Past, present, and future: Non-consumptive effects of the ectoparasitic mite Macrocheles subbadius on adult and larval Drosophila nigrospiracula

Abstract

Parasites can indirectly impact hosts through non-consumptive effects (NCEs) via changes in behaviour, morphology, and/or physiology. These responses can be understood in terms of the ecology of fear framework. My study system involves a cactophilic fruit fly (Drosophila nigrospiracula) and a naturally occurring ectoparasitic mite (Macrocheles subbadius). Previous studies have shown that chronic mite exposure (without infection) in adult flies decreases fecundity and longevity, although the mechanism leading to that decrease is unknown. I tested the hypothesis that NCEs of parasite exposure (e.g., parasite avoidance and defense) trade off with fitness-related behaviours such as feeding and resting. I also posited that the magnitude of these NCEs would be amplified by either primary exposure (sans infection) or primary infection history. I found that secondary mite exposure (through free roaming mites in an observation arena) resulted in increased grooming and movement, but exposure history did not affect these behaviours. However, the interaction between primary and secondary exposure influenced host feeding and resting behaviours. Upon a secondary mite exposure, previously exposed flies increased feeding and decreased resting, suggesting an important role for exposure history in the expression of NCEs. I then tested the role of infection history on the NCEs of current parasite exposure. Regardless of prior infection status, flies increased defensive and ambulatory behaviour in the presence of mites, and consequently less time was spent resting but feeding was unaffected. None of the behaviours measured were affected by previous infection status. Moreover, these results showed that previous exposure (sans infection) to parasites may have an even stronger effect upon secondary exposure than infection history. Secondly, I investigated whether M. subbadius exert non-consumptive effects on fly larvae that persist through development. Even though mites do not infect larvae, previous work has shown that mite presence exerts NCEs by reducing pupation success. I hypothesized that exposure to mites during the larval stage has downstream effects, such that the NCEs persist through development into the adult stage. I predict that larvae exposed to mites will exhibit decreased body weight, fecundity, and longevity as adults. The results showed no evidence of a downstream effect on body weight, fecundity, or longevity. Since parasites were absent in adulthood, perhaps adult flies did not prioritize parasite defence under conditions of reduced risk. NCEs of parasite exposure on larvae did not carry-over into the next life stage adulthood. My study highlights the importance of the ecology of fear and the role that parasites play in generating non-consumptive effects of parasitism. The ecology of fear should be integrated into management decisions as parasites not only modify current host behaviour, but have the potential to affect future behaviours, even without active infection.