Bot fly parasitism of the red-backed vole: host survival, infection risk, and population growth

SEASONAL AND HOST-ASSOCIATED VARIATION IN BOT FLY (OESTRIDAE: CUTEREBRINAE) PARASITISM OF LAGOMORPHS ACROSS AN ARID-LAND ECOSYSTEM IN SOUTHERN NEW MEXICO

Bot flies (Oestridae: Cuterebrinae) are obligate mammalian parasites that complete the larval stage of their life cycle under the skin of their host. Most detailed studies of bot fly larval disease ecology have been conducted in temperate deciduous zone rodent systems. To understand the relative importance of seasonal and spatial factors, as well as factors intrinsic to the host, in underpinning the likelihood and extent of parasitism by bot flies in non-rodent hosts as well as in arid-land ecosystems, we examined the dynamic for black-tailed jackrabbit (Lepus californicus) and desert cottontail (Sylvilagus audubonii) parasitism by bot fly larvae (Cuterebra spp.) across 7 repeatedly sampled sites spread across approximately 500 km of the Chihuahuan Desert ecoregion of southern New Mexico. This environment is characterized by a climate that includes hot dry summers and cool to cold dry winters, as well as strongly seasonal summer monsoonal rains. Lagomorphs are a common mid-sized mammal in these landscapes. Bot fly parasitism was strongly seasonal, with peak prevalence and abundance in the spring, and there was spatial variation in the extent of parasitism between collection sites. Additionally, jackrabbits in better body condition were less likely to be parasitized (as indicated by kidney fat index). We did not find sex-based differences in bot fly parasitism between male and female jackrabbits. Thus, in arid-land ecoregions, abiotic factors are likely the primary driver of the bot fly-host interaction, whereas factors intrinsic to the host were of secondary importance for characterizing the interactions of bot flies and lagomorphs.

Factors influencing the presence of parasitic trombiculids on red-backed voles in a temperate archipelago

Parasites can influence host population health and dynamics and are often an overlooked aspect of the ecology of ecosystems. Understanding the impacts of ecological interactions between parasites and small mammals can provide insights into ecosystem dynamics. We live trapped small mammals within the Apostle Islands archipelago (2017-2020) and assessed factors influencing the presence of mites from the Trombiculidae family. Archipelagos provide unique research opportunities because in small mammal-parasitic trombiculid systems, they are essentially closed systems. We detected trombiculids on 47% of Myodes gapperi (Vigors, 1830; red-backed vole) individuals but rarely detected trombiculids on other species. We developed and ranked a set of a priori logistic regression models of trombiculid presence relative to habitat quality, host abundance, body condition, sex, and sexual maturity to identify factors significant in predicting trombiculid infection for M. gapperi. Parasitic trombiculids were more likely when M. gapperi abundance was high and body condition was poor, however it is unknown whether trombiculids affect condition or if trombiculids are more likely to parasitize hosts in poor condition. The significance of host abundance may indicate density-dependent transmission. Our work suggests that host density and body condition are important factors influencing parasitism by trombiculids in M. gapperi populations.

The effects of mite parasitism on the reproduction and survival of the Taiwan field mice (Apodemus semotus)

Background

The effects of parasitism on host survival and reproduction can be highly complex depending on the type of parasites, host sex and life-history characteristics, and ecological conditions. In this study, we tested sex-biased parasitism from Trombiculid mites (Acarina: Trombiculidae) and their sex-specific effects on host reproduction and survival, in a natural population of the Taiwan field mouse (Apodemus semotus). We performed surveys of A. semotus and their Trombiculid mites between April 2010 and August 2011 and again between June and September 2012 in a subtropical evergreen forest in Taiwan.

Results

Contrary to the commonly reported male-biased parasitism in mammals, we did not find sex-biased parasitism in A. semotus. We found that mite abundance was negatively associated with A. semotus reproduction and survival in both males and females. The mite abundance and rodent reproduction fluctuated seasonally, and the peak reproductive season coincided with the time period of relatively low mite abundance.

Conclusions

Trombiculid mites could potentially regulate A. semotus populations through reducing their reproduction and survival. The overlapping periods of peak reproduction and low parasitism implied that A. semotus may adjust their reproductive phenology to avoid periods of high parasitism or be constrained by parasites to reproduce only during periods of low parasitism. Although our results are correlational, host breeding season has been shown to increase in response to experimental reduction of parasitism. We suggest that parasites may shape host reproduction phenology through which they may influence host population dynamics.

Free-ranging eastern chipmunks (Tamias striatus) infected with bot fly (Cuterebra emasculator) larvae have higher resting but lower maximum metabolism

Given the ubiquity and evolutionary importance of parasites, their effect on the energy budget of mammals remains surprisingly unclear. The eastern chipmunk ( Tamias striatus (L., 1758)) is a burrowing rodent that is commonly infected by cuterebrid bot fly ( Cuterebra emasculator Fitch, 1856) larvae. We measured resting metabolic rate (RMR) and cold-induced Vo2-max (under heliox atmosphere) in 20 free-ranging individuals, of which 4 individuals were infected by one or two larva. We found that RMR was significantly higher in chipmunks infected by bot fly larvae (mean ± SE = 0.88 ± 0.05 W) than in uninfected individuals (0.74 ± 0.02 W). In contrast, Vo2-max was significantly lower in chipmunks infected by bot fly larvae (4.96 ± 0.70 W) than in uninfected individuals (6.37 ± 0.16 W). Consequently, the aerobic scope (ratio of Vo2-max to RMR) was negatively correlated with the number of bot fly larvae (infected individuals = 5.74 ± 1.03 W; noninfected individuals = 8.67 ± 0.26 W). Finally, after accounting for the effects of body mass and bot fly parasitism on RMR and Vo2-max, there was no correlation between the two variables among individuals within our population. In addition to providing the first estimate of Vo2-max in T. striatus, these results offer additional evidence that bot fly parasitism has significant impacts on the metabolic ecology of this host species.

Energetic cost of bot fly parasitism in free-ranging eastern chipmunks

The energy and nutrient demands of parasites on their hosts are frequently invoked as an explanation for negative impacts of parasitism on host survival and reproductive success. Although cuterebrid bot flies are among the physically largest and most-studied insect parasites of mammals, the only study conducted on metabolic consequences of bot fly parasitism revealed a surprisingly small effect of bot flies on host metabolism. Here we test the prediction that bot fly parasitism increases the resting metabolic rate (RMR) of free-ranging eastern chipmunks (Tamias striatus), particularly in juveniles who have not previously encountered parasites and have to allocate energy to growth. We found no effect of bot fly parasitism on adults. In juveniles, however, we found that RMR strongly increased with the number of bot fly larvae hosted. For a subset of 12 juveniles during a year where parasite prevalence was particularly high, we also compared the RMR before versus during the peak of bot fly prevalence, allowing each individual to act as its own control. Each bot fly larva resulted in a approximately 7.6% increase in the RMR of its host while reducing juvenile growth rates. Finally, bot fly parasitism at the juvenile stage was positively correlated with adult stage RMR, suggesting persistent effects of bot flies on RMR. This study is the first to show an important effect of bot fly parasitism on the metabolism and growth of a wild mammal. Our work highlights the importance of studying cost of parasitism over multiple years in natural settings, as negative effects on hosts are more likely to emerge in periods of high energetic demand (e.g. growing juveniles) and/or in harsh environmental conditions (e.g. low food availability).