White-footed mice (Peromyscus leucopus) are an abundant species in eastern deciduous forests. The objective of this study was to examine the ecology of botfly parasitism in a white footed mouse population in the Brockport Woods, Brockport, NY. I analyzed data collected by live trapping in May and September from 1993 to 2009. Males and females exhibited similar levels of botfly infestation. When a greater percentage of mice was infected with botflies, there was a significant increase in average body mass. I also found that as fall trap success increased, the proportion of mice with botflies decreased, but the number of infected mice remained relatively constant over time. This may be due to the fluctuation of P. leucopus populations, which is characterized by rapid increases and sudden collapses, so there may not be enough botflies to take advantage of all the available hosts at high densities. Botfly infection did not have an impact on overwinter survival. Lastly, spring abundance was most affected by trap success in the previous fall and two weather variables; spring abundance increased when fall trap success and mean January temperature increased and when total January snowfall decreased. These three variables, however, did not explain all of the observed variability in abundance. Population fluctuations in P. leucopus are complex, so future studies should look at other factors that could be responsible for driving abundance of this species

Pilakouta, Natalie

English

The College at Brockport, State University of New York: Digital Commons @Brockport

The College at Brockport: State University of New York
Digital Commons @Brockport
Senior Honors Theses Master's Theses and Honors Projects
5-2010
Ecology of Botfly Parasitism in White-Footed Mice
(Peromyscus leucopus)
Natalie Pilakouta
The College at Brockport, npila1@gmail.com
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Pilakouta, Natalie, "Ecology of Botfly Parasitism in White-Footed Mice (Peromyscus leucopus)" (2010). Senior Honors Theses. 27.
http://digitalcommons.brockport.edu/honors/27
 0  
 
 
 
 
 
 
 
 
 
 
 
Ecology of botfly parasitism in white-footed mice (Peromyscus leucopus) 
 
 
 
A Senior Honors Thesis 
 
 
 
Presented in Partial Fulfillment of the Requirements 
for graduation in the College Honors Program 
 
 
By 
Natalie Pilakouta 
Environmental Science and Biology Major 
 
The College at Brockport 
May 2010 
 
 
Thesis Director: Dr. Christopher Norment, Professor, Environmental Science and Biology 
 
 
 1  
ABSTRACT.—White-footed mice (Peromyscus leucopus) are an abundant species in eastern 
deciduous forests. The objective of this study was to examine the ecology of botfly parasitism in 
a white footed mouse population in the Brockport Woods, Brockport, NY. I analyzed data 
collected by live trapping in May and September from 1993 to 2009. Males and females 
exhibited similar levels of botfly infestation. When a greater percentage of mice was infected 
with botflies, there was a significant increase in average body mass. I also found that as fall trap 
success increased, the proportion of mice with botflies decreased, but the number of infected 
mice remained relatively constant over time. This may be due to the fluctuation of P. leucopus 
populations, which is characterized by rapid increases and sudden collapses, so there may not be 
enough botflies to take advantage of all the available hosts at high densities. Botfly infection did 
not have an impact on overwinter survival. Lastly, spring abundance was most affected by trap 
success in the previous fall and two weather variables; spring abundance increased when fall trap 
success and mean January temperature increased and when total January snowfall decreased. 
These three variables, however, did not explain all of the observed variability in abundance. 
Population fluctuations in P. leucopus are complex, so future studies should look at other factors 
that could be responsible for driving abundance of this species. 
 
INTRODUCTION 
White-footed mice (Peromyscus leucopus) are common in eastern deciduous forests (Cramer 
and Cameron 2007). Previous studies have shown that P. leucopus populations exhibit unstable 
dynamics (Terman 1968, Vandegrift and Hudson 2009). They increase when food is abundant, 
leading to greater survival during the winter and greater subsequent breeding (Jones et al. 1998, 
Falls et al. 2007). The eventual population decline is thought to be due to resource 
 2  
overexploitation. However, parasitism may also contribute to destabilization of populations 
(Vandegrift and Hudson 2009), because parasites can decrease body condition and breeding 
success (Vandegrift et al. 2008). P. leucopus are common hosts to the botfly (Cuterebra 
fontinella), an obligate parasite that completes its life cycle in mouse hosts (Jaffe et al. 2005, 
Cramer and Cameron 2007). Botflies lay their eggs in late spring and early summer (Cramer and 
Cameron 2007) and mice become infected when they come in contact with the eggs. The larvae 
develop in the host for about one month (Cogley 1991). 
It is still unclear how parasites influence population dynamics and whether they have a 
significant effect on host abundance and reproduction (Jaffe et al. 2005, Cramer and Cameron 
2007). According to Ewald (1994), a successful parasite needs to exhibit a level of virulence in 
its host that does not compromise its own survival and transmission. Therefore, the parasite 
would be expected to have little effect on its host. However, previous research has produced 
contradictory results in regards to the effect of botflies on P. leucopus (Munger and Karasov 
1994, Jaffe et al. 2005, Cramer and Cameron 2007, Vandegrift and Hudson 2009). 
The objective of this study was to examine botfly parasitism in a P. leucopus population to 
determine whether, and to what extent, botflies affect population abundance. I analyzed data 
collected by live trapping in the Brockport Woods in May and September from 1993 to 2009. I 
investigated the relationship between rate of botfly infection and overwinter survival, and also 
examined the relationship between population density and botfly infection, hypothesizing that as 
P. leucopus abundance increases, the proportion of individuals infected will also increase, due to 
increased crowding and thus a higher probability of contact with the parasite. I then examined 
the impact of botfly infection on body mass. Last, I looked at the effect of fall abundance, botfly 
infection, and weather conditions on white-footed mouse abundance in the spring, to develop a 
 3  
model with spring abundance as the response variable. This project is especially important, 
because of limited availability of long-term studies on P. leucopus populations. The analysis of a 
16-year data set hopefully will allow me to answer important ecological questions regarding 
botfly parasitism in this species. Even though other studies have found that botfly infection alone 
does not affect survivorship (Jaffe et al. 2005, Cramer and Cameron 2007), there have been no 
previous studies, to my knowledge, examining whether and how the interaction between botflies 
and weather or other conditions affect P. leucopus abundance. 
 
MATERIALS AND METHODS 
The trapping grid was located in the Brockport Woods, a sugar maple (Acer saccharum) 
dominated deciduous forest with down logs and a sparse understory, on the SUNY Brockport 
campus, Brockport, Monroe Country, NY. Mammal residents of these woods include white-
footed mice, white-tailed deer (Odocoileus virginianus), and eastern chipmunks (Tamias 
striatus). The size of our 10 x 6 grid was approximately 1 ha. The study employed Sherman live 
traps placed 15 m apart. Live trapping was done during 4-night trapping sessions every May and 
September from 1993 to 2009. Rolled oats were placed in traps in the evening and the traps were 
checked in the early morning. A wad of polyester batting was also placed in traps to provide 
insulation and prevent mortality at low temperatures. White-footed mice captured were ear 
tagged and released, after their mass, sex, age, reproductive condition, and botfly infection were 
recorded. Age classification was based on mass; mice that weighed ≤ 15 g were considered 
juveniles, mice that weighed between 15 and 20 g were considered subadults, and mice that 
weighed ≥ 20 g were considered adults. When other small mammals were captured, they were 
released after recording the species.  
 4  
I performed a χ2 goodness-of-fit test to determine whether there was a difference in the 
proportion of males and females in the population. I arcsin transformed infection rates of males 
and females and used a paired t-test to compare them. I then performed regression analysis to see 
how average mass of mice changed with percentage of mice infected with botflies. I also used 
linear regression to investigate the relationship between population density and botfly infection 
rate. Last, I performed best subsets regression to identify the model that best explained 
variability in spring abundance. The response variable was spring abundance and the covariates 
were fall trap success, botfly infection rate, and several weather variables: January snowfall, 
April snowfall, total snowfall from December to March, mean January temperature, mean 
February temperature, mean April temperature, mean December to March temperature, January 
snowfall/January temperature, heating degree days from December to March, snow depth on 
March 1, snow depth on April 1, and date of last snow in April. In all of these analyses, trap 
success was used as a measure of P. leucopus abundance. Trap success was calculated by 
dividing the total number of white-footed mice captured by the number of traps available for use 
by white-footed mice. Minitab 15 was used for all statistical analyses; p-values < 0.05 were 
considered statistically significant. 
 
RESULTS 
A χ2 goodness-of-fit test showed no statistically significant difference in the number of males 
and females captured (χ2 = 2.44, df = 1, P = 0.119), which was 269 and 234, respectively. There 
was also no statistically significant difference in botfly infection between males and females 
throughout the study period (t=-0.55, df=13, P=0.588). Regression analysis showed that as the 
 5  
percentage of mice infected increased, the average mass of the mice increased (F=11.80, 
R2(adj)=43.6%, P=0.004; Figure 1). 
I found that as fall trap success, and thus fall abundance, increased, the percentage of mice 
infected with botflies decreased (F=7.00, R2(adj)=28.6%, P=0.019; Figure 2). However, the 
number of mice infected did not change much as fall trap success increased (F=0.07, 
R2(adj)=0.00%, P=0.789; Figures 3 and 4). The range in number of infected mice throughout the 
study period was 1 to 17, whereas the average number of infected mice captured each fall was 
5.8. A change in the percentage of mice infected in the fall did not have an effect on spring trap 
success the following year (F=0.00, R2(adj)=0.00%, P=0.978; Figure 5). 
To evaluate the importance of botfly infection rate in driving P. leucopus abundance the 
following spring, I first used univariate regression. The results showed that botfly infection alone 
does not significantly affect spring abundance (F=0.09, R2(adj)=0.00%, P=0.773; Figure 6). 
Multivariate regression was then performed, using botfly infection rate as one of the predictor 
variables. None of the simple models that were generated explained the variation in spring 
abundance to a satisfactory degree. One of the models included eight variables and had an R2 
value of 86.6%, but it was considered too complex due to the large number of variables included 
in the model. Multivariate regression was performed again, with fewer variables as predictors. 
Based on the results of this analysis, the model determined to be the best included three 
variables: fall trap success in the previous year, total January snowfall, and mean January 
temperature (R2(adj)=31.4%, Mallows Cp=1.4). Stepwise regression showed that spring trap 
success increased with decreasing fall trap success (t=-1.65, P=0.129), increasing January 
temperature (t=2.19, P=0.053), and increasing January snow (t=1.34, P=0.211). 
 
 6  
DISCUSSION 
Males and females exhibited similar levels of botfly infestation. When mice are infected with 
botflies, they may experience a significant increase in body mass; even though this analysis 
showed that the increase was statistically significant, it is unclear whether it is also biologically 
significant. It should also be noted that some of the increase may be due to the mass of the botfly 
itself. Previous studies have reported that larva can grow to 5% of the host’s body mass (Munger 
and Karasov 1994, Jaffe et al. 2005). Cramer and Cameron (2007) also found that botfly 
infection increases male body size and thus improves their mating success, since females show a 
preference for larger males. 
As fall trap success increased, the proportion of mice with botflies decreased, but the number 
of infected mice remained relatively constant through time. This may be due to the fluctuation of 
P. leucopus populations, which is characterized by rapid increases and sudden collapses 
(Vandegrift and Hudson 2009). Therefore, there may not be enough botflies to take advantage of 
all available hosts at higher densities. Another possibility is that good conditions for mice are bad 
conditions for botflies, leading to an increase in the former and a decrease in the latter. Since 
changes in the percentage of mice infected in the fall did not affect spring trap success the 
following year, botfly infection did not have an impact on overwinter survival. This finding is 
consistent with previous studies, which found that botfly infection alone does not affect 
survivorship (Jaffe et al. 2005, Cramer and Cameron 2006). 
As far as the results of statistical modeling are concerned, botfly infection does not appear to 
affect spring abundance. Based on the results of the multivariate regression analysis, spring 
abundance is mostly driven by fall trap success, total January snowfall, and mean January 
temperature. Spring abundance tends to be higher when fall abundance in the previous year is 
 7  
lower, probably because of decreased intraspecific competition for resources in the winter and 
thus lower mortality. Spring abundance is also higher in years with higher total snowfall in 
January. This may due to the fact that P. leucopus use the subnivean zone as shelter during the 
winter, so a thicker layer of snow would provide increased insulation and protection from 
predators. Last, a higher mean January temperature leads to higher spring abundance, since P. 
leucopus need to expend less energy for thermoregulation at milder ambient temperatures. Also, 
when weather conditions are better, food resources may become available relatively earlier in the 
year. Kesner and Linzey (1997) found that the most important predictor of density in a given 
month was density in the previous month. A small amount of variance was also explained by two 
weather variables: deviation from mean precipitation with a 5-month lag and high temperature 
with a 2-month lag (Kenser and Linzey 1997). 
A potential problem of this study was that trap success was used as a population index, but 
factors other than abundance can lead to variation in trap success. For example, on a night with a 
clear sky and full moon, P. leucopus may be more wary due to increased predation risk and thus 
wander less, decreasing trap exposure. However, it is unlikely that this caused significant errors, 
so the results of this study are robust. 
 
LITERATURE CITED 
Cogley, T. P. 1991. Warble development by the rodent bot Cuterebra fontinella (Diptera: 
Cuterebridae) in the deer mouse. Veterinary Parasitology 38: 275-288. 
Cramer, M. J. and G.N. Cameron. 2006. Effects of bot fly (Cuterebra fontinella) parasitism on a 
population of white-footed mice (Peromyscus leucopus). Journal of Mammalogy 87: 1103-
1111. 
Cramer, M.J. and G.N. Cameron. 2007. Effects of bot fly, Cuterebra fontinella, parasitism on 
male aggression and female choice in Peromyscus leucopus. Animal Behaviour 74: 1419-
1427. 
 8  
Jaffe, G., D.A. Zegers, M.A. Steele, and J.F Merritt. 2005. Long-term patterns of botfly 
parasitism in Peromyscus maniculatus, P. leucopus, and Tamias striatus. Journal of 
Mammalogy 86: 39-45. 
Kesner, M.H. and A.V. Linzey. 1997. Modeling population variation in Peromyscus leucopus: an 
exploratory analysis. Journal of Mammalogy 78: 643-654. 
Munger, J.C. and W.H. Karasov. 1994. Costs of botfly infection in white-footed mice: energy 
and mass flow. Canadian Journal of Zoology 72: 166-173. 
Terman, C.R. 1968. Population dynamics in Biology of Peromyscus (J.A. King ed.). American 
Society of Mammalogists 2: 1-593. 
Vandegrift, K.J. and P.J. Hudson. 2009. Could parasites destabilize mouse populations? The 
potential role of Pterygodermatites peromysci in the population dynamics of free-living 
mice, Peromyscus leucopus. International Journal for Parasitology 39: 1253-1262. 
Vandegrift, K.J., T.R. Raffel, and P.J. Hudson. 2008. Parasites prevent summer breeding in 
white-footed mice, Peromyscus leucopus. Ecology 89: 2251–2258. 
 
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Figure 1. Regression line showing that as the percentage of infected mice increases, 
the average mass of the mice also increases.  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2. Regression line showing that as fall trap success, and thus fall abundance, 
increases, the percentage of infected mice in the population decreases. 
 
 
 
 
9080706050403020100
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21
Percentage of mice infected
A
ve
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 m
as
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(g
)
R2=43.6% 
P=0.004 
0.40.30.20.10.0
90
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60
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Fall trap success
Pe
rc
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 o
f 
m
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 R2=28.6% 
 P=0.019 
 10  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3. Regression line showing that number of infected mice does not change with 
changes in fall trap success. 
 
0
10
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30
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50
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1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
Number infected
Percentage infected
 
Figure 4.  Number and percentage of infected mice over time. The number of infected mice 
remained relatively constant throughout the study period. 
0.40.30.20.10.0
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Fall trap success
Nu
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 R2=0.0% 
 P=0.789 
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1.41.21.00.80.60.40.20.0
0.30
0.25
0.20
0.15
0.10
0.05
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Botfly infection rate
Sp
ri
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 tr
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uc
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 th
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fo
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 y
ea
r
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5. Regression line showing that spring abundance is not affected by the 
percentage of infected mice in the previous fall. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6. Regression line showing that botfly infection rate does not drive changes in 
the spring abundance. 
9080706050403020100
0.25
0.20
0.15
0.10
0.05
0.00
Percentage of mice infected in previous fall
Sp
ri
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 tr
ap
 s
uc
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ss
R2=0.0% 
P=0.978 
R2=0.0% 
P=0.773 


Ecology of Botfly Parasitism in White-Footed Mice (Peromyscus leucopus)

https://digitalcommons.brockport.edu/cgi/viewcontent.cgi?article=1026&context=honors

Ecology of Botfly Parasitism in White-Footed Mice (Peromyscus leucopus)

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