

be introduced experimentally to CPB infested crops. Labia species may also prove
important as CPB natural enemies in Mexico. should the CPB develop as a pest there.
No Species of coccinellid adults were observed in the visual count: Coleomegilla
macrdata DeGeer and Hippodamia corwergens Guer. Two other coccinellids common
in the plots. Cyclomeda sanguinea and Brachaoanta decora are not included in the
count because of negative results in the feeding trials (see Table 2.2). C. maculata and H.
convergens are polyphagous predators that commonly feed on CPB eggs and first instar
larvae in Michigan and Rhode Island. both as larvae and adults (Groden 1988).
Coccinellid larvae observed are assumed to be of these two species. although they were
not identified in the field. C. maculata and H. convagens are widely distributed
predators that occur abundantly in cotton (Whitcomb and Bell 1974). alfalfa (Hodek
1973). and sorghum (Rice and Wilde 1988). Predation on CPB by coccinellids observed
in this study may have been enhanced by high populations that I observed feeding on
aphids in adjacent sorghum fields. Coccinellid predation appeared less important to
CPB mortality in these study plots than pentatomids or carabids (Chapter 3). but may be
significant for its early appearance. preceding population peaks of these other key

predators (Table 2.1).

Predator Feeding trials. A summary of all predator feeding trials is presented in
Table 2.2.

Five of eight pentatomid species tested attacked CPB prey. These species are all
members of the predaceous subfamily Asopinae: negative results were obtained for
three common species of the subfamily Pentatominae. Trial indications asopines
attack both eggs and larvae are borne out by field observations (Table 2.1).

All foliar searching carabid species tested fed on both eggs and larvae of the CPB
(Labia and Callida species were not differentiated and are pooled as "Lebia spp. &

Callida spp " (Table 2.2)). Occasional refusal of CPB prey by the two Onypterygta species

33

suggests that they may be less receptive to CPB prey than the Labia species (Table 2.2).
Calasema is a ground dwelling carabid that was tested to determine if CPB pre-pupae on
the soil surface were acceptable prey to a typical generalist predator. CPB were readily
attacked by this predator. which consumed as many as five fourth instar larvae in one
day.

The four coccinellid species that were commonly observed in the study site (and
at other locations in Morelos) were tested on CPB eggs. We species known to be CPB
predators. C. maculata and H. cenuergens. generally consumed eggs. C. sanguinea and
B. decora to eat eggs or first instar larvae in six trials. Negative results strongly suggest
that these two species do not prey on CPB in the field.

Consumption rates of CPB eggs by difi‘erent species of the three major groups of
predators are compared in Table 2.3. Although these tests provide insufficient evidence
to demonstrate difl'erences between species within groups. several trends are evident. O.
dichrous consumed more eggs than S. anchorage . which consumed more than P.
cenfluens. This may be explained by differences in the body size of these species.
However. examining consumption of Epilachna cerrupta larvae in feeding trials with
the same three species. Plummer and Landis (1932) found the greatest consumption for
S. anchorage (and the least for P. cerdluens ). The difference in results may be
attributable to prey preferences. Comparison of feeding trials with carabids reveals
that consumption of Lebia was equal to or greater than that of Onypterygia. Despite the
much larger body size. Onypterygia accepted CPB eggs less readily. rejecting this diet in
3 of 7 trials. while Lebia fed on eggs in all 13 trials attempted (Table 2.2). Because Lebia
species studied to date are prey specialists (Groden 1988; Madge 1967). they may be more
receptive to CPB prey than are the generalist Onypterygia species.

Cantharid larvae readily consumed eggs in feeding trials (Table 2.3). Two

individuals consumed 15.3 and 12.3 eggs/day in a seven day period.

34

Feeding trials with spiders suggest that these abundant generalist predators at
least occasionally accept CPB larvae as prey. Positive results were obtained for
Misumena sp. and two other unidentified thomisid spiders. and for Peucetia virldans.

Three other groups of predators found in the Xoxocotla study plots rarely or
never consumed CPB prey in feeding trials. These are Dermaptera (the only species
tested. Deru taeniatum. was common). Nabidae. and Reduviidae (only one of several

species observed was tested. These species appear to be unimportant as CPB predators).

Parasitism. Parasitism of CPB egg masses observed in Morelos is presented in
Table 2.4. The only parasitoid encountered was a eulophid wasp. Edovum puttlerl
Grissel. Parasitism occurred late in the season and at relatively low levels. E. puttlerl
was first detected in samples collected August 2 1987 in the Cation de Lebos. E. puttlert
was detected in four of five samples taken from the Canon in subsequent weeks: the
maximum parasitoid incidence recorded reached 6% on September 6. Parasitism also
occurred in two other locations. The maximum incidence of parasitism observed was
28% at Xoxocotla. E. puttlerl was not observed in 1988. However. no samples were
taken from the Canon de Lobos. the site most favorable for E. puttlerl in 1987.

E. puttleri was originally discovered as a solitary parasitoid of L.
undecemlineata (Stal) from Colombia (Puttler and Long 1983). The parasitoid has
since been detected in the CPB. L. undecemlineata. and L. typegraphiea Jacoby in
Mexico (Logan et al. 1987). The ultimate success of E. puttleri in biological control
projects may depend on genetic material from new biotypes ((Obrycki. et al. 1985).
Laboratory studies by Ruberson. et a1. (1987) show that a Mexican E. puttleri biotype
differs in traits such as host age effects on parasitoid sex ratio and temperature
response. Virtually nothing is known of the natural history of E. puttlert The low
incidence of parasitism evident in the current study (2.3% of 1082 egg masses sampled

in 1987 after first appearance of the parasitoid). and data from prior collecting trips

35

(Logan et a1. 1987) suggests that E. puttleri is a minor mortality factor in natural CPB
populations in Mexico.

Incidence of larval parasitism observed is reported in Table 2.5. Two species of
the tachinid genus Myiepharus were encountered: M. amertcanus Riley and M.
dayphorae Riley. Tachinid parasitism was detected for most of the 1987 CPB growing
season. and ranged between 2 and 58% of fourth instar larvae. Myiopharus was present
at all four locations sampled. and on nearly every occasion sampled following the first
observed parasitism on July 12 1987 (18 of 19 samples contained parasitized larvae). In
1988. tachinids were present in all samples. but the maximum level of parasitism was
17%. substantially below the 28% average rate of parasitism for all sites in 1987.
Thirteen larval parasitism samples were collected at the main study site in Xoxocotla
in 1987. allowing construction of a parasitism incidence curve. This curve is graphed
with the density of fourth instar larvae available as hosts on the same plots (from
population count--Chapter 3) in Fig. 2.3. Parasitism by Myiepharus at this site shows
an apparent pattern of inverse density dependence.

Myiephams pupae contained two species of hyperparasite. 15% of tachinid
pupae were killed by a solitary perilampid wasp. Perilampus sp. . and 4.5% by an
unidentified chalcidid.

M. doryphorae is an abundant and widely distributed CPB parasitoid in U.S.
potato fields. causing up to 80% mortality (Riley.1869: Kelleher 1966; Tamaki et al.
1983). However. it has been of only limited significance as a biological control agent of
the CPB for two reasons. M. derypherae is poorly sycchronized with its host in the
northern parts of its range. In Manitoba. Washington. and Michigan. parasitism is low
during the first CPB generation. as CPB populations increase to damaging levels
(Kelleher 1966. Tamaki et al. 1983: E. Groden. unpublished data). However. Horton and
Capinera (1987). studying low density CPB populations on potato and S. sarracheides

in Colorado found parasitism by Myiepharus higher in the first than in the second

36

generation in 3 of 4 years studied. A second problem is the lack of a density dependent
response by Myiopharus. Both Kelleher (1966) and Harcourt (1971) found that percent
parasitism was unchanged or decreased with increasing CPB density.

Phenology of Myiepharus may be better synchronized with the CPB in Morelos
than in the northern U.S. or Canada. Myiophanrs appeared in both 1987 and 1988
during the first CPB generation. and at one location (El Rancho). first generation
parasitism reached 50% in 1987 (Table 2.5). Secondary hosts among other abundant
species of Doryphorini (see Chapter I) may also enhance CPB parasitism. M.
americanus was detected in L. tlascalana Stal 3 weeks prior to its appearance in the
CPB (3 of 34 third and fourth instar larvae collected at El Rancho. June 5 1987). and
adult flies reared from parasitized L. tlasealana later successfully parasitized CPB
larvae. Myiopharus has also been reared from L. undecemlineata (Stal) and L.
typographica Jacoby collected in Mexico (Logan et al. 1987). Further investigation of
Myiepharus species in Morelos should focus on such inter-species interactions.

Natural enemies reared from CPB adults included a tachinid parasitoid and a
fungal pathogen (Table 2.6). Strongyaster sp. was encountered at low levels in the
Canon de Lobos. This tachinid has been collected previously in Mexico (as
Hyalemyodes triangulifera (Leew)) from the CPB and from L. undecemlineata (Logan et
al.. 1987). H. iriangulifera is a widely distributed parasitoid with hosts that include
the alfalfa weevil Hypera postica in the eastern U.S. (Brunsen and Coles 1968). The
fungal pathogen observed was not identified but appeared to be Beaver-ta bassiana. a
common pathogen of CPB in potatoes (Clark 1980). The species B. bassiana
encompasses numerous strains that attack more than 200 hosts (Lipa 1967). Isolates of
B. bassiana are sometimes less virulent against the host from which they are reared
than against exotic hosts (Lappa 1978). Nevertheless. B. bassiana strains collected
from Mexico (from the CPB or other Doryphorini species) may contain genes that could

enhance pathogenicity against pest populations of the CPB.

37

The incidence of parasitism of adult beetles by the mite Chrysomelobia
labidemerae is presented in Table 2.7. In 1987. mite loads were quite low until the end
of the season. when as many as 49 mites/beetle were encountered. Mite densities were
lower in 1988. although the highest mean density observed was also at the end of the
season. on October 6 (Table 2.7). The biological control potential of C. labidomerae has
been examined by Drummond (1986). who found that parasitism by the mite is
insignificant as a direct mortality factor. although flight behavior may be strongly

inhibited.

Summary. In central Mexico. the center of origin for the CPB and its principal
host plants. the CPB is associated with a diverse complex of natural enemies.
Observations during 2 years at a single site in Morelos revealed 4 species of asopine
pentatomids. 7 carabids, and 2 coccinellids among the predators of the CPB.
Parasitoids include 3 species of tachinids and a eulophid egg parasitoid. Key natural
enemies are listed in Table 2.8. These natural enemies include several predators that

have not previously been reported attacking the CPB.

38

Table 2.1. Visual count of predators at Xoxocotla, Morelos.

 

Mean Count on Site

 

 

Predator 1987 4283
S . E l S' . I S' E I E l . ]
titanium
Pentatomidae 1.3 3.6 3.5 CPB: eggs (8). larvae (24)
lepidoptera lane (1), pentatomid (l)
Reduviidae 1.4 0.88 2.2 CPB: eggs (1), larvae (l)
Nabidae 2.4 0.36 0.0 CPB eggs (1)
Colman
Carabidae 0.50 2.3 0.53 CPB larvae (1)
Coccinellidae (Adults) 0.78 0.48 0.27 CPB eggs (1)
Coccinellidae (Larvae) 4.7 0.52 0.46 CPB: eggs (10), larvae (4)
Cantharidae --- 0.36 0.27 CPB eggs (8)
Forficulidae 1.7 0.70 0.07 CPB eggs (I)
Hamennmeta
W 42.6 9.2 5.0
Anna:
Thomisidae 1.0 3.8 22.3 CPB larvae (2), flea beetle (1),
ant (l), diptera (ll), leafhopper (1)
Others 1.3 5.3 18.5 CPB larvae (4), hemeptera (2),

nabid (l), hemiptera (3), diptera (2),
coccinellid (l)

 

a early site count from June 2 to July 25, 1987; late site count from July 25 to
October 30, 1987. 1988 data from August 16 to October 6.

39

Table 2.2. CPB feeding trials

 

 

 

CPB Life Stage: BCIE L-l L-2 L-3 L-4 AIL'IRIAIS
Predator + - + - + - + - + - + -

Eeniainmidae;
Perriloides confluens 3 l 2 l
Stireirus anchorage 21 1 l 1
Oplomus dichrous 4
Perillus bioculalus l
Apaieticus spp 1
Euschistus spp

Oebalus pugnax

Mormr'dea :pp 1

Carabidae;

Lebia spp &Callida spp l3
Onypterygia fulgens l
Onypterygia tricolor 3 1
Calasoma spp

C . ".1 .
Coleomegilla maculaia 6 l
Hippodamio convergens 6

Brachacania decora 5 l
Cyclomeda sanguinea 2

Arenas;

Peuceu'a app 1 1

Misumena :pp 3 2 l
Themisidae unk. 2 2

Tetragnathidae ukn. l l

Salticidae unk. l 1

Miscellaneous;

Cantharidae 2
Dermaptera 6 7 3 3
(Dom iaeniaium)
Nabidae l 6 9
Nabidae 2 5 l 2 l 7
Reduviidae 4 7

u—I&UJD-‘
p—e
bat—-

WN"

9’qu

p—e
MMQH OOOHHckNOi
03

fl

:3ch O‘NO‘HOO—‘F‘

OCNUIN OOOON
NNNHUI NO‘CH

ON
H
\O

 

N"

a ”+" indicates consumption of life stage offered. indicates no feeding observed.
Trial=two day exposure. Each individual entered once for each life stage offered.

40

Table 2.3. CPB egg consumption rates.

 

 

daily egg
_Eredator hawk—ML
Bematamidas;
Stiretrus anchorage 12 23.8i1.75 13.1-47
Perilloides confluens 3 12.9t1.73 11.6-14
Opiomus dichrous 3 41.4ill.l 32.5-46.4
Carabidae;
Lebia 10 2011.93 9-37
Onypterygia 5 16435.16 5.6-29.3
C . “.1
Hippodamia convergens 5 7.51:1.36 4.8-11.5
Coleomegilla maculata 6 7.81:1.48 5.6-14.6
Cantharid“
unidentified larva 2 l3.8i2.l 12.3-15.3

 

a number of individuals tested in seven day trial
b mean i 95% confidence interval

41

Table 2.4. Parasitism efCPB egg masses by Edovum puttleri in
Morelos, Mexico.

 

 

 

1987 1988
Sample Date, no. masses percent no. masses percent
11m; gaging; mesa ggllggjedpazgsijism sites ggllggjgd parasitism
June 21 KT 35,27 0
28 KC 33.13 0
July 5 XC 35,12 0
12 -- -- --
19 X.C.R 29.12.15 0 V.M 27.12 0
26 X,R 40.13 0
Aug. 2 X.C.R 30.15.21 0. 6.7. 0
9 KC 30.29 0. 6.9 X.V 20.39 0
16 X 22 0
23 KC 30.30 0.6.7 X.M.V 176.36.235 0
30 X,C,R,V 30,41.30.30 0 X 100 0
Sept. 6 X.C,V 130,121.83 0. 5.8, 0
13 X 51 0
20 X 124 0
27 H 42.24 4.8, 8.3 X 60 0
Oct. 3 X 80 0
10 X 39 28.2
17 X 30 0
24 X 19 0

 

a V = Cuemavaca, X = Xoxocotla. C = Cation de Lobos, T = Ticuman, R = Ranche, M = Moyotepec.

42

 

 

 

 

 

Table 2.5. Parasitism of Colorado potato beetle fourth instar
larvae by Myiopharus spp in Morelos. Mexico.
1987 198L
Sample Date. percent percent
Week gaging; sitgsl n11 parasitism sites all 931.35.113.21
June 21 - - - - - -
28 R 36 0
July 5 X 118 0
12 H 95.60 2.1. 6.7
19 X.C.R 60.18.26 23.3, 28.8. 50.0 V.M 300.200 1.0. 3.5
26 X 100 34.0
August 2 X.R 45.35 20.0. 54.3
9 X 24 37.5 V 128 7.0
16 X 30 16.7
23 X 36 58.3
30 R 26 46.1
September XC 104.9 21.2. 0 X 107 2.8
13 - - - - - -
20 - - - - - -
27 21 47.6 X 24 16.7
October 3 72 18.0
10 - - - - - -
l7 - - - - - -
24 X 60 3.3
31 X 48 18.8

 

a V = Cuemavaca, X = Xoxocotla. C = Canon de Lobos. T = Ticuman, R = Rancho. M a Meyotepec.
5 number of fourth instar larvae collected. per sample date. per site

43

Table 2.6. Mortality ofCPB adults caused by Strongyaster sp.
and unidentified fungal pathogen in Morelos. Mexico.

 

 

1987 Date. number
week ending; sitesa 11 killed (cause)?-
June 21 X,R 31.13 0
28 KC 30.30 0
July 5 CR 40.16 2(F).0
12 X.R 12.34 0
19 X.C.R 28.6.23 0
26 X 58 0
Aug. 2 R 30 0
9 KC 30.30 0
16 X 30 0
23 X,C 30.30 0.1(8)
30 X.C.R 30.19.30 0.1(F).0
Sept. 6 KC 60.60 0.2(F)&2(S)
13 X 30 0
20 X 60 0
27 H 30.48 0
Oct. 3 X 96 1(F)
10 - - - - - -
17 X 48 0

 

a V a Cuernavaca. X = Xoxocotla. C = Canon de Lobos. T a: Ticuman, R 8 Rancho, M = Moyotepec.
b F s unidentified fungal pathogen. S n Sirongyasier sp.

44

Table 2.7. Incidence of Chrysomelobia iabidomerae on adult
CPB in Morelos, Mexico.

 

 

__1.28_7__
Sample Date. beetles in W
Wing: sitesa smmmnniinum
June 14 CR 14 021158 0-2
21 XR 20 05511.0 0-4
28 X.C.R 20 0.051.22 0—1
July 5 C 10 0 0
12 X 8 16212.5 1-7
19 X 10 0 0
Sept. 6 XR 19 29514.0 0- 15
Oct. 17 X 23 31.019.86 12-49
_1.9_8.8_
June 28 X 36 0 0
July 19 V 63 0.271.88 0-5
26 M 51 0 0
Aug. 2 X 12 0 0
9 X 113 03411.08 0-6
V 5 7 0.051.29 0-2
16 M 47 0 0
23 R 32 0.161.63 0-3
X 62 07111.43 0-7
30 X 72 0 0
October 10 X 31 l.451l.69 0-4

 

a X = Xoxocotla. C = Canon de Lobos. R = Rancho.

Table 2.8. Natural enemies of the Colorado potato beetle: key

species observed at Xoxocotla.

EBEDAIQRS

Pentatomidae:

Apateticus lineolatus
Perilloides confluens
Perillus bioculatus
Optomus dichrous
Stiretrus anchorage

Carabidae:

Callida decora

Lebia cyane

Lebia guatamalena
Lebia quadrinotata
Lebia yufosutura
Onypterygia fulgens
Onypterygia tricolor

Coccinellidae:

Coleomegilla maculata
Hippodamia convergens

 

 

Morelos.

 

PARASITES

Tachinidae:
Myt'opharus americanus
Myt‘opharus doryphorae
Strongyaster Sp.

Acarina:
Chrysomelobia labidomerae

Eulophidae:

Edovum puttleri

‘46

 

 

0...
.o.. ..O’... O I.
:.:.O Q
5‘ ‘--. u a x I c o
:. .0. .00.? .00...... ...0. 0
: : a? i I "
I. " 0.“; II 0 R E L O S
O
O
’0'... ’f"-~—-~’mm
e’ I
‘ “’- ~ " “ ~ v - ~mm
’ I ~15w ureter:
M=Meyotepec
fiRancho
\
Jejutla \
\
\
Tequesquliengo \
\
l
'_ 1000 mien

  
   

 

 

 

Figure 2.1.
hdexico '

Parasite sample collection locations in Morelos.

47

 

 
     
  

 

 
 
  
    

       

 

% °‘° EARLY srre 1 937
7.0
1:
§ 3.0
II I O. dichrous ”7.
§ 5'0 m P. oonfluons £33557, \ _ I
o 4.0 77/- / I m/
LU // /, 1;;
2 3.0 / 8.
i— ,r s
S 2.0 .c // i
g 1 o 7‘I lll' ///1
D ' / -' se.-x...
o M //////7/ , / //////////,
o 2 4 s s 10 12 14 1 6 1a 20
June i July E August g Sept . October

 

. O. diohrous
m P. conflusns
I S. anchorage

September E October
I

      
         
 

 

   

CUMULATIVE OBSERVATIONS
a.

_!"/'/'/}/'/'//II

Figure 2.2. Phenology of pentatomid species at Xoxocotla.
Morelos, 1987 & 1988. The number of observations of each species
is indicated by the difference between the lowest and highest
points in the shaded area. Graphs are plotted from weekly
averages of the total count on plot.

48

 

u -o— spammsu .
--o-- mummusmoorm .500

% PARASITISM
i?

    

«a”"°'“r----- ’

O 2 4 6 8 1012141618 20 22
imgmgmgsmgeaow

 

5
FOURTH INSTAR COUNT

 

 

 

Figure 2.3. Tachinid parasitism in relation to CPB density.
Xoxocotla. Morelos. 1987.

Chapter in
IMPACT OF NATURAL ENEMIES ON THE POPULATION DYNAMICS

orrnncownanorormmm-oo

INTRODUCTION

The Colorado potato beetle (CPB). Leptinotarsa decemlineata (Say). has become
a classical example of the failure of conventional pest control (Casagrande 1987). Faced
with an imperative to develop ecologically based strategies for management of resistant
CPB populations. recent studies have focussed on the adopted host of the CPB. the
potato. However, until the mid- 1800's. the CPB was restricted to wild hosts distributed
in Mexico and the southwestern U.S.. These plants are weedy early-successional plants
typified by S. rostratum Dunal and S. angustifolium Mill. Studies of the CPB on its
ancestral hosts may ofi’er important insights into the ecology of this pest. In this study.
we consider effects of natural enemies on CPB populations on native host plants in
Mexico.

As the center of origin of the genus Leptinotarsa (Tower 1906). central Mexico
possesses a particularly rich complex of CPB predators and parasitoids. As in the U.S..
the most prominent groups of CPB predators are asopine pentatomids. foliar searching
carabids. and coccinellids. Parasitoids include tachinid flies and a eulophid wasp
(which is restricted to Mexico and Central and South America). In the only recent study
of CPB populations in Mexico. Logan et al. (1987) found low CPB population densities.
Food was apparently not a limiting factor. because defoliation of native hosts was
rarely observed. Noting an abundance of natural enemies. Logan et al. hypothesized

that these may regulate CPB populations at low levels.

49

50

The current study was initiated to examine interactions between the CPB and its
natural enemies in the state of Morelos in central Mexico. Because the study sites at
Xoxocotla. Morelos are in the area assumed to be the center of origin of the CPB (Tower
1906: Hsiao 1981). and because the study sites have abundant CPB populations. it is
assumed that the sites represent ancestral conditions. Additionally. abundant and
diverse natural enemy populations made the sites well suited to analysis of CPB-
natural enemy interactions. The predator complex included specialist predators
known to be significant CPB enemies (e.g.. Lebia spp.. Oplomus dichrous (H.S.).
Hippodamia convergens Guer.). and generalist predators (esp. Aranae. Nabidae.
Reduviidae). CPB parasitoids included two tachinid species and the eulophid Edovum
puttleri.

In the previous chapter the incidence of natural enemies observed in Xoxocotla.
Morelos. Mexico was reported. Here. I discuss the following questions: Are natural
enemies significant mortality factors? What are the seasonal patterns of mortality?
How well are natural enemies synchronized with CPB population cycles? The answers
to such questions may provide insight for the design of a potato growing system that
optimizes natural controls of CPB populations. Basic research on the CPB in Mexico
may also be applicable to pest control efforts that will be neccesary if the CPB develops

as a pest there (Chapter IV).

METHODS

Study Sites. The study sites were located at an agricultural school (CBTA No. 8)
near Xoxocotla in the Mexican state of Morelos (Fig. 1.1). The climate of the area is
semi-tropical. classified as A wo (w) (Koépen system. modified by Garcia 1966) The
mean annual temperature is ca. 22’ C. and mean annual rainfall averages 800-1000

mm. The five month growing season begins in early June. The altitude is 1 100 meters.

51

The landscape is characterized by fields of corn and sorghum interspersed with fallow
fields and pasture/Chaparral. Approximately 20% of the cultivated area receives
irrigation. allowing continuous cropping (R. Rodriguez. pers. comm.). Daily maximum
and minimum temperatures. relative humidity. and precipitation for the site were
obtained from a weather station maintained by the Instituto Nacional de
Investigaciones Forestales y Agropecuarios in Zacatepec. ca. two km from the study
sites. Weather data is summarized in Appendix 2.

Newly emergent adult CPB were first observed at Xexocotla in mid-June 1987. On
June 25. a representative 100 x 2.5 meter block of field edge habitat was chosen and all
established 8. angustifolium plants (i.e.. plants >20 cm) were marked and numbered.
Plant height was measured then and at 2 week intervals. By July 25. host plant
defoliation precluded continued sampling on this site ("early site"). A second location
("late site") approximately 50 m away was chosen and sampling continued until October
30. Both sites contained between 58- 78 plants distributed over 100 m of field edge
habitat adjoining sorghum fields. In 1988. CPB did not emerge at Xoxocotla until July
5. 42 S. angustifolium plants were marked in the same area as the late site of 1987. and
sampled from July 8 to August 16. These plants became heavily shaded by the sorghum
crop and attracted few beetles: thus sampling was continued after August 16 on 70
plants in an adjacent open field that had begun to be colonized by adult beetles.

Sampling at the second 1988 site was continued until October 6.

CPB Population Census. CPB were counted on numbered plants at 2-4 day
intervals. corresponding to 30-60 degree days (base 10" C). Counts were made between 9
and 12 am.. Data recorded for each plant included incidence of each CPB life stage.
number (1' egg masses. and total eggs.

In 1987 predator-damaged eggs and egg masses were also counted. Eggs removed

by predators were estimated by determining the difference between the average size of

52

damaged and undamaged masses. "Missing eggs" were calculated for each sampling
date as [(#Damaged Masses x Mean Size Undamaged Massesll-(Total Damaged eggs). In
the late site two causes of egg damage were quantified: consumption by mandibulate
predators (chorion partially consumed. typical of Coccinellidae and Carabidae). and
consumption by sucking predators (chorion collapsed and intact. typical of
Hemiptera). TWO other sources of visible damage that contributed < 10% of the
observed damage were noted but were not quantified: fungal growth (eggs discolored or
overgrown by hyphae). and CPB adult and larval cannibalism (chorion consumed

leaving base attached to leaf).

Determination of Mortality. Stage-specific mortality was estimated using an
area-under-the-curve technique (Southwood 1978). The residence time of CPB eggs and
larval instars assumed for these calculations was the developmental time of CPB as
determined by Walgenbach and Wyman (1984) for CPB feeding on potato in Wisconsin.
CPB deve10pmental times have been shown to be similiar for CPB from Europe
(Walgenbach and Wyman 1984). Although these times may vary with different
geographic areas or difi'erent hosts. it is assumed for mortality calculations that the
relative duration of different CPB life stages is similiar between CPB populations.
Degree day accumulations above 1090 were determined from daily mean temperatures
recorded at Zacatepec (Appendix 2). Mortality estimates based on the Southwood
method depend on the within-stage distribution of mortality. and are most accurate
when mortality is low at the beginning of the instar and high at the end (Southwood
1978). Because CPB larval mortality is probably greater shortly after ecdysis (Groden
1988). the Southwood method will tend to underestimate stage-specific densities.

Mortality of the pupal stage was measured in cage studies in 1987 on three
occasions corresponding to degree days 340. l 150. and 1750. Mature fourth instar

larvae were confined to four 1 m3 cages enclosing two or three host plants. and the

53

proportion emerging as adults were counted to determine survival. The density of caged
larvae was chosen to approximate maximum densities observed in the field. Thus in
the early site. where average per-plant fourth instar production was 27. an average of 57
larvae was placed in each of four cages: at the late site. an average of 4 fourth instar
larvae was produced per plant. and 12 larvae were placed in each cage (four cages at
degree day 1150. 2 cagesat degree day 1750).

Mortality of CPB adults in diapause during the November to June dry season was
measuredinsix 1 m3 screencages. OnOctober 15 1987. thecageswereestablishedover
S. angustifolium plants and 15 CPB adults were confined in each cage. The cages were
monitored for emergent adult CPB every 4 days. from June 21 to August 1 1988.

In 1987 mortality on the sample plots was calculated for three discrete time
periods: The first CPB generation sampled at the early site (DD 0-636). and for CPB
cohorts that correspond roughly to second (DD 591-1430). and third (DD 1430-2067)
generations of CPB on the late site. Additionally. stage-specific mortality was
calculated for individual plants on the late site (n=58) for the entire interval DD 591-
2067. In 1988 mortality was calculated only for the period from August 16 to October 6

(early season population density was too low to permit life table analysis).

Predator Impact. Relationships between predators. their CPB prey. and egg
damage levels were analyzed for 1987 on both the early and late sites using time series
cross-correlation (TSCC)(Box and Jenkins 1976). This technique allows the
determination of a maximum correlation coefficient between dependent variables.
based on correction for lags along a common independent variable. usually time. In
this analysis. time-corrected predator-prey and predator-prey damage correlations
were used to infer the response of predators to changes in prey density. The time lag

required for optimization of the correlation provides an estimate of the response time.

54

TSCC was computed using PROC ARIMA (SAS Institute 1984). and the output was
screened for the maximum significant positive correlation occurring within a delay of
up to 10 units of time lag between the independent variable (prey density or egg damage)
and the dependent variable (predator abundance). (Ten units of time lag correspond to
ca. 350DD ontlre earlysite. andca. 460DDonthelate site.)

Relationships between predator intensity and CPB egg mortality. integrated
over all sampling dates. were analyzed on the late site by multiple linear regression.
Predator intensity was taken as (Number of Predators)/ (1‘ otal Egg Count X 1000). The
units of observation were individual plants: thus positive regression relationships
between predator and prey indicate association on a small spatial scale. Egg mortality
was measured in two ways: by the total incidence method (Seuthwoed 1978). and as
damage to egg masses. From the perspective of predators. the effective egg density on a
plant would depend on the foliar area over which a given number of eggs were
distributed. To minimize the confounding effect of plant size on predator-mortality
relationships. the efi'ect of total egg count on egg predation levels was analyzed
separately for three size classes of plants. where size was determined from the average
of six measurements made at two week intervals.

Regression analysis was performed using PROC ST'EPWISE (SAS Institute 1982).
Significant explanatory variables were determined using the backward elimination
procedure. A critical value of p=0. 15 was selected for acceptance of variables in the

model.
RESULTS

CPB Population Dynamics. Incidence curves for CPB eggs and larvae in 1987 are
presented in Fig. 3. 1 (early site) and Fig. 3.2 (late site). Egg and larval counts on the

early site define a discrete first generation: second generation adults emerging on the

55

site fed briefly on the nearly defoliated plants. and dispersed before the onset of
oviposition. The rising egg curve on the late site (Fig. 3.2). coincident with the dispersal
of second generation adults from the early site. appears to represent recruitment of
these adults to newly emergent host plant stands. although late-emerging CPB from
within the late site may have contributed to oviposition. The incidence of larvae on the
late site shows two peaks: an initial cohort. and a late season resurgence. The second
peak. beginning with a five-fold increase in first instar larvae. follows a more limited
oviposition increase of ca. 50%: thus the second peak results more from a release from
mortality than from increased egg input. In order to differentiate mortality’occurring
during these two phases. mortality was calculated separately for the periods before and
alter DD 1530. as described under "Methods."

In 1988 early season CPB populations were at very low density. CPB were not
present at Xoxocotla until the beginning of July. and the population sampled on the 42
plants followed after July 8 was extremely low (egg counts on the plot ranged from 0 to 4
egg masses). Beetles at a density comparable to that observed on plots in 1987 were
finally encountered on August 16. in an open field stand of 200-300 host plants. CPB
population dynamics were thus analyzed for the period from August 16 to October 6

only. Incidence of CPB on 70 plants in this host stand is represented in Fig. 3.3

Stage Specific Mortality. Note that mortality stated for any life stage is a
function of the difi'erence between total incidence at that stage and the subsequent stage:
thus. e.g. mortality for the egg stage includes both egg and first instar deaths.

A life table for the CPB in 1987 at Xoxocotla is presented in Table 3. I.
Calculations include pupal mortality determined from cage studies. displayed in Table
3.2. First generation mortality (early site) was concentrated in the first instar and egg
stage (81%). and overall mortality. egg to adult. was moderate (88%). Mortality in

the second generation (late site. DD 591-1430) was relatively high in all life stages. and

56

very high overall (99.8%). Third generation (late site. DD 1430-2067) mortality
decreased. to a level of 97.2%. Egg and larval mortality calculated for 1988 is displayed
in Table 3.3. Mortality was again highest in the egg stage (77%). Overall mortality was
greater than 91%. calculated up to the fourth instar. Fourth instar to adult mortality
was not determined in 1988. Emergence of adults in 1988 from diapause cages
established in November 1987 was 13%. All but one of 1 1 emerging adults was
recovered between June 22 and July 1: a single adult emerged on July 22.

Mortality (for eg through fourth larval instar). calculated for individual plants
on the late site in 1987. ranged from 86 to 100%.

Natural Enemies. Significant mortality of fourth instar larvae was contributed
by tachinid parasitoids. In 1987, tachinid parasitism on the early site accounts for
most of the 9.2% fourth instar mortality calculated in Table 3. 1. In fact. the weighted
average of fourth instar parasitism. based on parasitism samples collected in the field
(Table 2.5). was 15.6%. Fourth instar to adult mortality is thus probably
underestimated in Table 3. I. On the late site the weighted average of fourth instar
parasitism (from Table 2.5) was 33.0%. somewhat less than the 60-70 % mortality
calculated in Table 3. l. A low level of egg parasitism by Edovum puttleri (less than 1%.
based on Table 2.4) was recorded in the late site. In 1988 eg parasitoids were not
detected. and larval parasitism was low. Parasitoid samples in 1988 were insufficient
to permit an estimate of their efi‘ect on mortality.

Relationships between CPB population dynamics and the incidence of predators
suggests that the largest component of mortality is attributable to predators. The
relative abundance of the major predator groups. based on visual counts at Xoxocotla
(Chapter 2) is exhibited in Fig. 3.4. In 1987 observations at the early site are dominated
by coccinellids (H. convergens and C. maculata ). Coccinellids are at much lower

densities on the late site. appearing briefly between 900 and l 150 degree days. Foliar

57

searching carabids are relatively abundant at the late site. but essentially disappear at
the end of the CPB oviposition curve (Fig. 3.2). Pentatomids are also relatively
abundant in the late site. A late season decline of pentatomids is followed by a final
resurgence (accounted for by Optomus dichreus--see chapter 3. Fig 2). In 1988. only

pentatomids were frequently observed at Xoxocotla (Fig. 3.4b).

Predation: Variation Over Time. Six species groups of CPB predators were
considered as potentially significant based on field observations and on positive
results from laboratory feeding trials (Chapter 2). These include: 1) Asopine
pentatomids. 2) Foliar searching carabids. 3) Coccinellids (Coleomegilla maculata and
Hippodamia oonvergens ). 4) Reduviids. 5) Crab spiders (Thomisidae). and 6) Cantharid
larvae (late site only). Relative abundance of these groups is displayed in Table 2.1 . The
relationships between occurrence of these predators and various measures of egg
density and egg predation in both the early and late sites are presented in Table 3.4.
Reported values are the maximum positive correlation obtained by optimizing the time
lag between variable 1 (eggs. or egg damage) and variable 2. the predator.

It should be noted that use of TSCC for analysis of predator response to CPB prey
is complicated by the fact that several of the predator species may attack several CPB
life stages: for example. it is impossible to distinguish between a delayed predator
response to CPB eggs. and an immediate response to second instar larvae. However. I
have chosen the egg stage as the independent variable for TSCC for several reasons. On
the early site. the most common predators (the Coccinellidae) are probably exclusively
egg and early instar specialists (Groden et al.. in press) On the late site. the most
abundant CPB predators. the pentatomids and carabidae. consume all CPB life stages
but probably feed primarily on eggs and first instar larvae due to the very low incidence
of older life stages (Fig. 3.2).

58

In order to estimate the relationship between predator abundance and predation
rates. TSCC between predators and egg damage is also reported. Egg damage is measured
as "missing eggs." calculated for each sampling date as [(#Damaged Masses ‘ Mean Size
Undamaged Masses)l-l'l‘otal Damaged eggs]. In the late site. eg damage from sucking
and mandibulate predators is analyzed independently (causes of egg damage were not

accurately determined on the early site).

Inter-correlation of CPB Life Stages. In the early site the total egg incidence
correlates highly and significantly with each of the larval instars (r>0.86). and time
lags correspond roughly to the duration of larval instars. This serial symmetry of life
stage incidence curves. apparent in Fig. 3. l . implies a consistent pattern of mortality
between life stages. By contrast. total egg incidence in the late site does not shows
- significant correlation with any larval instar (Table 3.4). This is a result of a change in
the pattern of stage-specific mortality during the period of observation: the increase in
larval density late in the season thus appears to result not from an increase in prior

oviposition. but from a release from mortality.

CPB-Predator Correlation. On both early and late sites. most CPB predators
exhibit a positive correlation with CPB egg incidence-only coccinellid adults and
Cantharid larvae fail to show a significant positive relationship (Table 3.4).

No differences between the early and the late site are evident in the TSCC
analysis. First. the correlation between predator and CPB egg abundance increases at
the late site for total predators. and five of the six predator classes reported. Secondly.
peak predator abundance follows peak oviposition much more closely later in the
season: the delay between peak egg incidence and peak predator abundance is 245 DD on
the early site. and 92 DD on the late site. Thus the maximum potential contribution to

mortality by predators in the early site occurs well after the oviposition peak. The most

59

abundant egg predator in the early site. larval Coccinellidae. also exhibit a 245 DD
delay in numerical response. This delay means that most coccinellid predation
pressure occurs afier the majority of eggs and first instar larvae have metamorphosed.
escaping the predation of coccinellids. The shorter lag between egg and predator
abundance en the late site implies relatively greater predation intensity on the egg
stage. and is perhaps part of the explanation for much higher egg mortality on the late
site.

Missing eggs were correlated with total eggs in both the early and late sites. and
time lags were small (0 and 46 DD respectively): thus predator-induced egg damage
would appear to be synchronous with egg abundance. The relationship between egg
density and the rate of egg damage provides an indication of the density dependence of
CPB egg predation. This relationship was analyzed by simple linear regression of total
egg incidence on the percentage of eggs missing. Regression on the early site was
negative but nensignificant (p=0.24). A significant positive relationship between egg
density and egg damage was observed on the late site (p=0.09. n=33). Lew values for R2
(early site: R2=0.10: late site: R2=0.09) suggest that the density dependence of egg
predation is weak over the range of densities observed in the two sites. or that there are

other factors which obscure the relationship between egg density and egg mortality.

Predation: Variation Between Plants. Relationships between CPB eggs or egg
mortality and intensity of six CPB predators are presented for the late site in Table 3.5.
Plant size (mean height during period of observation) was included as an independent
variable likely to have confounding effects on predator response. Regression of total
eggs against predator intensity and plant size reveals that plant size accounts for 28 %

of the variability in egg counts between plants (Table 3.5). Thomisids and coccinellid

60

larvae. also included in the significant model. are negatively correlated with plant size.
but account for only 6 and 4 % of the variability in egg count. respectively (Table 3. 5).

Regression of predator intensity on egg mortality revealed very low correlation;
the two variables in the model. Reduviidae and coccinellid larvae. account for less than
1% of the variability in mortality. Ne relationship between predator intensity and egg
mortality calculated for individual plants is evident. Mortality was uniformly high--
the range was from 86 to 100%. Thus differences due to predation intensity may be
masked by the overall high mortality.

A final attempt to look at the density dependence of predation mortality on
individual plants involved regression of total egg count on the percentage of egg masses
damaged by predators. To minimize the confounding variable of plant size. the
analysis was run seperately for three size classes of plants. The result showed a weak
but significant relationship (R2=0.25) for the largest class of plants (>73cm). However.
whether egg predation increases with egg density is not directly indicated. as total eggs
do not necessarily correspond to density on the plant; the foliar area of the plants. over
which the total eggs are distributed. is highly variable even within plant height

classes.

DISCUSSION

CPB Population Dynamics. CPB incidence data for 1987 and 1988 show
significant between-year variability in phenology. In 1988 beetles appeared at
Xoxocofla ca. three weeks later than in 1987. the same pattern that was observed
throughout the state of Morelos (Chapter 1). Another apparent difi'erence in 1988 was
the lower density of early season populations. The CPB population on the early site in
1987 was high enough to defoliate the host plant stand. whereas early season beetle

populations at Xoxocotla were barely detectable (R. Herrera. pers. com.). This

61

difi'erence may be attributable to physical factors delaying emergence, or high mortlity
ofeggs and larvae during the 1987 season (see below). During thegrowing seasoninboth
years. heat unit accumulation at Xoxocotla was sufficient for the development of four or
five CPB generations (assuming ca. 400 degree days/generation. base 10°C: Walgenbach
and Wyman 1984). However. the actual number of generations for individual beetle
cohorts may be less if emergence is delayed or diapause occurs prior to the senescence of

host plants. These factors were not investigated in this study.

Mortality. Mortality of the CPB may be attributable to 1) environmental
factors. including wind. rain. temperature extremes. and host plant quality. 2) density-
dependent. intraspecific factors including starvation. fertility . and cannabalism. and.
3) attack by natural enemies including predators. parasites. and pathogens. The

relative contribution of each of these factors is considered below.

Environmental Mortality Pactors.- In a six year population study of CPB in
potato fields in Ontario by Harcourt (1971). rainfall was estimated to contribute from 0
to 36% mortality to young larvae. and 4% mortality to the egg stage. In Harcourt's
study. rain-related egg mortality was determined by direct observation of egg masses
damaged by mud splash. This type of damage was not observed at the Mexico study
sites. where dense ground cover absorbs the impact of precipitation and prevents egg
damage. Thus rain-related egg damage is considered minimal. Significant mortality
due to temperature extremes seems unlikely at the Xoxocotla site. Maximum
temperatures show little day to day variance. and the native CPB would presumably be
adapted to typical local temperature regimes. Furthermore 1987 temperature maidma
were highest in June (see Appendix 2) when mortality was low (Table 3. 1).

Host plant quality efl'ects on CPB mortality are a distinct possibility. Hare

( 1983) found that a seasonal increase in mortality of CPB on S. dulcarnara was

62

attributable to increased glycealkaleids and decreased protein in late season plants.
Seasonal variation in the quality of S. angustU'olium. or CPB hosts other than S.

dulcamara has not been investigated.

Intraspecifie Mortality Factors. When pest populations of the CPB defoliate
potato crops. starvation causes significant mortality (and/or emigration) (Harcourt
1971). Defoliation of native hosts in Mexico has been reported to be rare (Logan et.al.
1987). However. in the current study. the early 1987 site experienced substantial
defoliation as fourth instar larvae matured. with 40 of 86 plants on the plot >90%
defoliated. Starvation (or mortality due to predation associated with large larvae
searching on the soil for food) may have contributed to the mortality of CPB fourth

instar and pupae at this site.

Predation: Variation Over Time. Significant correlations were observed between
CPB egg incidence and the specialist predators most likely to feed on CPB eggs:
Coccinellid larvae (more abundant in the early site). and pentatomids and carabids
(more abundant on the late site. Predator numbers thus appear to track prey
abundance. but correlation may be explained by several factors. The numbers of
predators observed in visual counts might be expected to correlate with the duration of
the observation period. which is itself correlated with CPB density. This seems
unlikely. as predator counts exhibit a marked decline in the late site while CPB egg
counts (and on-site observation time) remain relatively high. Predators may also
respond to increasing plant size and foliar area. However. again late site predator

counts decline during a period when host plants are increasing in size. Thus a

63

relationship between egg and predator abundance does appear to exist in both early and
late sites.

Missing eggs were correlated with total eggs. and the time delay between the two
categories was negligible (Table 3.4). This finding is somewhat at odds with the stated
evidence of a time delay between egg incidence and predator abundance. and suggests
that as eg density increases. the few predators present respond functionally. by
switching from other prey. or by increasing consumption rates. Such a functional
response has been demonstrated for C. maculata (Groden et al.. in press) and 0.

dichrous (Drummond et al. 1987).

Predation: Variation Between Plants. Predator intensity did not increase with
the number of eggs on plants (Table 3.5). This result appears to suggest that predators do
not exhibit a density-dependent response to CPB eggs. However. the lack of correlation
between total eggs and predator intensity may be explained by sampling errors. Visual
observations of predators on plants do not neccesarily correspond to predation
pressure-cg. a noctumally active predator may be observed during the daytime counts
on plants other than those on which it forages. Also. the probability of observing each
of several predators in regions of high egg density is probably less than the probability
of observing the single predator feeding alone on a small plant. This sampling bias
would obscure a real density dependent increase in predator numbers. Finally.
predators may change their foraging range over time--e.g.. coccinellids may be
associated with CPB eggs early. and switch to other prey on host plants not colonized by
CPB later. Nevertheless. the lack of a clear predator-prey abundance relationship on
individual plants does suggest that predators do not respond functionally to prey on the
spatial scale of the plant.

The uniformity of egg mortality calculated for individual plants also suggests

that predation does not vary with egg abundance on individual plants. Such a

64

conclusion is consistent with the fact that CPB predators are highly mobile. and

individuals may respond to the density of plant groups or patches.

Summary. This study was the first to investigate CPB life history in the
conditions that likely represent the evolutionary milieu of the species. In 1987.
population dynamics of the CPB on its native host in Mexico followed a pattern
similiar to that observed for the CPB on unsprayed potatoes in Michigan and Rhode
Island--an early. first generation increase in population. ofi'set by increasing mortality
later in the growing season (Groden 1988).

In Mexico. early season mortality was highest for the egg stage. when the most
abundant CPB predators were the egg-specific coccinellids C. maculata and H.
convergens. Time series analysis reveals that the response of these predators is
delayed sufficiently to allow the early-laid eggs to escape predation. Late season
mortality was substantially higher. both in the egg stage and in all larval stadia. Late
season mortality correlates with increased abundance of natural enemies that include
the tachinid parasitoid Myiopharus. the asopine pentatomids. thomisid spiders. and
foliar searching carabids. Time series analysis suggests that the phenologr of CPB
predators is more closely synchronized with prey abundance in the late season.

The investigation of the density dependence of CPB predators on a per-plant basis
revealed no clear pattern. a result of the many factors contributing to between-plant

variability. and of the high mobility of CPB predators.

65

 

n« .. .- .. « -. .. .. n: 8.5%
«.3 m.«o we 3. «do «.8 n n «.3 «.o «— R— v «53
o.«o «.2 3 oo n.oo . «.3 o 3 «.5 v.3 me «2 n 552.—
oéo «no? o- . S o.oo 99. o o" «.2 n.» on «2 « <>m<4
oéo «.3 «a no: duo flow 2. no . «.3 adv c2 3n — 45g
Ema 5.3 on» .3 Eco «do go .2. «.8 «do «no i. g
.8 83 8 mus. .8 :50 so owe: we :50 8 one: 8

gang Sagan”— aaaaflgdumuuq gag

 

.ocon £39.32 6:3..on 2. 9:92— .582. 23.3.60 2: Ce 5:83.: 9592.. one.» in 939—.

66

Table 3.2. Emergence of CPB adults at Xoxocotla. Morelos.

 

 

 

 

 

 

W4- W W
Cage L4 Adult L4 Adult L4 Adult

1 58 58 12 l 12 5
2 7O 56 12 5 12 4
3 74 67 12 6
4 26 28 12 3

mean survival mean survival mean survival

= 91 % = 31 % =37%

 

Table 3.3. Stage specific mortality of the CPB

at Xoxocotla. Morelos. 1988.

 

 

 

CPB stage % cum %

I 'E S l 1 l' 1'
HI} 1000 772 77.2 77.2
LARVA 1 278 111 48.8 88.3
LARVA 2 117 29 24.4 91.2
LARVA 3 88 -2 -2 91.0

LARVA4 9o -- -- --

67

Table 3.4. Time series cross correlations between the CPB and

its predators at Xoxocotla, Morelos, 1987.

 

 

EARLY $11115 LATE SITE
Correlation Correlation
WIND-W
Total Eggs
larva 1 0.91 l 0.78 0
larva 2 0.87 3 (0.40) 1
larva 3 0.86 4 (0.13) 8
larva 4 0.86 5 (0.39) 10
total preds 0.48 7 0.77 2
pentatomid 0.56 6 0.76 l
carabid 0.52 4 0.66 l
cocc larvae 0.51 7 0.59 l
cocc adult (0.05) 6 (0.49) 2
reduviid 0.55 7 0.53 5
thomisid 0.68 0 0.74 2
cantharid ---- - (0.35) 4
Eaten Eggs
total preds (0.40) 5 0.61 l
pentatomid 0.58 7 0.64 0
carabid 0.61 5 (0.43) 0
cocc larvae (0.36) 7 (0.33) l
cocc adult (0.41) 6 (0.39) 0
reduviid (0.39) 7 0.57 4
thomisid (0.13) 0 0.67 0
cantharid ---- -- 0.51 3
Sucked Eggs
pentatomid ---- -- 0.61 0
Chewed Eggs
carabid ---- -- (0.40) 2
cocc larvae ---- -- 0.49 2
cocc adult ---- -- 0.55 0
reduviid ---- -- (0.50) 4
thomisid ---- -- 0.81 l

 

a Values in parentheses not significant; K 1 two standard errors
9 One time lag = ca. 35 degree days (base 10°C)
c One time lag = ca. 46 degree days (base 10°C)

68

Table 3.5. Regression relationships between the CPB and its
predators, Xoxocotla, Morelos, 1987.

 

All Plants. n=58:

 

All Variables

Dependent Independent Variables In Model Variable Model

Variable Variables R112 §§<0.15) RE...2 Ra“...2

Egg Mortality Predator Intensity 0.12 Reduviid 0.04 0.0008
Plant Size Cocc Larvae 0.08

Total Eggs Pred Intensity 0.40 Plant Size 0.28 0.34
Plant Size Themisid 0.06

Cocc Larvae 0.04
21am: 2 13 cm ":18.
% Eggs Cons. Total Eggs 0.25 Total Eggs 0.25 0.25
55 cm 2 Plants 2 23 cm 11:29.

% Eggs Cons. Total Eggs 0.10 ........... ---- ----

% Eggs Cons. Total Eggs 0.0 ----------- - - - - - - - -

 

 

 

69

 

COUNT

 

 

 

 

 

.2 ' I '1- . g .
0 200 400 600

DEGREE DAYS (June 19—July 28)

 

COUNT

 

 

 

DEGREE DAYS (June 19-July 28)

Figure 3.1. CPB incidence at Xoxocotla, Morelos. Early site,
1987. a) eggs and first instar larvae. b) larvae.

COUNT

5
§

8000
A
60001
+ L-1
—O— m
4000'
20001
01 --..U. ”L .¢.1 -_ .. -J_m—aL-.w_w
500 700 900 1100 1300 1500 1700 1900 2100
700
B
600‘ —-g— L.1
““0““ L-2
500' ""I'" L-3
400‘ -"." L4
300' ‘
3
200- '7 1‘
100.1 : l I ' 7 ‘K ‘k‘
0- (In! A: 3": "has: 1‘3 . :a. J‘
500 700 900 1100 1300 1500 1700 1900 2100

70

 

 

 

 

 

 

 

DEGREE DAYS (July 25-0ctober 30)

Figure 3.2. CPB incidence at Xoxocotla, Morelos.
a) eggs and firstinstar larvae. b) larvae.

1987.

 

Late site.

71

 

 

 

 

 

5000
' —a— L-1
4000 - _._ am
3000 -
2000 -
1000
o « ...—U- . «A i 1 I ‘ f I
o 200 400 600 800
DEGREE DAYS (August 16-October 6)
800

COUNT
§

 

 

 

 

 

200 400 see 000
DEGREE DAYS (August 1 6-October 6)

Figure 3.3. CPB incidence at Xoxocotla, Morelos. 1988. a)
eggs and first instar larvae. b) larvae.

72

 

 

 

 

 

 

 

 
    
   
   
 
       
  

      

 

   

(D

Z

.9

§

c:

u.)

8.?

0

Lu

E

S

:1

2

D

0

8

:

§

0:

Lu

8

O

E

.—

S

D

2

D .

0 590 790 990 1190 1990 1590 1790 1990
DEGREE DAYS (July 25—0ctober 30)

<2 15 C

8 - OOOCADULT

< - I OOOCLARVA

E 10 . a CARABID

g I PENTATOMID

§ 5- . ..

g o: /////%

o 900

DEGREE DAYS (AUGUST 16-OCTOBER 6)

Figure 3.4. Relative abundance of key CPB predators,
Xoxocotla, Morelos. The number of observations of each species is
indicated by the difference between the lowest and highest points
in the shaded area. Graphs are plotted from weekly averages of
the total count on plot. a) Early site, 1987. b) Late site, 1987. c)
1988.

ChapterIV
THEOGDRADOPOTAT‘OBEETIEINMEXIOO:

DOOONDITIONSEXISTFORITSDEVELOPMENTASAPEST?

INTRODUCTION

The Colorado potato beetle (CPB). Leptinotarsa decemlineata (Say). is the major
insect pest of potatoes in many regions of the United States. Canada. Europe the eastern
USSR. and Turkey (deWilde and Hsiao 1981). Efforts to control the CPB have been
characterized by increasing reliance on synthetic pesticides. countered by the
development of pesticide resistance in CPB populations. Since intensive use of
chlorinated hydrocarbons began after WWII. the CPB has developed resistance to
virtually every major class of insectide. and in the northeastern United States. it new
threatens the erdstence of commercial potato production (Ferro 1985).

Potatoes are currently the world's fourth largest food crop. and are of increasing
importance. particularly in developing countries (Swaminathan and Sawyer 1982). In
Mexico. commercial potatoes are a relatively recent introduction. but production has
increased steadily since national breeding programs began in 1946. Potatoes are
presently grown on some 80.000 ha in five major production zones (M. Villarreal. pers.
com.). The CPB has not yet been recorded as a pest in Mexico. although it is widely
distributed there.

The CPB is believed to have originated in south-central Mexico (Tower 1906:
Jacques 1972). The original range of the beetle was defined by the distribution of its
host plants and by geographic barriers. In southern Merdco. the beetle is found on S.
angustifolium Mill. (=S. comutum) as far south as the Isthmus of Tehuantepec. an

efl'ective geographic barrier to many insects (I-Ialfiter 1987). The northern distribution

74

of the CPB extended to the U.S. southwest on S. elaeagnifolium L.. and into the U.S.
Great Plains on S. restratum Dunal (Hsiao 1978: Whalen 1979).

Within its original range. the host plants of the CPB are limited to a complex of
closely related species of Solomon section mama. However. several factors about
CPB biology suggest a propensity for adaptation and host race formation. These
include broad environmental tolerance. high intrinsic rate of increase (Harcourt 197 I).
and high genetic variability (J acobsen and Hsiao 1983). Such characteristics might be
predicted from the apparent evolutionary history of the CPB: its endemic hosts are
patchily distributed successional plants with a high rate of extinction (Whalen 1979).
The section Androceras is also characterized by limited gene flow and rapid speciation.
Furthermore. genetic heterogeneity of the CPB and its food plants is likely enhanced by
altitude gradients that greatly limit east-west dispersal in Mexico (Helmer 1987). The
CPB has thus evolved under conditions favoring extensive within-population variation
and effective mechanisms of dispersal to cope with frequent changes in food and
habitat. These pre-adaptations then set the stage for the enormous success of pest
populations of the CPB.

In about 1820. the potato. Selanum tuberosum. was first introduced to
Nebraska. at the northeastern limit of the CPB's range. The adaptation of the CPB to
this new host occurred during the 40 years following the first encounter. resulting in the
establishment of a pest population by 1859 (Riley 1871: Casagrande 1985). The
geographic range of the pest biotype of the CPB rapidly expanded. following the
cultivated potato east across the U.S.. into southern Canada. and ultimately across
most of Europe (Hsiao 1981). As a result of its expansion into new habitats the CPB
colonized several additional species of Solanaceae including the cultivated tomato.
eggplant. and pepper. CPB host plants now include at least 20 wild and cultivated

species of Solomon (Hsiao 1985).

75

As with the potato. the colonization of novel hosts by the CPB has been
accompanied by rapid adaptation and the emergence of biotypes with increased
virulence on specific hosts. For example. the tomato, Lycopersicon esculentum. has
been shown to be an inferior host for the CPB. and pest problems have not developed in
many areas with sympatric CPB and tomato populations (Latheef and Harcourt 1974).
However. local populations of the CPB have recently overcome the resistance
mechanisms of the tomato. and the CPB is of growing importance as a tomato pest.
particularly in the northeastern U.S. (Ferro 1985). Other geographic host races of the
CPB have exhibited genetically based host-specific adaptations to S. elaeagntfelium in
Arizona (Hsiao 1978) and S. carolmense in North Carolina (Hare and Kennedy 1987).

The conditions for a similiar pattern of host shifts by the CPB may now exist in
Merdco. During the summers of 1987 and 1988. we conducted experiments at Xoxocotla.
Mexico. and in the laboratory at the State University of Morelos (UAEM) to assess the
pest potential of the CPB in Mexico. This paper focuses on how changes in potato

cultivation combine with factors intrinsic to CPB biology to favor the development of a

pest.

Materials and Methods

All experiments were conducted in Morelos. Mexico. an area with abundant
populations of the CPB at elevations below 2000 meters (Chapter I). Beetles have been
reported from a single host species in Morelos. S. angustifolium Mill.. although
potential hosts including S. elaeagnifolium Cav. and S. cardiophyllum Lindl. overlap
the range of the CPB in the state (Rezdowski 1985: Correll 1962). Potatoes are grown in

Morelos but occur at elevations above the range of the CPB.

76

UAEM Best Choice Experiments. Laboratory studies examining host choice
were conducted at the experimental station of the UAEM in Cuernavaca. Morelos. No
experimental designs were employed:

1. W In this cage study. CPB adults were given free choice of
S. tuberosum or S. angustifolium hosts that were physically isolated from each other.
Preferences exhibited thus reflect both pre- and post-alighting components of selection.

CPB adults were collected from natural stands of S. angustifolium at Xoxocotla.
Morelos and transferred to the experimental station. After being held overnight. 25
beetles were introduced into a 1 m3 screen cage containing three S. angustifolium and
three S. inbemsum plants. The potted plants were arranged in two same-species groups
along either side of the cage. with the foliage of the two species groups separated by 30
cm.. Plants were chosen so as to form species groups of approximately equal foliar
area; plants were also of approximately equal age. 6- 10 weeks post-emergence. S.
angustifolium plants were transplanted from the CPB collection sites and measured ca.
35-40 cm. in both height and diameter. Each potato plant contained 2-5 stems and
measured 30-50 cm. Two potato cultivars. obtained from the potato research center of
INIA in Toluca. Mexico. were used: Alpha (subspecies 19112112511111) and Lopez (subspecies
33min). S. tuberosum plants were grown from seed pieces under ambient conditions
at the UAEM experimental station (elevation: 1800m).

Following introduction of adult beetles. each cage was examined at two or three
day intervals on six occasions. Numbers of adults and egg masses on S. angustifolium.
S. tuberosum. and on the substrate or cage screenning were recorded. All observations
for each cage were averaged and host difi'erences determined with an unpaired t-test.
The experiment was replicated on June 28. July 14. and August 4 1987. S. tuberosum cv.

Alpha was used in the first replicate. and c.v. Lopez was used in replicates two and three.

77

2. W A second study conducted at UAEM examined differences
in host utilization by both CPB adults and larvae freely colonizing a mixed planting of
S. angustifolium and S. tuberosum. The experimental design was intended to simulate
conditions that may occur in a weedy field where the two hosts are sympatric.

Potted plants were arranged with foliage touching in a six plant group with S.
angustifolium and S. tuberosum (var: Lopez) alternating. These plants were freely
colonized by CPB adults and large larvae moving from potted S. angustifolium plants
located within 2 m of the test plants. Counts of adults. larvae. and egg masses present
on each plant species were made three times. at two day intervals. The experiment was

conducted on July 25 1987. and included three replicates.

Field Colonization Study. In 1988 colonization of potatoes by CPB was evaluated
under field conditions at Xoxocotla. Morelos. The study compared rates of
colonization of small potato plots with colonization of natural stands of S .
angustifolium Seventy potatoes (c.v. Alpha) were planted on 40 cm centers in each of
two plots on June 21. One ounce of 10-10-10 fertilizer was applied to the base of each
plant on August 16. Potato plantings were located ca. 50 meters apart along the edge of
sorghum fields. in an area that had CPB populations in 1987 (chapter III). Each of the
plantings was paired with a natural stand of S. angustifolium located within 5 meters.
S. angustifolium stands contained 15-20 plants with a foliar area approximately equal
to that of the potato plots. On June 18. when host plants had attained 20-40 centimeters
in height. 16 adult CPB (8 female and 8 male) were introduced to each of the potato plots
and native host stands. Density of all CPB life stages was then measured on all plots. on

eight occasions between July 22 and August 16.

78

UAEM survival experiments. The developmental performance of CPB larvae on
potato was initially examined in cage studies at the UAEM experimental station. Three
each of potted S. angustifolium and S. tuberosum (cv. Alpha) plants were evenly spaced
in a one cubic meter screen cage. as described in cage studies above. Each plant was
innoculated with 30 one day old first instar CPB larvae applied gently with a brush to
the upper leaf surfaces. Larvae were selected individually from a petri dish containing
the larvae pooled from 10 eg masses collected at Xoxocotla. and placed in alternation
on plants of each species. Plants were examined at five. ten. and twelve days. and
numbers and developmental stage of larvae remaining on each plant were recorded.

A second experiment was conducted to assess whether declining counts of larvae
on the potato plants indicated mortality. or whether they included successful
emigration to the adjacent S. angustifolium plants. Three caged S. tuberosum plants
were innoculated with 50 first instar larvae. and numbers and developmental stage
recorded on six occasions during 21 days. To measure emigration. the larvae were also
counted on an initially beetle-free S. angustifolium "trap" plant. placed with leaves not

touching the potatoes. 30 cm from the potato plants.

Field survival study. Survival and developmental rate of CPB larvae on potato
and S. angustifolium were evaluated under field conditions at Xoxocotla in 1988.
Potato plants were established in the field in a small plot (described above). and
similiar sized S. angustifolium plants in an adjacent stand were identified. All plants
were 30-40 centimeters in height at the beginning of the experiment on August 20. To
exclude natural enemy mortality. plants were enclosed within cylindrical steel screen
cages. Cageswere40cmhighby30cmdiameter.withmesh size of2by2mm. Ten

potato plants and seven S. angustifolium plants were innoculated with 20 randomly

79

selected newly eclosed first instar CPB larvae. applied to plants as described above. The
larvae were pooled from 20 egg masses collected from Xoxocotla and Cuernavaca. FNery
three days surviving larvae were counted and developmental stage determined. Pre-
pupal fourth instar larvae were collected from each host and maintained without food
in petri dishes to determine adult emergence. The behavioral response of larvae to
different hosts was also assessed by observing all surviving larvae in each treatment
for one minute. These observations were made at three and six days after introduction
of the larvae to plants. Behavior was classed as quiescent. feeding. short distance
movement. or long distance movement. Short distance movement was defined as travel
5 one larval body length. and travel > one body length was considered long distance

movement.

RESULTS AND DISCUSSION

A host shift by an insect herbivore may involve many kinds of sensory.
physiological. and ecological adaptations. The insect must be able to efi'ectively locate
and colonize its host; it must be temporally synchronized with host phenology; it: must
be capable of assimilation. growth. and maturation on its new diet; and finally.
environmental conditions in the new host habitat must be adequate for development.
However. although the population gene pool may include the genes that would permit
survival on a new host. adaptation will occur only under conditions where encounter
with the new host is frequent enough to expose those genes to selection. In the case of
the potato in Mexico. I will describe how recent changes in its distribution and
cultivation practices have greatly increased the rate of encounter between the CPB and
potato. 1 will then discuss our experimental evidence that suggests that these

encounters may create the series of genetic accidents that will result in a pest biotype.

80
12W

Expansion of the Potato into the Range of the CPS. Although Mexico is one of
two major centers of speciation of the tuber-bearing species of Solomon (Correll 1962).
the cultivated potato S. tuberosum is a relatively recent introduction. The earliest
record ofpotato in Mexico is by Humboldt (181 I). who suggested that it had been brought
from South America by the Spaniards during the period of conquest. Until recently. the
potato has been a minor crop. and production has been essentially limited to the
Central Volcanic Cordillera. including parts of the states of Mexico. Michoacan.
Puebla. and Veracruz (Ugent 1967). The traditional growing areas are typically at
elevations above 2900 m. well above the upper elevation limit of the CPB in Mexico.

The CPB is distributed across much of Mexico. in diverse habitats ranging from
costal lowlands in the Rio Grande Valley. to the arid northwestern plateau. to the
relatively temperate Valley of Mexico (Tower 1906. pers. obs.). Several surveys have
demonstrated that the CPB occurs in abundance only at elevations below 2200 meters.
Drummond (pers. comm.) has recorded CPB incidence along highway transects
approaching potato production areas in the Toluca Valley and Puebla. and found that
CPB populations were absent above 2000 m. despite an abundance of S. restratum.
Cappaert (chapter I) and Anaya (1988) found the same result in surveys of the states of
Mexico and Morelos. In the U.S.. Riley (1877) also recorded an elevation barrier to CPB
dispersal in the Rocky Mountains of Colorado.

The factors that limit the high elevation distribution of the CPB are not clearly
understood. but cool temperatures that inhibit larval development are likely involved.
Daily heat accumulations in Toluca (2638 meters) in the warmest month of May are
6.09 C degree days. base 10" (1975-1985 average. Estacion Toluca. Instituto Nacional de
Investigaciones Agricolas). Based on CPB developmental rates determined by

Walgenbach and Wyman (1984). one CPB life cycle would require 67 days. This is 2.7

81

times the developmental time required in July at Zacatepec. Morelos. a location
supporting abundant CPB pepulations (developmental time = 25 days. based on mean
temperature of 269C. Institute Nacional de Investigaciones Agricolas). Estimated
developmental times for the CPB in July are also relatively rapid at U.S. locations on
the northern edge of the CPB's distribution (developmental time at Midland. Michigan =
23 days: Toronto. Canada = 34 days. based on U.S. Dept. of Commerce records). Retarded
growth rates at high elevations would likely reduce the overall vigor of larvae and
increase exposure to predation and disease. Furthermore. stress during development is
probably increased where low daily temperature maxima fall well short of heat
required for optimal CPB growth--Ferro (1985). and Grafius (pers. comm.) suggest that
optimal larval development occurs at 25-30’ C. These temperatures are rarely attained
in the temperate conditions of traditional Mexican potato growing areas.

In August of 1987 I investigated survival of egg cohorts placed on S. rostratum at
several sites in the vicinity of Toluca. Egg masses were fixed with an insect pin to each
of 75 plants at three sites. Of 2690 total eggs. 15 fourth instar larvae were recovered. and
the highest survival to fourth instar in any site was 1.2% (Table 4.1). High mortality
was most likely attributable to weather conditions. as natural enemies were never
observed. and the host plants have been shown to be suitable as larval food (Cappaert.
unpublished data: Casagrande. unpublished data). Development time to the fourth
instar was ca. 28 days (Table 4.1). roughly as predicted from average temperatures for
the area. The occasional occurrence of CPB specimens collected in the Toluca
Valley (Coleccion Entomologica del Instituto Nacional de Investigaciones Agricolas:
Tower 1906) demonstrate that the CPB is not excluded by geographic barriers. and lend
support to the hypothesis that high elevation conditions do not allow for adequate
larval development.

Both experimental data and the historical record suggest that the overlap of the

CPB range with areas of potato cultivation has been limited in Mexico. However. since

82

World War II. potato production has expanded. both within the traditional growing
areas. and to new sites at lower elevations. This expansion has been motivated by a
vigorous national potato program (Niederhauser and Villarreal 1986). and by
increasing recognition of the food value and nutrient conversion efficiency of the
potato (Swaminathan and Sawyer 1982). The potato harvest in Mexico increased 128%
over the period 1965 to 1980 (1981 FAO Production Yearbook). and current annual yield
is over one million metric tons. produced on 80.000 ha (M. Villarreal. pers. comm.).
Aside from the overall expansion in area devoted to potato production. several
associated factors increase the probability of a CPB host shift to potato. A large part of
new hectarage devoted to potatoes is in warmer regions of Mexico that are characterized
by endemic CPB populations. Only 47% of 1978 production was in the traditional
growing areas of central Mexico. while 42% of production was recorded for the northern
states of Sinaloa. Chihuahua. Nuevo Leon. Guanajuato. and Coahuila (Ferrorri 1981).
CPB populations in or near potato fields have been observed in Guanajuato (A. Marin.
pers comm.). Chihuahua (C. Garcia. pers. comm.). and Coahuila (D. Cappaert. pers. obs.).
The widespread distribution of cultivated potatoes not only increases the probability of
colonization by dispersing beetles. but also exposes potatoes to many genetically
distinct subpopulations of L. decemlineata. Hsiao (1985). in an allozyme study.
demonstrated that a population of the CPB collected in Morelos was genetically distinct
from 11 populations in the U.S.. Canada. and Europe. Further study of the CPB in
Mexico. where the beetles have colonized diverse habitats on genetically differentiated
host plants over evolutionary time. is likely to reveal equally great differences in host

utilization characteristics.

Effect of Potato Production Techniques on Colonization by the CPB. Potato
production trends in Mexico. particularly in the recently developed northern zone

involve several features likely to facilitate a CPB host shift:

83

1) 85 % of current cultivation is of S. tuberosum subspecies tuberosum ("papa
blanca") varieties (principally alpha). in contrast to the traditional varieties of subsp.
andigena ("criolla"). The andigena varieties investigated to date have not been shown
to have exceptional resistance to the CPB (Schalk et al. 1975). However. different
potato varieties have been shown to vary in CPB resistance (Moreau 1971 1980). and the
presence of new potato cultivars increases the possibility of genes that may enhance
susceptibility to CPB attack.

2) A related factor concerns the genetic uniformity of the commercially produced
potatoes accounting for most of the production increases. particularly in the northern
production zone (Ferroni 198 1). In traditional potato cultivation. described by Ugent
(1968) for the Nevado de Toluca and Pico de Orizaba areas, farmers maintain a complex
mixture of andiggna cultivars-- 17 cultivars are known from these areas. N on-edible
wild potato species including 8. edinense and S. demissum Lindl. are also present.
within and adjacent to potato fields. Wild plants contribute both to the diversity
present in the individual field and ultimately. through hybridization. to the genetic
diversity of S. tuberosum germplasm. Ugent (1968) hypothesizes that multiple-variety
plantings contain varying degrees and mechanisms of resistance to disease. and are
thus an ef'fective means of limiting the spread of disease. particularly late blight.
Diverse plantings may also vary in susceptibility to the CPB. and may reduce the
potential of serious infestations on potato plantings. Planting diversity may also
inhibit CPB infestation through the effects of natural enemies or resource
concentration. as described by Root (1973). Horton and Capinera (1987) found that CPB
in intercropped potato plots reached lower density. and had lower fecundity than
beetles in monocultures. It is interesting to note that S. demissum. a common associate
of traditional potato plots. has been studied for its high glykealkaloid content and
associated CPB resistance (Tingey 1984). By contrast. commercial plantings are

typically monocultures of one cultivar. The exclusively vegetative propagation of

84

potato seed stock insures that such fields provide a genetically uniform. consistent
resource to a CPB population that has adapted to exploit it.

3) The extensive (and increasing) use of irrigation associated with cultivation of
S. tuberosum varieties creates distinct selective advantages for CPB colonists. In
drought conditions that kill native hosts prematurely. an irrigated potato crop would
likely be "forcibly" colonized by adults and larvae displaced from defoliated hosts.
Even in normal years. the brief wet season limits multiple CPB generations. By
shifting to the irrigated host. CPB colonists would be able to increase their fitness by
completing additional generations. and by avoiding natural enemies phenologically

synchronized with native hosts.

WW

Given that the CPB is now broadly sympatric with the potato in Mexico. what is
the potential for founder populations of the CPB to actively colonize the new host?

Results of host colonization experiments are summarized below.

UAEM Best Choice Experiments. In both the isolated and mixed plant groupings.
adult CPB were encountered five times more often on the native host S. angustifolium
than on the potato (Table 4.2: p=0.04 and 0.02. respectively). However. in both
experiments. beetles were observed on potato and minor feeding damage was evident.

Oviposition was more frequent on S. angustifolium in the isolated plant
experiment (p=0.08). and the higher level was proportional to the number of adults
recorded on the two hosts (i.e.. ca. five times more frequent on S. angustifolium )
Oviposition on the cage screening and substrate occurred frequently. ca. 30 % of

oviposition on S. angustifolium In the mixed plant experiment. oviposition on potato

85

was more frequent than in the isolated plant experiment. and higher oviposition on S.
angustifolium was not demonstrable (p=0.67).

These results demonstrate that. although S. angustifolium probably receives
more CPB eggs than potato. in proportion to adult preference. the potato is accepted as
an ovipositional substrate. The apparently higher frequency with which eggs are laid
on the mixed plant group suggests that host discrimination occurs. in part. prior to
alighting on the host plant. In the isolated plant experiment. beetles may have rejected
the potato host based on olfactory or visual cues perceived at a distance. while in the
mixed plant experiment. beetles alighting on the preferred host (or responding to
olfactory cues generated by the two-species assemblage) may have subsequently arrived
on the potato by random non-discriminatory movements. CPB do in fact often
"misplace" egg masses on surfaces adjacent to their intended host. as recorded in the
isolated plant experiment.

Third and fourth instar larvae. counted in the mixed plant experiment (Table 2).
exhibited no demonstrable host preference. and were clearly observed feeding on both
hosts. Because these larvae had invariably emigrated from adjacent S. angustifolium
plants. this result suggests that effects of larval conditioning to the host species (if any)
are not strong enough to prevent colonization of potato by larvae reared on S.
angustifolium. There may. however, be physiological costs for larvae moving between
hosts. Hsiao (1978) found that conditioning of larvae on one host may diminish the

reproductive performance of adults raised on a second host.

Field colonisation study. CPB colonization of S. tuberosum and S. angustifolium
in small plots is compared in Fig. 4.1. The 16 adult beetles initially introduced to S.
angustifolium dispersed but were replaced by immigrants that reached populations of
16 and 20 (ca. 1 beetle/plant) after 30 days (Fig. 4.19). Egg mass counts on the S.

angustifolium plots peaked at 20 and 28 on the two plots (Fig. 4.1b). On the paired S.

86

tuberosum plots. one CPB adult and two egg masses were observed on Plot 1 at the first
observation. Both S. tuberosum plots remained free of beetles and egg masses for the
next 30 days (Fig. 4.1).

Colonization experiments demonstrate that under field conditions S.
angustifolium is colonized by Mexican CPB adults more frequently than S. tuberosum .
This host plant preference was particularly strong in the Xoxocotla field experiment.
The low incidence of CPB adults on potatoes may be attributable to decreased
immigration. or increased emigration rates. The attraction of CPB adults to host
plants is mediated by a balance of green leaf volatiles common to several solanaceous
species (Visser and Ave 1978). Visser and Nielsen (1977) showed that even unsuitable
solanaceous hosts such as Nicetiana tabacum and Petunia hybrida induced positive
odor-conditioned anemotaxis in adult CPB. Thus it seems likely that Mexican CPB
would be attracted to potato plants. Rapid emigration from potato patches by Mexican
beetles may reflect a negative response to the array of chemosensery stimuli that
mediate post-contact host acceptance (Hsiao 1969; Hsiao and Fraenkel 1968: May and
Ahmad 1983). Nevertheless. although a preference for S. angustifolium foliage is
apparent. it is also clear that adults do not strictly avoid potato foliage. and Mexican
beetles will colonize potato plants. especially in close associations of S. angustifolium
and S. tuberosum. as in the UAEM experiments. Furthermore. once beetles are present
in potatoes. oviposition is likely. In each of the three colonization experiments
reported here. the ratio of ovipositions to adult beetles present is very similiar in
potatoes and S. angustifoliwnute. there is no evidence for a barrier to oviposition in
potato. These experiments demonstrate that potato plants in mixed plantings would

likely receive significant exposure to all CPB life stages.

87

 

Given that the CPB lays eggs on potatoes. why have pest populations not become
established in Mexico? Preliminary evidence suggests that the potato is a suboptimal
host for growth and development of the Mexican CPB. Morelos experiments comparing
larval growth and survival on potatoes to survival on S. angustifoliwn are summarized

in Figs. 4.2 - 4.5.

UAEM survival experiments. Nelve days after introducing 30 first instar CPB
larvae to potato plants. mortality (and/or emigration) had eliminated all cohorts.
while mean survival (and/or immigration) on S. angustifolium was 61% (18/30; Fig.
4.2). A second experiment was conducted to determine whether apparent larval
mortality was a result of failure to develop on the potato host (physiological
mechanism). or of non-acceptance of the potato and consequent emigration from the
plants (behavioral mechanism). This experiment revealed that part of the apparent
mortality on potato probably resulted from emigration. A total of five larvae (of 150
total in the three potato replicates) had appeared on the S. angustifolium trap plant by
the fourteenth day; others had likely left the potato plants and died (Fig 4.3). For the
first experiment. development rates observed on potato and S. angustifolium are
compared in Fig. 4.4. Mean developmental stage attained is lower for potato at each
observation date. This slow rate of development of larvae that remain on potato plants
suggests limited feeding by larvae. In the UAEM experiments. survival of CPB on potato

was extremely low: only one of 240 first instar larvae survived to fourth instar.

Field survival study. Fourth instar survival of CPB on S. tuberosum and S.

angustifolium was not significantly different in the field survival study (i=-

88

0.81.p=0.43; unpaired t-test). As in the UAEM survival experiment. there was a geater
rate of mortality on potato during the first several days (Fig. 4.5). and this difi'erence in
survival was significant (t=-2.615.p=0.02: unpaired t-test). Survival from fourth instar
to adult was similiar on the two hosts. 43% on potato (21/49). and 57% on S.
angustifolium (6/14). Growth rate of larvae was retarded slightly on the potato at each
observation (Fig. 4.6). Behavioral observations of larvae foraging on the two hosts
revealed significant differences. The proportion of larvae feeding was compared to the
proportion not feeding (short travel. long travel. and quiescent categories were
combined into a nonfeeding class to avoid classes <5 observations) in a 2 x 2
contingency table. A larger proportion of larvae on potato were classed as feeding. both
at 3 “2:54. 1:002) and 6 days “2:325. p=0.0001) after being placed on the hosts. This
result is interesting in light of the trend toward lower growth rate among larvae on
potato (Fig. 4.6). despite their apparently greater proportion of time spent feeding. This
result suggests that supression of larval performance on the potato (if this effect is
significant) is not a consequence of feeding deterrence. but is more likely a result of
poor assimilation or toxic efi'ects.

In the field survival study. larval survival on S. angustifolium decreased (from
61 to 21%). and performance on potato increased (from <1 to l 1%) relative to the UAEM
survival experiments. These differences may be attributable to differences in plants.
ambient conditions. and/or CPB used. Plant material in the two experiments came
from similiar sources: potatoes were c.v. Alpha. and S. angustifolium plants were
obtained from Xoxocotla. Nevertheless. plant quality may have been influenced by
differences in growing conditions. Genetic differences in host afilnity of larvae used in
the experiment may also be important. as larvae used in the field experiments were
collected in Cuernavaca. while the source of larvae in the UAEM experiments was
Xoxocotla. Difi‘erences in host performance of difi'erent CPB races has been extensively

documentedbstiao (1978. 1981. 1982. 1985).

89

Previous studies investigating performance of mexican races of the CPB on the
potato have yielded similiarly contradictory results. Logan (unpublished data) found
first instar to adult suvival of CPB collected in Xoxocotla Morelos to vary from 5.6 to
38% in different experiments. Hsiao (1982) reported no difference in the rearing success
of mexican CPB (collected from Yautepec. Morelos) raised on S. rostratum or S.
tuberosum : however. the beetles used in the experiment were the progeny of mexican
CPB that had experienced several generations of selection on potato in the laboratory
(T. Hsiao. pers. comm.). Arizona CPB. collected from their native host S.
elaeagnifolium . have also exhibited increased survival on the potato following
laboratory selection. First generation mortality. resulting from non-acceptance of
potato foliage by young larvae. exceeded 80%. However. afier five generations of
selection on potato. mortality had decreased to <20% (B. Bishop. pers. comm.).

Collectively. the evidence suggests that Mexican CPB can survive on potato. that

survival varies with growing conditions. and that survival increases markedly under

selective pressure.

 

The case presented thus far suggests that the CPB has the potential to adopt the

potato as a host in Mexico. Why has a pest outbtreak not yet been reported?

Genetic factors. It is possible is that the Mexican CPB is in fact a distinct
species. or biotype that lacks the genetic capacity to adapt to S. tuberosum Mexican
races of L. decemlineata were initially classified into six different species based on
morphological characters and coloration (Tower 1906). Features that seem to separate

all mexican CPB from the pest biotypes include the pale yellow coloration of larvae. the

90

dark venter of adults. a heightened propensity to flight. and of course. host affinity
(pers. obs.). Nevertheless. current evidence indicates that the Mexican CPB is
conspecific with the pest biotype. Jacques (1972) unified the species complex described
by Tower into the single species L. decemlineata. based on careful analysis of genital
morpholog. Hsiao (1982). and Tower himself (1906) ofi'er more definitive evidence of
the relationship between Mexican and pest biotypes of the CPB: Mexican beetles
collected from several locations are completely inter-fertile in crosses with beetles
collected from potato in the U.S..

Finally. although the Mexican population of the CPB may represent a distinct.
host associated biotype (Bush and Diehl 1984). it is probably not a true host race
(Jaenike 1981). The high capacity of the CPB for dispersal from temporary habitats
militates against the theory that mexican biotypes are genetically isolated from pest
populations. and therefore lack the genes that would allow a host shift. That there is
gene flow between Mexican and pest beetle biotypes is borne out in a cytogenetic study
by Hsiao (1985). This study revealed a stably inherited pericentric inversion on the
second chromosome of the CPB. Combining analysis of the distribution of the three
chromosomal races of the CPB (the original metacentric. the acrocentric mutation. and
the heterozygote) with the historical record. Hsiao theorizes that the pest populations of
the CPB evolved from the metacentric race in Texas. The acrocentric race is restricted
to the non-pest biotypes of the CPB in Mexico and the U.S. southwest. However. the
existence of heterozygotes across the U.S. Great Plains and into the northeast indicates

gene flow between pest and non-pest populations of the CPB.

Pesticide use practices. It has already been argued that adaptation of the
Mexican CPB to the potato is most likely to occur in commercial fields. particularly in
the irrigated northern growing areas. At the same time. the use of pesticides in these

same fields may have delayed the appearance of pest populations. Delay of the potato

91

crop harvest. practiced in Nuevo Leon and Coahuila. allows farmers to take advantage
of late season price increases but necessitates the use of systemic carbamates (e.g..
Temik. Furadan) for nematode control (Oswaldo Garcia. pers. comm.). Carbamates and
pyrethroid insecticides are also used for control of tuber moths (Terymea ). aphids. and
leafhoppers in virtually 100% of fields (M. Villarreal. pers. comm.). Pesticide use
would eliminate immigrant CPB and forestall the chance encounter of environmental
conditions and host adaptive genes that might create a pest population. Within this
scenario. the appearance of pesticide resistant genes. through selection of resident
beetles. or through their introduction from pest populations. might allow for the rapid

development of pest biotype in Mexico.

Conclusions

The conditions that will allow development of a CPB pest population appear to
be present in Mexico. The primary factor limiting a host shift may well be time. The
initial host shift of the CPB in the U.S. occurred over a 40 year period. approximately
the same period that beetles have been exposed to the potato in Mexico. Lew densities
of the CPB have in fact been reported in commercial potato fields in the states of
Mexico. Chihuahua. and Zacatecas (A. Marin. pers. comm.; C. Garcia. pers. comm.; J.
Mena. pers. comm.). These populations may represent the colonization events that can
be expected to result in a virulent CPB biotype. given the appropriate genetic and
environmental circumstances.

Several strategies available to growers and agricultural workers might lower the

probability of a CPB pest outbreak in Mexico:

92

1 . Pesticide use on commercial fields should be minimized to avoid selecting for
pesticide resistant genes in natural populations. Pesticides would then remain
available to eradicate evolving pests under outbreak conditions.

2. Rotation of potato crops should be encouraged. This would greatly decrease the
selective advantage of genes for adaptation to potato.

3. Maintain diversity in the potato cropping system. Biological control in native
plants appears to be effective in limiting CPB populations (Chapter III; Logan. et
al. 1985). and CPB natural enemies are enhanced in diverse systems (Groden
1988). Traditional potato cultivation practices that incorporate several potato
varieties on small plots are probably resistant to CPB infestation. owing to
augmented natural enemy activity and relatively low plant apparency.

4. Growers and extension personnel should be aware of the pest potential of the
CPB and be prepared to control potato-adapted populations. Emergency control
measures could include crop rotation. trap cropping on the preferred native
host (S. rostratum or S. eleagnifelium). and insecticide. applied at a
sufficiently high rate to eliminate resistant genes.

5. The National Institute of Agriculture (INIFAP) should be alert to the potentially
disastrous result of accidental introduction of the CPB pest biotype from the

U.S. or Europe.

In most of its range. the CPB has proven to be one of the most damaging pests in
agriculture. Its current status is a testament both to its highly adaptable biology and to
misguided management practices that have resulted in nearly total pesticide resistance.
In Mexico. presently innocuous CPB populations appear likely to become pests in the
near future. By monitoring this process. and responding to incipient pest populations
with cultural and biological as well as chemical management. Mexico may be able to

prevent an epidemic outbreak of this pest.

93

Table 4.1. Survival of CPB egg cohorts at Toluca. Mexico.

 

 

 

1987 CPB Life Stage-Total Count
Count
SiiLanaiinn Date eggs L-l L-2 L-3 L-4
Toluca INIA stn 8/1 964 0 0 0 0
(25 egg masses) 8]? 646 0 0 0 0
8/ 1 7 0 7 0 0 0
8/ 29 0 0 0 0 0
6 Km E. Toluca 8/1 852 0 0 0 0
(25 egg masses) 8/ 7 362 22 0 0 0
8/ 17 0 0 0 0 0
8/ 29 0 0 0 0 l
20 Km N. Toluca. 8/ l 874 0 0 0 0
Hwy 55 8 l7 5 66 101 0 0 0
(25 egg masses) 8/17 0 l3 l3 0 0
8/ 29 0 0 0 3 1 l

 

94

Table 4.2. UAEM host choice experiments.

 

 

   

 

ISOLATED PLANT GROUPS:

S. tuberosum S. angustr'folr'wn Cage wall P
W Wa—
Number Adults 1.9310.6 10.111.6 --- 0.04
Egg Masses 0.55103 3.011.0 1.1 0.08
GRUPSO:

S. tuberosum S. angustifolium P
CATEGQRX Wi—
Number Adults 6313.3 36018.4 0.02
Egg Masses 3.013.0 4.712.0 0.67
Number larvae 2.011.5 4.013.6 0.48

 

a unpaired t test

95

 

 

 

 

 

 

 

:2
.J 19.5 ' —D— S.1uberosum P1011 I“
8 . —Q— S. tuberosum Plot 2 i.
< ----o--- S.angustltollum P1011 r ..... 7""
E “-5' -----e- s. angustifolium H012
o
(J:
DJ
on
2
D
2
o 10 20
DAYS FOLLOWING ADULT RELEASE

{fl
8 29.5- q
g ‘ 1. ------D S.tubarosum P1011

24-5' z. -—e— stuberosum Plot 2
(D ‘ o" ,e ------o s. angustifolium P1011
(La 19-5' -------o s. angustifolium Plot 2
E 14.5 - x. j.
0
u.
0
n:
UJ
CD
2
D
Z

 

Figure 4.1. Colonization of host choice plots by CPB,
Xoxocotla, Morelos. 1988. a) egg masses. b) adults.

96

 

 
   
 

 

 

 

 

40
a) 35 --O-- S.angustifoium
a: —I— Stuberosum
g 30
— 25-
> A 1 ~ ------------
II E 20- } }“‘~{
3 a) .
CD $115"
2 .‘
fi 10‘
0 10

5
DAYS ON PLANT

Figure 4.2. Survival of CPB larvae on two hosts. UAEM
experiments, 1987.

 

—'I— S. Tuberosum
- - '0- - trm plant

MEAN SURVIVORS
(iSEM)

 

 

 

 

 

Figure 4.3. Survival of CPB larvae on S. tuberosum and S.
angustifolium trap plant. UAEM experiments, 1987.

97'

 

(n-51)

4-0 " —‘I— S. angustifolium
- '0'- - S. tuberosum

 

 

 

WEIGHTED MEAN INSTAR

 

Figure 4.4. Development rate of the CPB on two hosts. UAEM
experiments, 1987.

 

 
  

 

 

 

 

 

(I) 20.0
a: —D— S.tuberosum
g - - o- - S. angustifolium
_ 15.0-
> A
g a
100- \
<0 ‘19., E __________
z u-.. ~~~-
a 50" ~~~~~~
z . +
00 ‘ f T ‘ T ‘ I ' ' 1 ' ' I
O 2 4 6 8 10

DAYS ON PLANT

Figure 4.5. Survival of CPB larvae on two hosts. Field study,
Xoxocotla, Morelos, 1988.

study,

WEIGHTED MEAN INSTAR

98

 

 

 

   
  
 
   

 

  

Figure 4.6.
Xoxocotla,

4 0 - ' ' 0‘ ' S. angustifolium (It-30) 4

° + S. tuberosum - - ‘0

q p - .. o ("-1 7)
1”
3.0 -
2.0 -
I

1.0 # —1r 1 . I j . f

DAYS OF DEVELOPMENT ‘°

Development rate of the CPB on two hosts.
Morelos, 1988.

 

Field

CONCLUSIONS

Recent studies of the CPB in Mexico (Logan et. al.. 1987). the first since the early
work of Tower (1906). suggested the potential importance of natural enemies as
mortality factors in native CPB populations. In the present study. life table analysis
revealed moderately to extremely high mortality of CPB eggs and larvae. The seasonal
pattern of predator incidence. predation levels indicated by eg damage. and direct
observations strongly suggest that predators are a primary cause of mortality.
Population studies in 1987 indicate that natural enemies are poorly synchronized with
CPB populations early in the season. but exhibit increasing numerical response to CPB
density late in the season. Low CPB poulation density precluded an assessment of early
season patterns in 1988.

This study analyzed CPB population dynamics in one location during two years.
Mortality due to natural enemies was clearly high but variable. Interactions of the CPB
and its predators depend on many factors not yet investigated. Most natural enemy
species encountered were observed atacking other Doryphorini hosts. which may
ultimately determine natural enemy abundance. Density dependent response of CPB
natural enemies may also be important. and it may occur at efi'ective densities above or
below densities observed in this study. Abiotic factors may be the ultimate
determinants of CPB populations in native plant stands. For example. both the pattern
of distribution of the CPB host plant. and the character of adjacent non-host habitats
may influence immigration and emigration rates of adult beetles. and thus contribute
to observed patterns of population dynamics. Further study is necessary to establish
the extent to which natural enemies regulate CPB populations in Morelos.

The study at Xoxocotla revealed a surprisingly diverse complex of CPB natural
enemies (Table 2.8). In the U.S.. two species of Pentatomidae are common CPB

99

100

predators in potatoes: at a single site in Morelos. five species were observed to prey on
the CPB. Seven species of foliar searching carabids were collected from the CPB host in
Morelos. whereas two are reported from potatoes in the U.S. No coccinellid species
known to be effective CPB predators were present. Parasitoids included the egg
parasitoid Edovum puttlert and a tachinid parasitoid of CPB adults (at Canon de Lobos
only). as well as the tachinid larval parasitoid species known from the U.S.. Generalist
predators that feed on CPB. particularly Reduviidae and Aranae were also abundant. In
summary. natural enemy mortality of the CPB on its native host appears to depend on a
diverse complex of species. This finding suggests that study of biological control of the
CPB in cultivated hosts should also recognize the role of the natural enemy complex (see
also Tamaki. 1981. and Groden, 1988). Several natural enemy species. previously
unknown as CPB predators. should be further investigated as potential biological
control agents. for import to the U.S.. or for use in Mexico should the CPB develop as a
pest there (Lebia spp. are currently being evaluated by E. Aranda. University of Rhose
Island).

Wdence presented here demonstrates that there are no clear barriers to the
emergence of a CPB pest population in Mexico. The rate of encounter between the CPB
and potato in Mexico has increased dramatically with the geographic expansion of
potato production since World War II. The CPB in Morelos has been shown to colonize.
feed on. and survive at least one generation on potato. Potato production in Mexico
should now be monitored closely for the appearance of an incipient CPB outbreak.

Several questions pertaining to the host plant relations of the CPB in Mexico.
not addressed in this study. need to be pursued. CPB in difl'erent regions of Mexico
utilize difl'erent host plants and would almost certainly differ in their behavioral and
physiological response to the potato. For example. CPB from the state of Zacatecas that
are known to infest Solanum cardiophyllum (J. Mena. pers. comm.). taxonomically

related to S. tuberosum (Correll. 1962). may be pre-adapted to the potato. Comparison

101

of the host use characteristics of Mexican CPB populations could suggest which areas

are mmt likely to experience pest problems. and provide clues to the origin of the pest
biotype.

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Ugent. D. 1967. The potato in Mexico: Geography and primitive culture. Econ. Bot.
22(2): 112-122.

Villarreal. M. Personal Communication. Programa Regional C00perativo de Papa.
INIA. Toluca. Mex.. Mexico.

Visser. J H and DA Ave. 1978. General green leaf volatiles in the orientation of the
Colorado potato beetle. Ent. exp. appl. 24: 538-549.

Walgenbach. J .F.. and J A. Wyman. 1984. Colorado potato beetle development in
relation to temperature in Wisconsin. Ann. Entomol. Soc. Am. 77: 604-609.

Whalen. MD. 1979. Speciation in Solanum . section Androceras. In W
W G. Hawkes. Ed. Academic Press. London.

Whitcomb. W.H. 1974. Natural populations of entomophagous arthropods and their
efi’ect on the agroecosystem. In Proceedings of the summer institute on biological
control of plant insects and disease. Univ. Press Miss. Jackson. Mississippi.

108

Whitcomb. W.H. and K. Bell. 1964. Predaceous insects. spiders. and mites of Arkansas
cotton fields. Florida Entomol. 62: 1- 181.

White. RE. 1983. W. HougitonMifilinCo" Boston. 368pp.

Wilcox. J .A. 1972. A review of the North American Chrysomeline leaf beetles
(Coleoptera: Chrysomelidae). Bulletin #42 1. New York State Museum. Albany New
York.

Wright. E.J.. and J .E. Laing. 1978. The efi'ects of temperature on development . adult
longevity and fecundity of Coleomegilla maculata and its parasite. Perilitus
coccinellae. Proc. Entomol. Soc. Ont. 109: 33-47.

APPENDICES

109
APPENDIX 1

Distribution of chrysomeline beetles and host plants in Morelos,
Mexico. Census at 45 sites, 1987-1988.

 

June 1987

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1900 C,I.B,P,W-- -- -- -- -- -- .. -- -- -- -- --
1700 C.B,F -- -- -- -- -- -- -- -- -- -- -- --
1600 QB -- -- -- -— -- -- -- -- -- -- -- --
1400 B,W -- -- -- -- -- -- -- -- -- -- -- --
1200 B.W.F + -- + -- -- +4- + -- -- -- -- --
1100 C,I.B.W + + + + -- -- +
1100 B -- + -- -- -- -- +
1100 QB -- + -- -- -- + --
1000 B,F + -- -- -- -- -- .. .. -- -- -- --
950 C,I.B,P,F ++++ -- -- -- 4+ -- + -- -- -- --
900
900

1000 + -- -- -- -- -- -- -- -- -- ... --
1000 -- -- -- -- -- -- -- -- -- --
1000 -- -- -- -- -- -- -- -- -- -- -- --

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1050
1000
1000
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950
1000

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23 1100 -- -- —- ..- -- -- -- -- -- -- -- --
24 1100 I,B,P,F -- + -- -- Sn -- -- + + + -- --
25 1500 B H + + + -- ++ -- -- -- -- ... --
26 1500 B -- + -- -- -- -- -- + -- -- -- --
27 2000 B,W -- -- -- -- -- -- .. -- -- -- -- --
28 2400 P,W -- -- -- -- -- -- -- -- -- -- -- --
29 2500 C.B.W -- -- -- -- -- -- -- -- -- -- -- --
30 2600 QB -- -- -- -- -- -- -- -- -- ... -- --
31 2700 CB -- -- -- -- -- .. -- -- -- -- -- --
32 2750 -- -- -- -- St -- -- -- -- -- -- --
33 2700 ,P -- -- -- -- St -- .. -- -- -- -- --
34 2500 -- -- -- -- -- -- -- -- -- -- -- --

35 2400
36 2150
37 2000
3 8 2000
39 1900

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1900
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1700
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1200
1100
1100
1100
1000

950

900

900
1000
1000
1000
1050
1000
1000
1000

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1000
1000
1100
1100
1500
1500
2000
2400
2500
2600
2700
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41 1900

B -- -- -- -- -- -- -- -- -- -- -- --
42 1800 C -- -- -- -- -- -- -- -- -- -- -- --
43 1700 s -- -- -- -- -- -- -- -- -- -- -- --
44 1700 as + -- -- -- -- -- -- -- -- -- -- --
45 1800 B -- -- + -- -- -— -- -- -- -- -- --

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2 1700 C,B,F -H- -- -- -- -- «H- -- -- .. +4. -- --
3 1600 GB + -- -- -- -- -- -- -- -- -- -- --
4 1400 B,W -- -- -- -- -- -- -- -- -- -- -- --
5 1200 B.W.F ++ + + -- -- ++ + -- -- + -- --
6 1100 C,I.B.W +1- + + + -- ++ + -- -- -- + --
7 1100 B -H- -- -- -- -- + -- .. -- -- -- --
8 1100 QB ++ + -- + -- -H- -- + -- -- + ..
9 1000 B,F H -- -- + -- -- -- -— -- -- + --
10 950 C,I.B,P,F -H- + -- -- -- -H- -- -- + + + Ldil
11 900 ----------------------------- NO DATA -----------------------------
12 900 ----------------------------- NO DATA -----------------------------
13 1000 CB ++ + -- + -- + -- -- -- + + --
14 1000 CB ++ + -- + -- + -- -- -- -- ... --
15 1000 CB +1- -- -- -- -- ++ -- -- -- -- -- --
16 1050 CB +4 + -- -- -- + .. .. -- -- -- --
17 1000 B + -- -- -- -- -H- .. -- -- -- -- --
18 1000 B + --- -- -- .. -- -- -- -- -- -- --
19 1000 B,F -- -- -- -- -- -- -- -- -- -- -- --
20 950 C,I.B,P,F -H- + -- -- -- ++ -- -- -- -- -- --
21 1000 B -H- -- -- -- -- + ..- -- .. .. .. --
22 1000 C.I,F ++ -- -- + -- ++ -- -- + + + --
23 1100 B -- -- -- -- -- -- -- -- -- -- -- --
24 1100 I,B.P.F -- + -- + -- -- -- -- -- + + --
25 1500 B + -- -- + -- + -- -- .. -- + --
26 1500 B -H- -- -- + -- ++ + -- -- -- -- --
27 2000 B,W -- -- -- -- -- -- .. -- -- -- -- --
28 2400 P,W -- -- -- -- -- -- -- -- -- -- -- --
29 2500 C,B,W -- -- -- -- -- -- -- -- -- -- -- --
30 2600 QB -- -- -- -- -- -- -- -- -- -- -- --
31 2700 CB -- -- -- -- .. -- -- -- -- -- -- --
32 2750 C -- -- -- -- St -- -- -- .. -- -- ..
33 2700 GP -- -- -- -- St -- -- -- -- -- -- --
34 2500 B -- -- -- -- -- -- -- -- -- -- -- --
35 2400 B -- -- -- -- -- -- -- -- -- -- -- --
36 2150 B.W -- -- -- -- -- -- -- -- -- -- -- --
37 2000 B.W -- -- -- -- -- -- -- -- -- -- -- --
38 2000 C -- -- -- -- -- -- -- -- -- -- -- --
39 1900 B -- -- -- -- -- -- -- -- -- -- -- --
40 1900 B H -- -- -- -- ++ -- .. -- .. .. ..
41 1900 B -- -- -- -- -- -- -- -- -- -- -- --
42 1800 C -- -- -- -- -- -- -- -- -- -- -- --

43

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1500
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2400
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2000
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1900
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C.B.F 4+ -- -- 4- --
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C,I.B.W 4+ -- -- + --
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B,F 4- -- -- + --
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-- -- -- -- + + --
+ -- -- -- -- -- --

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+ ---- -- + +IAH

-- -- + -- 4- + --
DATA ------------------------------
DATA ------------------------------
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DATA ..............................

43 1700 B -- -- -- + --
44 1700 CB -- -- -- + --
45 1800 B -- + 4- -- --

 

a C=corn or com/bean; I=irrigated crop: B=brusll;
F=fallow field.

9 Sa=Solanum angustifolium Mill; K=Kallstroemia
L.f.; Tt=Tithonia tubiformes (Jacq.); Sn=Solanum
L

P=pasture; W=woods;

spp ; S.m.=S. marginatum
nigrum L.; St=S. tuberosum

c Ld=Leptinotarsa dccemlineata (Say): Lu=L. undecemlineata Stal; Lt=L.
tlascalana Stal; Lh=L. haldemani (Rogers); Ldil=L. dilecta Stal;
Cspp=Calligrapha spp : Zs=Zygogramma signatipennis (Stal)

d single '+' indicates presence of S.angustifolium,
indicates plants >40cm height.

e single '+' indicates presence of L. decemlineata.,
>20 adults.

plants <40cm height. '++

< 20 adults. '++' indicates

APPENDIX 2

Weather data for Zacatepec Experiment Station.

 

   

May

1 36. 5 12. 0 10 8 0.0 13.7 0.0
2 36 5 13.0 9 2 0.0 17 38.0 16 5 12.2 0.0
3 36 5 15.0 114 0.0 18 38.5 15 0 17.2 0.0
4 36 0 12.0 9 9 0.0 19 34.0 18 0 18.3 0.0
5 38 0 13.0 11 7 0.0 20 37.0 18 0 16.6 0.0
6 38 0 16.0 13 3 0.0 21 37.5 18 5 13.5 0.0
7 38 0 16.0 13 7 0.0 22 39.0 18 0 13.5 0.0
8 36 5 18.0 14.1 0.0 23 38.0 19 5 13.7 0.0
9 36 5 19. 0 14.4 0.0 24 38.0 18. 0 13.7 0.0
10 36.0 16. 0 12.7 4.8 25 35.5 16.0 14.6 0.0
11 37. 0 17. 0 15.5 0.0 26 34. 0 17. 5 17.0 0.0
12 36. 5 16.5 18.3 0.0 27 36.0 20 0 18.3 0.0
13 38.0 16.0 14.6 0.0 28 38.0 20 0 19.3 4.3
14 38. 6 17. 0 14.1 0.0 29 36.5 21 0 19.6 18.3
15 38. 0 18. 5 14.1 0.0 30 34. 5 17.0 20.0 27.5
31 34. 5 17.0 19.5 0.0

June 1 34.5 20.0 18.8 0.0 16 32.5 19.5 20.1 22.7
2 35.0 19.0 19.9 1.8 17 31.0 20.5 20.5 0.4
3 34.0 17.5 18.7 18.3 18 32.5 18.0 19.7 0.0
4 33.0 18.0 18.0 0.0 19 33.5 19.0 19.3 0.0
5 34. 0 19. 0 19.8 2.7 20 34.0 17.0 17. 5 34.3
6 33. 0 18. 5 20. 6 4.0 21 34.0 20.0 18. 0 12.0
7 28.5 18. 5 19. 0 1.4 22 33.0 20.0 20. 2 0.8
8 29.5 17.0 19. 6 0.0 23 33.5 19.0 21.5 0.0
9 34.0 19.0 20. 4 5.5 24 34.0 21.0 21.7 0.0
10 34.0 19.5 19.1 1.1 25 33.5 20.0 19.5 0.0
1 1 32.0 19.5 20. 6 0.2 26 32.5 21.0 20.0 0.0
12 34. 0 20.0 20.6 5.8 27 32.5 19.0 20.3 6.5
13 33. 0 20.0 19. 6 7.1 28 33.0 19.0 19.6 0.0
14 35.5 20. 3 20. 0 0.0 29 34. 0 21.5 20. 5 16.1
15 34. 5 20. 0 20.1 0.3 30 35. 0 18.5 20. 6 0.5

114

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1 0.0 0.0
2 38 23 NA 0.0 17 37 15 NA 0.0
3 21 37 NA 0.0 18 38 18 NA 0.0
4 39 22 NA 0.0 19 37 16 NA 0.0
5 37 20 NA 0.0 20 38 14 NA 0.0
6 38 22 NA 0.0 21 40 16 NA 0.0
7 38 22 NA 0.0 22 40 19 NA 0.0
8 39 23 NA 0.0 23 39 20 NA 0.0
9 39 23 NA 0.0 24 37 20 NA 0.0
10 39 19 NA 0.0 25 37 23 NA 0.0
11 40 21 NA 0.0 26 37 20 NA 0.8
12 38 21 NA 0.0 27 37 19 NA 0.0
13 37 21 NA 0.0 28 35 18 NA 0.0
14 37 20 NA 0.0 29 35 19 NA 0.0
15 36 20 NA 0.0 30 35 22 NA 4.0
31 38 19 NA 0.0

June 1 38 21 NA 0.0 16 35 18 NA 0.0
2 39 20 NA 0.0 17 31 21 NA 0.5
3 38 19 NA 0.0 18 33 21 NA 23.0
4 38 20 NA 0.0 19 39 20 NA 4.0
5 38 19 NA 0.0 20 36 23 NA 0.0
6 36 16 NA 0.6 21 35 24 NA 0.0
7 38 17 NA 0.0 22 35 24 NA 0.0
8 38 18 NA 6.0 23 30 20 NA 19.5
9 36 16 NA 0.0 24 29 20 NA 46.8
10 35 18 NA 1.4 25 33 20 NA 31.0
11 36 17 NA 0.0 26 31 21 NA 0.0
12 37 21 NA 0.0 27 32 21 NA 1.1
13 35 19 NA 13.6 28 34 20 NA 0.9
14 34 20 NA 8.1 29 34 20 NA 25.6
15 36 19 NA 0.0 30 33 20 NA 48.6
July 1 -- 21 NA 0.0 16 31 20 NA 13.5
2 33 18 NA 3.2 17 32 19 NA 0.0
3 33 22 NA 0.7 18 32 21 NA 0.0
4 33 20 NA 0.0 19 31 20 NA 3.5
5 31 22 NA 0.0 20 31 20 NA 69.9
6 34 21 NA 0.4 21 32 21 NA 104
7 34 20 NA 0.0 22 32 20 NA 105
8 34 20 NA 17.7 23 32 17 NA 0.0
9 33 21 NA 17.1 24 32 18 NA 0.0
10 33 21 NA 0.0 25 33 16 NA 0.0
11 33 20 NA 5.0 26 34 18 NA 0.0
12 33 20 NA 0.0 27 34 18 NA 4.7
13 33 20 NA 4.0 28 31 17 NA 0.0
14 33 20 NA 2.0 29 32 14 NA 0.0
15 33 21 NA 7.0 30 34 20 NA 0.0
31 35 22 NA 0.0

117

 

0.0 16 32 19 NA 6.8

2 34 20 NA 1.2 17 32 19 NA 0.4
3 34 21 NA 0.6 18 31 20 NA 0.0
4 33 19 NA 8.1 19 30 19 NA 0.0
5 33 22 NA 0.9 20 34 20 NA 11.8
6 34 22 NA 0.9 21 33 21 NA 3.5
7 33 20 NA 1.0 22 33 19 NA 0.0
8 32 20 NA 0.0 23 31 20 NA 2.5
9 32 20 NA 1.9 24 32 21 NA 0.0
10 33 20 NA 16.5 25 32 21 NA 0.0
11 33 20 NA 15.9 26 33 21 NA 0.0
12 32 21 NA 21.0 27 33 20 NA 0.0
13 32 21 NA 1.0 28 34 20 NA 0.0
14 32 20 NA 0.0 29 33 19 NA 0.0
15 30 20 NA 17.1 30 35 20 NA 0.0
31 33 19 NA 0.0

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