MUTUALISTIC TRANSFER OF EPICHLOE-SPERMATIA BY PHORBIA FLIES

We studied the interactions between insects and the spermatial stromata of Epichloe spp. they visit in North America and Great Britain. We found that Phorbia flies formed a close association with Epichloe at all study sites. Adult flies visited Epichloe stromata for feeding and egg laying. Stromata upon which Phorbia deposited eggs produced perithecia more frequently than those that lacked eggs. Furthermore, when we accounted for feeding on perithecia, stromata possessing eggs produced 5-7 times more perithecia as those lacking Phorbia eggs. Excluding insects from spermatial stromata with mesh bags prevented perithecia from being produced. A laboratory test gave no support for the hypothesis that spermatia are transferred by wind. Field observations of natural stromata revealed little visitation by non-Phorbia insects. Laboratory and field observations of Phorbia on stromata showed that flies display a stereotypical behavior in which they drag their abdomen immediately following deposition of an egg. Microscopic inspection of fly feces revealed that spermatia pass through the gut of the fly intact. Our results show that Phorbia is the primary vector of Epichloe spermatia and that the mode of transfer is likely through the gut of the fly.