Scared of the dark? Phototaxis as behavioural immunity in a host-parasite system.

Behavioural immunity describes suites of behaviours hosts use to minimize the risks of infection by parasites/pathogens. Research has focused primarily on the evasion and physical removal of infectious stages, as well as behavioural fever. However, other behaviours affect infection risk while carrying ecologically significant trade-offs. Phototaxis, in particular, has host fitness implications (e.g. altering feeding and thermoregulation) that also impact infection outcomes. In this study, we hypothesized that a fly host, Drosophila nigrospiracula, employs phototaxis as a form of behavioural immunity to reduce the risk of infection. First, we determined that the risk of infection is lower for flies exposed in the light relative to the dark using micro-arena experiments. Because Drosophila vary in ectoparasite resistance based on mating status we examined parasite-mediated phototaxis in mated and unmated females. We found that female flies spent more time in the light side of phototaxis chambers when mites were present than in the absence of mites. Mating marginally decreased female photophobia independently of mite exposure. Female flies moved to lighter, i.e. less infectious, environments when threatened with mites, suggesting phototaxis is a mechanism of behavioural immunity. We discuss how parasite-mediated phototaxis potentially trades-off with host nutrition and thermoregulation.