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

Epichloe fungi interact with Botanophila flies in a co-evolved mutualism th at appears to be analogous to angiosperm pollinator-parasite systems. Femal e flies act as vectors of fungal sports as they lay eggs on the fungi which grow within and on grass stems. To understand selective pressures operatin g on choice of host Fungi by flies, we monitored the interaction of two spe cies of Epichloe with Botanophila at two sites in southern England. At one site (Ashurst), the hosts Epichloe clarkii and Epichloe boconii co-occurred , while at a second site (Farm Gate), only E. clarkii was present. Flies ov ipositing on E. baconii showed preferences for large, somewhat isolated fun gi 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 Far m Gate site tended to prefer large fungi that occurred within small clumps of grass. Fungal characteristics preferred by the flies showed no correlati on with egg/larval survivorship. Insect survival on E. clarkii varied betwe en sites and was related to differences in egg dispersion among fungi at th e two sires. The absence of an alternate host (E. baconii) was associated w ith greater aggregation of eggs on E. clarkii at the Farm Gate site and the se insects experienced higher levels of mortality at this compared to the o ther site.