Host selection by a mycophagous fly and its impact on fly survival

Epichloe fungi interact with Botanophila flies in a co-evolved mutualism that appears to be analogous to angiosperm pollinator-parasite systems. Female flies act as vectors of fungal spores as they lay eggs on the fungi which grow within and on grass stems. To understand selective pressures operating on choice of host fungi by flies, we monitored the interaction of two species of Epichloe with Botanophila at two sites in southern England. At one site (Ashurst), the hosts Epichloe clarkii and Epichloe baconii co-occurred, while at a second site (Farm Gate), only E. clarkii was present. Flies ovipositing on E. baconii showed preferences for large, somewhat isolated fungi occurring within small clumps of grass. Flies ovipositing on E. clarkii at the Ashurst site did not select hosts based on any physical features of fungi that we measured. However, flies ovipositing on E. clarkii at the Farm Gate site tended to prefer large fungi that occurred within small clumps of grass. Fungal characteristics preferred by the flies showed no correlation with egg/larval survivorship. Insect survival on E. clarkii varied between sites and was related to differences in egg dispersion among fungi at the two sites. The absence of an alternate host (E. baconii) was associated with greater aggregation of eggs on E. clarkii at the Farm Gate site and these insects experienced higher levels of mortality at this compared to the other site.