Mitey Costly: Energetic Costs of Parasite Avoidance and Infection

Parasites reduce host fitness via perturbations to host energy allocation, growth, survival, and reproduction. Here, we investigate the independent effects of parasite exposure and infection on host metabolic rate. Our study focuses on Drosophila hydei and a naturally occurring ectoparasitic mite, Macrocheles muscaedomesticae. We use flow-through respirometry to measure the metabolic rate of flies during the period of exposure (preinfection) and during mite attachment. Flies were exposed to mites either indirectly (through a mesh screen) or directly, allowing for physical contact between the fly and the mite. We predicted that fly metabolic rate would increase with the level of parasite exposure: unexposed flies < flies with indirect exposure to mites < flies with direct contact with mites < flies actively infected with mites. As expected, flies indirectly exposed to but not in direct contact with mites produced 70% more CO₂ than unexposed flies. Flies in direct contact with mites produced 35% more CO₂ than flies with indirect contact, and this was more than double the amount of CO₂ produced by unexposed flies. However, infected flies-those actually carrying mites-did not produce significantly more CO₂ than uninfected flies. Our results show that simply being exposed to mites, either indirectly or directly, was sufficient to elicit a response from the host in terms of elevated CO₂ production. Our results show that the costs of parasitism can potentially extend beyond the physiological costs of infection per se to include the energetic costs associated with parasite avoidance. Although studies have shown energetic costs associate with predator-avoidance behaviors, no study to our knowledge has measured the metabolic cost of parasite avoidance.