Insects harbour a diversity of endosymbionts, and a majority of insects are host to at least one species. Although vertical transmission can explain the relatively close phylogenetic links between some endosymbionts and their hosts, bacterial symbionts have repeatedly transferred between species. One hypothesis is that ectoparasitic mites vector hemolymph dwelling endosymbionts on contaminated mouthparts after feeding on an infected host. Although mites can take up and transmit Spiroplasma poulsonii between flies in the lab, the likelihood of mites vectoring endosymbionts depends on mite preferences for infected or uninfected flies, and the ability of infected hosts to resist mite attack. In this study we test the effects of S. poulisoni MSRO on the behaviour of Drosophila melanogaster and the potential vector Macrocheles subbadius, a natural mite parasite of flies. We hypothesised that mites will preferentially infect Spiroplasma+ flies over Spiroplasma- flies. We also hypothesised that Spiroplasma+ flies will 1) have poorer endurance and be less able to sustain resistance (e.g. grooming, kicking, bursts of flight) and 2) show decreased avoidance of mites. We tested these hypotheses using y-maze preference experiments, geotaxis endurance assays, and behavioural monitoring respectively. Contrary to our expectations, mites preferred Spiroplasma- flies (69% infection bias in pairwise trials). On the other hand, Spiroplasma infection reduced anti-mite behaviour of flies (45% reduction in endurance). Preliminary behavioural trials suggest Spiroplasma may provide a protective effect against infection counteracting impaired endurance. Understanding the behavioural consequences of endosymbiont infection for insects helps elucidate ecologically significant secondary interactions with potential vectors.
