ABSTRACT

MICROTINE DAMAGE IN MICHIGAN APPLE ORCHARDS
By

Craig Steven Killian

The object of this study was to provide an estimate of
the financial losses caused in Michigan's apple orchards by
members of the genus Microtus, and, if possible, to discover
some of the factors affecting the amount of damage. Infor-
mation was collected from apple growers and from direct ob-
servation of 1,520 Michigan apple trees. A

Data was collected concerning the winters of 1972-73
and 1973-7U. Both of these years were characterized by rel-
atively low populations of microtine rodents in Michigan.

Mice, presumably microtines, were most frequently named
as the most destructive pest. Deer and rabbits were ranked
second and third respectively. Mice appear to be most de-
structive in the southern half of the apple growing region.

Damage rates were greatest for newly planted trees and
decreased as trees aged. By the age of ten, apple trees at-
tain minimum damage rates.

Ninety—seven percent of Michigan's apple growing acreage
is treated with rodenticide or other rodent damage preventa-
tives at an annual average cost of $166,UOO. In spite of
such control measures, however, microtines are responsible
for a minimum annual loss of $322,500 from damaged trees.

Contrary to expectations, damage rates were not lowest

Craig Steven Killian

in the cleanest orchards. Elimination of undergrowth had no

significant effect on the amount of microtine damage.

MICROTINE DAMAGE IN MICHIGAN APPLE ORCHARDS

By

Craig Steven Killian

A THESIS

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

MASTER OF SCIENCE

Department of Fisheries and Wildlife

197A

ACKNOWLEDGEMENTS

I would like to express my appreciation to the person-
nel of the Apple Pest Management Program at Michigan State
University for their valuable assistance in this research
and to the Michigan State University Extension Service for
the financial aid in the preparation and publication of this
report. I would also like to acknowledge Dr. Walt Conley
for his continued assistance and support and my wife Kathi

for her many hours of typing.

ii

ACKNOWLEDGEMENTS
LIST OF TABLES.

LIST OF FIGURES

INTRODUCTION
PROCEDURES
RESULTS
DISCUSSION
SUMMARY
BIBLIOGRAPHY
APPENDICES

APPENDIX A
APPENDIX B

TABLE OF CONTENTS

iii

Page
ii

iv

16
23
28
3O

3O
37

Table

B1.

B2.

B3.

BU.

LIST OF TABLES

Response to Question 29 "What species of
mammal do you feel is the most damaging
in your orchard(s)?"

Current rodent control procedures in Mich-
igan apple orchards

Varieties of trees examined by field scouts

Contingency table data for Question 7 "Are
mice a serious problem in your orchards?"
versus region (x2 = 7.05 df = 5
0.5 > p > 0:1)

Contingency table analysis comparing sever-
ity of damage during winter of 1972-73 as
determined from the questionnaires versus
geographical area. There was no signifi-
cant difference (x2 = 1.58 df = 5
0.975 > p > 0.900). . . .

Contingency table analysis of damage rates
between orchards where grass cover occur—
red and orchards where it was eliminated.
Data was collected by Apple Pest Manage-
ment field scouts (x2 = 2.91 df = 3
0.5 > p > 0.1) . . . . . . .

iv

Page

10

37

38

39

HO

LIST OF FIGURES

Figure ' Page

1. Michigan's Lower Peninsula with major apple
producing region indicated. Southern
region (stipled) demonstrated most severe
damage rates. . . . . . . . . . . . 7

2. Relationship between annual damage rate and
cost of annual rodent control program for
each of the six age categories considered . . l2

3. Histogram indicating the relationship between
the age of a Michigan apple tree and its
chance of being attacked by Microtus sp. . . 1U

AA. Effects of various amounts of girdling on
harvest during first harvest season after
girdling . . . . . . . . . . . . . 19

AB. Effect of various amounts of girdling on pro-
duction of an average Michigan apple tree
during successive years following girdling. . l9

5. Partitioning of annual expenses which Michigan
apple growers sustain due to microtine
rodents . . . . . . . . . . . . . 26

INTRODUCTION

Many species of the genus Microtus are considered agri-

cultural pests. The European field vole, Microtus agrestis

 

has caused significant damage to pine plantations, apple or-
chards, nurseries, and field crops such as sugar beets, hay,
and oats. Kanervo and Myllymaki (1971) report that during
1954 to 1966 M. agrestis caused an estimated $A0,860,000 da—
mage in Finland alone. A

California and Nevada suffered from unusually dense pop-
ulations of microtine rodents during the early 1960's which
caused heavy damage to fruit trees, and field crops such as
alfalfa and sugar beets (Anon. 196A). This extreme damage
occurred in spite of heavy applications of rodenticide. In
some cases, fallow fields were virtually stripped of standing
vegetation under snow cover (Foster 1965).

In eastern and midwestern United States there are three
species of the genus Microtus which are of concern. These

species are the field vole, M. pennsylvanicus, the prairie

 

vole, M. ochrogaster, and the pine vole, M. pinetorum. All

 

 

three species occur in Michigan.

The pine vole is a common inhabitant of orchards in New
York, Pennsylvania, Ohio, Indiana, Virginia and the Carolinas.
In these regions it can cause serious destruction in orchards
by girdling and cutting roots and underground portions of the

stem (Davis 19A3). In spite of the fact that M. pinetorum is

 

known to feed heavily on the bark and roots of apple trees,

few estimates have been made of the economic impact of this

1

 

 

2

species due to its highly subterranean habits. Scattered re-
ports prove the occurrence of the pine vole in Michigan (Burt
1957) but nowhere in the state does it appear abundant.

The prairie vole, M. ochrogaster may be locally abundant,
but is concentrated primarily in Cass, Berrien, and Van Bur-
ren counties in the southwest corner‘of Michigan. Biologi—
cally, M. ochrogaster is very similar to the meadow vole, M.
pennsylvanicus; thus, these two species will not be differ—
entiated in this report.

The meadow vole, M. pennsylvanicus is the species of
primary importance to Michigan apple growers since it is es-
tablished throughout the state. Like the European field vole,
M. pennsylvanicus becomes extremely destructive during the
peak phase of its demographic cycle. During 1938 and 19U2 M.
pennsylvanicus destroyed approximately 50% of the Scotch and
Douglas Pine seedlings planted in a reforestation project in
New York (Littlefield, Schoonmaker, and Cook 19u6).

While reports of heavy damage are common (Littlefield,
Schoonmaker, and Cook l9A6, Hayne 1950, Biser 1967) few at-
tempts have been made to assess the economic damage caused by
M. pennsylvanicus or similar species.

In this study, I have estimated the economic impact of
the field vole, M. pennsylvanicus upon Michigan's apple grow-
ing industry. The extreme fluctuations in densities of vole
populations create difficulties in estimating and controlling
their impact. Application rates of appropriate pesticides

and other associated control procedures, such as mowing, clean

3
cultivation and use of herbicides, which may be efficient at

low densities may be ineffective when dealing with population
densities of 150 or more animals per acre which may be attain-
.ed during peak years (Krebs 1973). Conversely, application
rates and procedures which are warranted during peak years may
represent wasted insurance during periods of low densities.

In an industry with a small profit margin, savings of several

dollars per acre would be a significant benefit to the grower.
PROCEDURES

Data for this research were gathered by three different
methods. A questionnaire was sent to 118 apple growers in
the state of Michigan. Growers were identified through the
Apple Pest Management Program at Michigan State University.
The questionnaire requested information on the size of the
orchard, varieties and ages of trees, current and past mam-
mal control procedures, costs and effectiveness of control
procedures, estimated annual loss due to small mammals, esti-
mated value of trees of various ages, past experiences, and
related topics. A sample questionnaire is included in Appen-
dix A.

‘vFollowing questionnaire returns, personal interviews
were conducted with 14 orchard owners. The purpOse of these
interviews was to gain insight into topics which received
poor response on the returned questionnaires, and to give or-
chard owners a better chance to express their opinions and

experiences.

A
In the spring of 197A a field survey was conducted with

the cooperation of the Apple Pest Management Program person-
nel. Field scouts were instructed in the appearance of damage
inflicted by microtines. Each scout was then provided with a
data sheet for each 10 acre orchard block which was to be in-
spected. In each block the scouts selected 10 trees at ran-
dom. At each of the 10 trees scouts recorded the variety of
tree, percent of the circumference which exhibited girdling
by Microtus s23, diameter of the tree at ground level, dis-
tance from the tree to the nearest visible runway, and the
depth of the litter layer. Scouts also indicated whether or
notvegetative manipulations had been practiced under the
trees or between rows of trees. The vegetative manipulations
which were considered included the use of herbicides, mow-
ing, and clean cultivation. In addition, scouts indicated
whether or not there was any corn at the base of the tree
(corn is the standard vector of zinc phosphide, currently the
_ major rodenticide used in Michigan orchards), location of the
orchard block and trapping success from a related program. A

»sample data form is shown in Appendix A.
RESULTS

Forty—seven of the 118 questionnaires were returned for
analysis. Mice (presumably Microtus sp.) were most often
named as the most destructive pest in Michigan orchards (Table
.1). Deer ranked a close second while rabbits were third. In
many cases growers indicated more than one species for ques-

tion 29. In these cases the species were considered equally

TABLE 1. Response to Question 29 "What species of mammal

do you feel is the most damaging in your or-
chard(s)?"

 

 

 

No. of times % of

Pest Recorded Responses

Mice 28 “5.9%
Deer 21 ‘ 3u.u
Rabbits 6 7 9.8
Woodchuck 3 4.9
Raccoons 1 1.6
Subterranean rodents 1 1.6
Birds 1 ' 1 6

Total 61 99.8

6

destructive and a single reSponse was accorded to each.

‘Deer appeared most often as the most destructive mammal
in the more northern reaches of Michigan's fruit growing re-
gion, while only a few growers from the southern region indi-
cated that deer were the most destructive mammal. In the
southern regions, mice were the most destructive mammal as
indicated by grower responses.

References to subterranean rodents may indicate the ac-

tivity of M. pinetorum or of ground squirrels (Spermophilus

 

tridecemlineatus). Several growers indicated a belief that
pine voles were present in their orchards. However, of sev-
eral hundred mice trapped by field scouts of the Apple Pest
Management Program in the last two years, there have been no
specimens of M. pinetorum. I conclude, therefore, that the
pine vole is not causing substantial damage in Michigan or-
chards. I

The majority of Michigan orchard owners feel that mice
are a significant pest in their apple orchards. There is,
however, a significant difference in the opinions of the or—
chard owners when the state is divided into northern and
southern regions (Figure 1). In the northern portion of the
fruit growing region 28.6% of respondents felt that mice cur-
rently were a serious pest in their orchards and an addition-
al 21.A% qualified their response with such phrases as "at
times" or "moderately",etc. In the southern portion of the
state 66.7% of the respondents indicated they considered
mice to be a serious peSt and an additional 3.3% qualified

their response. The difference is statistically significant

FIGURE 1. Michigan's Lower Peninsula with major apple pro-
ducing region indicated. Southern region (stipled)
demonstrated most severe damage rates.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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8
(x2 = 7.05 df = 5 0.05 > p > 0.01).

Data collected by field scouts of the Apple Pest Man-
agement Program supports this trend. In southern counties,
1.5% of 960 trees examined exhibited damage by Microtus s2,
while only 0.9% of 560 trees in the northern region had been
damaged. The difference, however, is not statistically sig-
nificant (x2 = 1.22 df = 3 0.9 > p > 0.5) and I have there—
fore used the state-wide mean of 1.3% in my analyses.

The increased concern with mice among orchard growers
in the southern half of the state is also reflected in the
cost of control programs. The median per acre cost of vole
control programs in the southern half of the state is $3.00.
In the northern regions the median cost is $1.75 per acre.
Because of the larger number of responses from the southern
region $3.00 per acre is also the state median. Median val-
ues are utilized rather than mean values because the res-
ponSes were skewed strongly towards higher costs. These fig-
ures, however, are generally conservative since in many cases
interviews disclosed the fact that growers indicated only the
cost of the poisoned grain. Other growers included operating
costs of vehicles and wages. Many who treated their own or-
chards did not include their own wages.

Differences in the responses received from northern and
southern regions do not necessarily indicate inherent differ-
ences between the regions. They may simply indicate that
during recent years vole populations were at different stages
of their demographic cycle in the two regions. To test this

possibility growers were asked to rate damage during the

9
~winter of 1972-73 as being light, average, or severe. How—
ever, there was no significant difference between the two re-
gions (x2 = 5.7M df = 5 0.5 > p > 0.1). Overall, 76.u% of
the growers rated the winter of 1972—73 as a year of light
damage; only 5.3% rated it as a year of heavy damage. The
remaining 18.3% rated the year as one of average damage.
Table 2 summarizes the various control procedures re-
ported on the questionnaires. When the number of orchard
owners responding is weighted by the amount of acreage each
has devoted to apple orchards approximately 97.6% of Michi—
gan's 56,600 acres of bearing age apple trees are treated
with rodenticide at an average annual cost of $166,A00.
This figure is conservative for a number of reasons. First,
the 56,600 acres of trees represents only those trees which
are commercially productive. Another contributing factor is
the rising cost of gasoline, labor, and poisoned grain. Only
two growers reported that they practice no small mammal con-
trol in their orchards. In addition to the application of
zinc phosphide treated grain, the majority of Michigan apple
growers eliminate or reduce ground vegetation in their or-
chards through clean cultivation, mowing and/or the use of;
herbicides. While this practice is usually aimed at reducing
competition for water between the trees and the ground cover,
most growers indicated that they believed part of this cost
should be assessed to their rodent control program. This a-
gain raises the total cost of rodent control within the state.
However, no attempt was made to determine what portion of

this cost would be applicable to rodent control.

10

TABLE 2. Current rodent control procedures in Michigan
apple orchards.

 

 

 

 

. No. of % of
Control Procedure Responses Responses

Zinc phosphide on corn 12 25.0%
Bait (unspecified) . 29 60.A
Bait and guards 2 A.2
Guards 1 2.1
Unspecified 2 A.2
None 2 A.2

 

 

Total A8 100.1

11

When questioned about the effectiveness of their entire
rodent control program 75.6% indicated that they felt it was
effective or better. Only 2A.A% of the growers who respond-
ed indicated that they felt it was less than effective. In
spite of the differences in costs between the northern and
southern portions of the fruit growing regions there was no
significant difference in the growers' opinion of control ef-
ficacy. Six orchard owners who returned questionnaires did
not express an opinion on this subject.

Comparisons were made between amount of money spent on
rodent control programs and the reported damage rate in the
associated orchards. Histograms of damage rates versus con-
trol costs for each age group exhibit an initial segment of
low damage rates with low control costs followed by peak dam-
age rates at intermediate expenditures. Further increases in
control costs provide reduced damage rates (Figure 2). It
appears that those growers who believe they have low damage
rates maintain relatively low key, inexpensive rodent control
programs. Conversely, those growers who spend large amounts
of money on rodent control have relatively low average annual
damage rates. Peak damage rates for trees in each age cate-
gory occurred at intermediate control costs. Even at an es-
timated annual expenditure of $50.00 per acre for rodent con-
trol programs, the estimated annual damage rate is as much as
3% of newly planted trees and 1% for trees 6 to 10 years old.

With only one exception growers indicated that the annual
damage rate decreases with increasing age of the tree. How-

ever, most growers indicated damage rates only for the age

FIGURE 2. Relationship between annual damage rate and cost
of annual rodent control program for each of the
six age categories considered.

N

Moan Annual Damage Rate (9’s)
[0 b 0’ Q

090

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Cost of Annual Rodenl Control Program (3)

13
classes presently in their orchards. Estimates of damage

rates for the first five age classes indicate that damage
rates decrease as trees grow older (Figure 3). The only ex-
ception to this tendency occurs among trees greater than twen-
ty years of age. The sixth age class (greater than 20 years)
had a mean damage rate which was slightly higher (but not
statistically significant) than the two preceding age class-
es. Beyond approximately ten years of age, apple trees are
relatively free from microtine attack. Newly planted trees

in the whip stage are approximately three times as likely to
be damaged as trees ten years old or older.

The damage rate of 1.0% to 1.5% for mature trees as de-
termined from the questionnaires was compared to the infor-
mation received from the Apple Pest Management field scouts.
This latter source indicated a damage rate of 1.3% (£0.01)
for individual 10 acre orchard blocks. A total of 1,520
trees were examined and recorded.

It is interesting to note that all damaged trees were re-
stricted to 12 growers. One grower who had 5 ten acre blocks
inspected by the Apple Pest Management field scouts had 1A%
of inspected trees damaged by Microtus gp. during the pre-
vious winter. Growers with multiple blocks tended to have
similar damage levels in all blocks. Of 38 growers with more
than one block only two exhibited damage in any block while
36 had no visible damage in any block. One grower had one
tree damaged in A of 5 blocks.

Mowing, herbicides and clean cultivation have long been

considered effective means of reducing microtine populations

\
g
/’/ .
7
FIGURE 3. Histogram indicating the relationship between the

age of a Michigan apple tree and its chance of be-
ing attacked by Microtus gp.

 

 

 

 

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75 2am mooEoo .0234 3834

 

Age of Tree (years)

15
through the destruction of suitable habitat (Elton 1951, Holm,

Gilbert and Haltvick 1959). These procedures appear to be of
dubious value in reducing damage to apple trees. While re-
ducing or eliminating ground cover under the trees signifi—
cantly reduced the number of trees which had visible runways
within their driplines there was no decrease in the number of
trees which were damaged. Trees which received no vegetative
manipulations around their base (192 total) showed no evi—
dence of damage while of the-1,308 trees examined which had
been treated to reduce grasses and herbs, 20 received some
amount of visible damage.

Ihile there was no significant difference between these
two categories (x2 = 2.91 df = 3 0.5 > p > 0.1) these re-
sults are contrary to my a priori expectations. It was felt
that orchards in which ground vegetation had been reduced
would exhibit lesser damage rates (Littlefield, Schoonmaker,
and Cook 19A6). Many growers also felt that elimination of
ground cover was an important aspect of their rodent control
'program. The lack of a significant difference, however, may
prove to be of considerable import to growers as the elimi-
nation of vegetative control would represent a savings of
several dollars per acre.

Mice do not seem to prefer any particular variety of
apple tree despite growers' indications of a slight prefer-
ence for the red delicious variety. Contingency table analy-
sis indicated no significant difference in damage levels a-
mong the varieties examined.

Question 18 on the questionnaire concerned growers'

16

estimates of their expenses and losses incurred because of
small mammal damage, including not only the cost of rodent
control programs, but also damage in the form of girdled or
partially girdled trees and the resulting loss in production.
The mean response weighted by acreage was $5.31 per acre per
year. There was no significant difference between the north-
ern and southern regions in the state. State—wide, this fig-
ure indicates a loss of approximately $301,000. I suspect,
however, that growers' estimates are low. In many cases, the
answer to question 18 indicated a loss which was less than
the amount spent on rodent control alone. Furthermore, many
growers ignore the fact that damaged trees may produce a re-
duCed crop, preferring to ignore fruit that never developed

rather than to estimate its potential value.
DISCUSSION

Microtines are a significant pest throughout the major
apple growing region of Michigan. The region south of and
including Muskegon county may incur slightly more damage than
the more northern regions.

Microtines seem to prefer young apple trees to mature
trees. This preference, however, is not absolute and approxi-
mately 1.3% of Michigan's mature apple trees are damaged by
voles annually.

Economic losses due to microtine damage are difficult to
estimate for several reasons. Beside the fact that damage
rates vary from year to year, the effect of observed damage

to the tree's trunk on apple production has never been

17

ascertained except in the extreme case where complete gird-
ling causes the death of the tree.

Even in the case of complete girdling, trees are often
saved by bridge grafts. Many growers, however, prefer to re-
move such trees and replace them. Furthermore, some growers
remove all developing fruit from extensively damaged trees to
redirect the trees' energy budget.

Small amounts of girdling probably have no significant
effect on apple production. However, such injury may facili-
tate the infection of the tree by other factors.

Since little data is available on the effect of physical
damage to the tree's trunk on apple production, a series of
"best estimates" have been made based on information obtain-
ed during interviews with orchard owners. When a tree is
girdled on less than 10% of its circumference, there is
little or no decrease in production. However, when a tree is
girdled over 80% or more of its circumference, bridge grafts
are usually required to prevent the death of the tree. At
this point, production is usually temporarily halted either
through the inability of the tree to produce fruit or the ac-
tion of removing developing fruit. Little information is a—
vailable on the effect of intermediate amounts of damage but
for the sake of simplicity the relationship is taken as lin-
ear between these two points.

Furthermore, little information is available on the ef-
fect of damage during subsequent years. Again, estimates
were based on information gathered during interviews. For

any amount of damage it was assumed that recovery takes

18

approximately two years and that any decrease in production
caused by girdling is halved during the second year. This
may not be entirely accurate for trees which sustain only
small amounts of damage or for trees which are severely dam-
aged but I feel it is the most appropriate estimate based on
available information. A graphic explanation of these effects
is given in Figure AA and AB. This estimate probably under-
states the effect of microtine damage on apple production
since trees which are severely girdled are apt to require
more than two years to fully recover, if they recover at all.

Applying these estimates to the data collected by field
scouts, 0.86% of Michigan's apple trees of bearing age were
damaged sufficiently during the winter of 1973-7A to reduce
production. Of the trees which were damaged, including trees
damaged less than 10%, the mean portion of the circumference
which was girdled was 31%. According to the damage/harvest
schedule developed earlier, the average harvest reduction on
damaged trees would thus be 28% during the harvest season
following the damage and 1A% during the second harvest sub—
sequent to the damage.

By multiplying the appropriate annual damage rate of
1.3% by 3,170,000, the number of trees of bearing age, I have
calculated that in the winter of l973-7A, approximately Al,210
Michigan apple trees of bearing age were damaged by micro-
tines.

An average Michigan apple tree of bearing age produced
221 pounds of apples in 1972 (Anon. 1973). In 1972 then,

the harvestable apple crOp in Michigan was reduced by an

.\
. [’1’
7' L"
FIGURE’AA. Effects of various amounts of girdling on harvest
during first harvest season after girdling.

FIGURE AB. Effect of various amounts of girdling on produc—
tion of an average Michigan apple tree during

successive years following girdling.

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estimated 3,825,112 pounds (Number damaged trees X lbs. har-

vest/tree X reduction of harvest during first season after
damage X 1.5) with a market value of $1A6,500. This is an
average of $2.58 per acre of producing trees and is incurred
in spite of prevailing control techniques.

Younger trees, however, bear the brunt of the damage.
To protect these trees many growers provide additional pro-
tection to new orchards in the form of clean cultivation,
wire or aluminum collars around the base of the trees, or the
eradication of all undergrowth through complete herbicide
treatment. Young trees, however, are seldom repaired by
bridge grafting in the case of severe damage, as it is us-
ually easier and cheaper to replace them. Assuming that
there are equal numbers of trees planted every year and that
an orchard has an average life expectancy of approximately
50 to 60 years (from personal interviews) approximately
57,600 new apple trees are planted in the state every year.
This figure is probably somewhat low because of the recent
trend towards high density dwarf trees. Thus, while the to-
tal acreage planted to apples in Michigan remains essentially
constant, the number of trees planted every year is increas-
ing. Using the damage rates indicated by the growers and
their estimates of the Value of trees at various ages, it is
possible to estimate the financial loss which growers suffer
before their trees are old enough to produce fruit commer-

cially as

Loss = §° {number of tPGES) ( damage rate among \Olvalue of a )
1=:\ aged 1 years , ( trees aged 1 years) (tree aged 1

21
Using the appropriate values I have estimated the loss to be

$175,797. This figure assumes that all damaged trees are de-
stroyed and subsequently replaced. If destroyed trees are
not immediately replaced the loss is higher since subsequent
production is lost. In this case, it becomes much more dif—
ficult to estimate the loss as many of the maintenance tasks
such as trimming, harvesting, spraying, etc., need not be
continued while others such as mowing are often performed..
In order to be consistent with other estimates of financial
costs, I will continue to use a minimum estimate.

Thus, in spite of better than $166,A00 spent on the con—
trol of microtines in Michigan orchards, they continue to
inflict a minimum of $322,290 worth of annual damage in
Michigan. It is important to note that these damage esti—
mates were based on the winter of 1972—73 and 1973—7A which
were both conceded to be relatively light years in terms of
microtine populations. While no estimate of the state-wide
damage rates during years of high microtine densities were
possible during this research, extrapolation of a few iso-
lated orchards which suffered unusually high damage rates in-
dicates that damage levels can go considerably higher. One
grower in the southern portion of the state suffered rela-
tively severe damage during the winter of l973-7A. Of 50
trees examined 7 exhibited girdling for a damage rate of
1A%. Similarly, another grower indicated that during the
winter of 1972-73, 250 of 30,000 trees were damaged severely
enough to require bridge grafting. While it is difficult

to extrapolate on such limited information, microtine

22

densities are well known to vary as much as two orders of
magnitude from years of low density to periods of peak den-
sities. Thus, the damage rate during years of peak microtine
populations may easily be increased to five to ten times as
much as my data indicate.

The increased damage rate among young trees bears further
consideration. While the total cost of the damage to young
trees is considerably less than the value of the decreased
production estimated to result from girdling damage to older
trees, the current trend towards dwarf fruit trees may in—
crease this figure. Dwarf fruit trees start bearing earlier
and also stop producing at a commercially profitable rate at
earlier ages. As older orchards are replaced by dwarf trees,
a larger portion of Michigan's apple trees will be under ten
years of age and therefore, subject to higher damage rates.

The effects of manipulating ground vegetation were sur-
prising. The initial purpose of recording the type of clear-
ing used was to determine whether the use of herbicides pro-
duced a significantly lower damage rate than just mowing.

The lack of a significant differenCe (and the trend towards
lower damage rates in orchards where no weed control was
practiced) is, in itself, biologically important since mow-
ing and herbicide treatments are generally considered to be
destructive to good microtine habitat (Elton 1951). Micro—
tines may move into the orchards during the winter, after

the application of poisoned baits, under the cover of a snow
blanket and utilize the trees as a food source where no other

vegetation is available.

23

An alternative explanation may be that growers with a
previous history of little damage from microtines do not feel
the need to mow or spray in their orchards. Further research
on this question is warranted since if the first explanation
is valid, it may be possible to reduce the costs of weed con-
trol in orchards as well as microtine damage. An added bene—
fit would be the decreased use of unselective herbicides.
Competition between trees and ground vegetation, however, may
warrant continued use of herbicides, particularly early in
the summer when apples are first developing on the tree.

Growers felt that current control procedures are gener-
ally effective. During the interviews, however, I learned
that zinc phosphide is applied to the corn with either an oil
or paraffin base. While the corn treated with oil is gen-
erally less expensive, if application of the orchards is fol—
lowed by rainy weather, the zinc phosphide may be washed from
the corn or become detoxified. This may account for the fact
that several growers indicated that periods of severe mouse

damage are often associated with rainy weather.
SUMMARY

0f 52 orchard owners responding to the questionnaire and/
or contacted for a personal interview, a majority considered
rodents to be significant pests in their orchards. Ninety—
seven percent of the apple producing acreage covered in this
survey was treated with poison bait, presumably zinc phos-
phide. .

The median cost to apply zinc phosphide treated corn to

2A
orchards was $3.00 per acre. State-wide, the projected an-

nual cost of applying poison bait to commercial apple or—
chards is in excess of $166,A00. This cost is based on the
acreage of mature trees. The cost of rodent control in
younger orchards is higher than in mature orchards because
of increased protection usually given to new trees.

In spite of the rodent control procedures currently being
used in Michigan apple orchards, damage still occurs. Re-
sponses to questionnaires indicated that growers estimated a
A% damage rate among trees three years old or younger. This
damage rate declines until the tree reaches an age of approx-
imately ten years at which time the average annual damage
rate was estimated to be 1.3%. These data agree closely
with data collected by field scouts during the spring of
l97A. Information gathered on 1,520 apple trees in the state
indicated a damage rate of 1.3% among mature trees. This was
in a year of apparent low microtine density.

1 Based on estimates of the effect of girdling on‘a tree's
production of apples, the yearly harvest loss attributable
to microtines is approximately 3,825,000 pounds. At an aver-
age value of 3.82 cents per pound as determined by the Michi-
gan Department of Agriculture, this can be converted to a
minimum annual loss of $lA6,500 due to Microtus gp. Fol-
lowing peak years apple production may be reduced by as much
as $732,500 or more.

In addition to the loss due to decreased production,
microtines also cause the death of trees. Damage of this

severity is relatively infrequent among mature trees, and

25

when it does occur, it is often repaired by bridge grafting.
In younger orchards, however, microtines can destroy a sig-
nificant number of trees. Estimates based on responses to
the questionnaire indicate that field voles may cause
$176,000 annual damage among young apple trees.

Based on my estimates, microtines are responsible for a
minimum annual cost of approximately $A89,000 in Michigan
apple orchards. This estimate includes the minimum esti-
mates of the costs of rodent control, the value of lost pro-
duction due to girdled trees, and the value of destroyed trees
less than 10 years old (Figure 5).

The damages may well be greater than this during years of
peak populations. Estimates based on two growers who sus—
tained higher than average damage rates indicate that in peak
microtine years, damage levels may be increased five to ten-
fold.

Zinc phosphide appears to be an effective means of rodent
control under suitable weather conditions. The efficacy,
however, is difficult to determine from my data. Dimmick
(1972) reported that both 1.0% and 1.5% applications of zinc
phosphide on oat groats were significantly effective in re-

ducing populations of M. ochrogaster and M. pinetorum in

 

 

Tennessee. Dry weather following application appears neces-
sary for adequate Control. Severe damage during peak years
often occurs in spite of the use of rodenticides.

The effect of eliminating or reducing ground vegetation
in orchards bears further study. While the purpose of herbi-

cides is to reduce competition for ground water, in view of

FIGURE 5. Partitioning of annual expenses which Michigan
apple growers sustain due to microtine rodents.

27

recent evidence that permitting ground cover is beneficial

in attaining biological control of European Red Mites (Croft
and McGoraty 1973), and the evidence that I have gathered
which indicates that girdling damage is most prevalent in or-
chards where ground cover was eliminated, it may be more ben—
eficial to the apple grower to permit grasses and herbs to

grow under his trees.

BIBLIOGRAPHY

BIBLIOGRAPHY

Anon. 1973. 'Michigan State Agricultural Statistics.

Anon. 196A. Vertebrate Pest Control. Calif. Dept. of Agr.
Bull. 5A: 116-118.

Biser, Daryl, L. 1967. Mammal damage to orchards in New
York State. MS Thesis. Cornell University.

Burt, William H. 1957. Mammals of the Great Lakes Region.
The University of Michigan, Ann Arbor. pp. 2A6.

Croft, B. A. and D. McGoraty. 1973. Model study of a
Caricide resistance, spider mite outbreaks, and biologi-
cal control patterns in Michigan apple orchards.

Env. Ent. 2: 633.

Davis, L. L. 19A3. The Pine Mouse in your orchard. Proc.
Tenn. State Hort. Soc. 38: A8—50.

Dimmick, R. W. 1972. Zinc phosphide and Prolin for con-
trolling prairie voles in Virginia pine plantations.
Proc. Ann. Conf. S. E. Game and Fish Assoc. 26: 293-295.

Elton, Charles S. 19A2. Lemmings, mice and voles. The
Oxford Press, London. pp. A96.

Foster, Ronald B. 1965. Effect of heavy winter rodent in-
festation on perrenial forage plants. J. of Range
Mgmnt. 18(5): 286-287.

Hayne, Don W. 1950. Mouse populations in orchards and a
new method of control. Mich. Ag. Exp. Sta. Quart.
Bul. 33(2): 160-168.

Holm, Leroy, Franklin A. Gilbert, and Ernst Haltvick. 1959.
Elimination of rodent cover adjacent to apple trees.
Weeds. 7(A): A05-A08. -

Kanervo, V. and A. Myllymaki. 1970. Problems caused by the
field vole, Microtus agrestis (ML) in Scandinavia.
EPPO Public Ser. A. 58: 11—26.

 

28

29

Krebs, Charles J., Michael S. Gaines, Barry L. Keller,
Judith H. Myers, and Robert H. Tamarin. 1973. Popu-
lation cycles in small rodents. Science. 179: 35-A1.

Littlefield, E. W., W. J. Schoonmaker, and D. B. Cook. 19A6.
Field mouse damage to coniferous plantations. J. of
Forestry. AA: 7A5-7A9.

Myllymaki, A. 1970. Population ecology and its importance
for the control of the field vole, Microtus agrestis
(E;)- EPPO Public Ser. A. 58: 26.

 

APPENDICES

APPENDIX A

30

SAMPLE QUESTIONNAIRE

1. Do you have fruit trees other than apples?
If yes, please list the number of trees of each
type and the number of acres devoted to each.

 

Kind Acres # Trees

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. How many acres of apple trees do you presently manage?

 

3. How many apple trees do you presently manage?

A. Do you have more than one variety of apple tree in
your orchard?
If yes, list the number of trees of each variety
and the number of acres devoted to each.

 

Variety Acres # Trees

31

SAMPLE QUESTIONNAIRE - continued

5.

10.

ll.

 

Are all of your trees the same age?
If yes, how old?
If no, please list the number of trees and acres in
each age category.

 

 

0-3 3-6 6—10 10-15 15—20 >20
# Trees
# Acres
Estimate the net worth of a tree in each of the above

age categories.

0—3 3—6 6-10 10-15 15-20 >20

Are small mammals a pest problem in your orchard
currently? in the past?

 

 

Do you practice small mammal pest control in your
orchard?
If yes, how often?
semi-annually? dates
annually date
as required

 

 

 

 

 

How much does this method cost per acre?

 

What procedures have you used in the past?

 

currently

 

In your opinion, how effective is the method of small
mammal pest control which you are currently using?

very effective effective moderately

effective of little value

32
SAMPLE QUESTIONNAIRE - continued

 

12. Do you eliminate ground vegetation in your orchard(s)
either thru the use of herbicides or by plowing?

If yes, directly under the tree? how

between rows of trees? how

 

 

 

13. Do you feel that small rodents are damaging any
particular variety(ies) of tree?
If yes, which variety or varieties?

 

1A. Do you feel that dwarf trees are more susceptible to
small mammal damage than larger stocks?

 

15. Estimate the percentage of trees in each age category
which are damaged by small mammals during an average
year.

0-3 3-6 6-10 10-15 15—20 >20

16. Do you practice different small mammal control methods
in newly established orchards?
If yes, what methods?.

 

 

17. What portion of your resources (time and money) is
devoted to controlling:

‘time money
pests of all kinds pests of all kinds
small mammals only small mammals only

(Note: include maintenance and depreciation on
equipment)

18. Estimate the amount of money you lose each year to pests
of all kinds -small mammals only
(Be sure to include money spent on pest control)

 

33

SAMPLE QUESTIONNAIRE - continued

19.

20.

21.

22.

23.

2A.

25.

 

If a tree is damaged seriously enough to require a

bridge graft, by what proportion is its productivity
reduced during the following years?

 

What % of your trees require bridge grafts during an
average year?

 

Of the small mammal damage incurred during the course
of a year what % occurs during each season?

spring % summer % fall % winter %

Do you feel that high levels of small mammal damage are
associated with any particular climatic conditions?

If yes, what conditions?

 

Are there any sections of your orchard which suffer

excessive small mammal damage more consistently than
other areas?

 

Do your records indicate the severity of small mammal

damage which occurred in your orchard during the last
10 years?

 

If yes, please list
1963 196A 1965 1966 '1967 1968

1969 1970 1971 1972 1973

Rate the severity of small mammal damage in your orchard
during the year preceding the '73 harvest season.

heavy average light

3A

SAMPLE QUESTIONNAIRE - continued

26.

27.

28.

29.

‘30.

31.

32.

33.

3A.

 

In your experience, when did you suffer the greatest
damage by small mammals? -

year amount in % of trees damaged

 

amount in $

 

Did you practice small mammal pest control at the time?

If yes, what method?

 

 

Do you consider large mammals such as deer to be pests
in your orchards?

 

What species of mammal do you feel is the most damaging
in your orchard(s)?

Do you hunt wild species in or around your orchards?
deer rabbits pheasants
grouse‘ squirrels

What effect does your entire pest control operation
have on these animals?

Would you be willing to participate in a program de—
signed to reduce this problem?

 

Do you feel a descriptive brochure on the small mammals
found in Michigan orchards would be helpful to you?

At the present time, what information regarding small
mammals would be most helpful to your operation?

35
SAMPLE QUESTIONNAIRE - continued

 

35. Please add any comments you feel would be useful to
our survey of small mammal damage in Michigan orchards.

36
Return to:
Walt Conley
Dept. Fisheries & Wildlife
Michigan State University
East Lansing, MI A882A

/’/
/ ///
Variety //l / / .
(if known) / // ,/ / /’ // ,
g293 ' 6f7 8F9l

I

 

 

 

 

 

. Tree # J 1 A g 5 : 10 .
% Girdling % %i % % % %? % %. %§ %
1
Distance from i i
Trunk to - 1 - .
Nearest Runway _ in. iru inA 1JL_ in. in. iJn, in. in. in

 

i 5
Diameter of ’ g i

Trunk ' in in. 11L? in, in. in. 11L in. in, in:

Evidence of
Cracked corn . ‘
at Base of Tree . . . _l m A '- , -!

 

 

Was Ground Around .
Base of Tree ' , i 1 , i
Sprayed with . ; E i ‘ f F
Herbicide i . ‘ ?

 

Grass Mowed ( 7 i f . A

 

._._--- 1.. .-.. -o.. .

or Disced

Soil Tilled j
Between Rows f

 

.—
- .._.. ..._._____x- ...__....__ .

 

 

 

 

 

I

l

l

a !
Depth of Litter 1 i i l
Layer on Ground 7 l t ( (

 

 

Orchard Owner County

/

 

Have you trapped mice in this Orchard?
If Yes, how many Microtus did you catch?
(NOTE: Microtus are dark grizzled brown on top with short ears,
tail, and legs)

How many trap nights did you spend?

(NOTE: Trapnights = # traps x # nights they were set)

 

 

 

APPENDIX B

37
TABLE Bl. Varieties of trees examined by field scouts.

 

 

Variety of Tree Exgmined Ingfired Injfred
Red Delicious 398 9 2.3%
Jonathan 31A A 1.3
Macintosh 113 l 0.9
Greening A7 1 2.1
Grimes 10 0 0.0
Golden Delicious 95 2 2.1
Rome 97 l 1.0
Ida Red 26 l 3.8
Hyslop Crab ‘ 10 0 0.0
Spy 55 0 0.0
Courtland 6 0 0.0
Stamen 1 1 100.0
Wealthey 6 0 0.0
Winesap . 27 0 0.0
Wolf River 1 0 0.0
Yellow Transparent 1 0 0.0
Fenton A 0 0.0
Macoun 2 0 0.0
William's Early Red 1 0 0.0

Delicious (unspecified) 20 0 0.0

38

 

 

 

 

 

TABLE B2. Contingency table data for Question 7 "Are mice
a serious problem in your orchards?" versus
region (x2 = 7.05 df 0.5 >‘p > 0.1).
NORTH SOUTH TOTALS
OBSERVED A 20
YES EXPECTED 7.6A 16.36 2A
x2 1.73 .81
OBSERVED 3 1
SOME EXPECTED 1.27 2.73 A
X2 2.36 1.10
OBSERVED 7 9
NO EXPECTED 5.09 10.91 16
2
X .72 ~33
TOTALS 1A 30 AA

 

 

 

39

 

 

 

 

 

 

TABLE B3. Contingency table analysis comparing severity of
damage during winter of 1972-73 as determined
from the questionnaires versus geographical area.
There was no significant difference (x2 = 1.58
df = 5 0.975 > p > 0.900).

NORTH SOUTH TOTALS
OBSERVED 9 20
. LIGHT EXPECTED 7.63 21.37 29
x2 0.2L: 0 09
OBSERVED 1 6
MEDIUM EXPECTED 1.8A 5.16 ‘7
x2 0.39 0 1A
OBSERVED 0 2
HEAVY EXPECTED 0.53 1.A7 2
x2 o 53 0 19
TOTALS 10 28 38

 

 

 

A0

 

 

 

 

TABLE BA. Contingency table analysis of damage rates bem
tween orchards where grass cover occurred and
orchards where it was eliminated. Data was
collected by Apple Pest Management field scouts
(x2 = 2.91 df = 3 0.5 > p > 0.1).

GRASS COVER NO GRASS COVER TOTALS
OBSERVED 0 20
DAMAGED EXPECTED 2.53 l7.A7 20
x2 2 53 0 35
OBSERVED 192' 1,308
NOT
DAMAGED EXPECTED 189.A7 1,310.53 1,500
X2 0.03 0.00
TOTALS 192 g 1,328 1,520

 

 

 

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