Natural Release of Epichloe typhina Ascospores and Its Temporal Relationship to Fly Parasitism

Abstract

Epichloe typhina (Clavicipitaceae), a fungal endophyte of Elymus virginicus and other grasses, is parasitized by larvae of the fly Phorbia phrenione (Anthomyiidae). Ascospores released from E. typhina stremata on intact, uncut E. virginicus culms were collected and counted at two field sites in Missouri to determine when ascospores were naturally discharged from perithecia. Consumption of perithecia by larvae of P. phrenione was also monitored. The time period over which larvae consumed perithecia was compared with the timing of ascospore release in order to assess the impact of larvae on the numbers of ascospores discharged. Ascospores were liberated from stromata on uncut host plants as intact filaments and underwent iterative germination under nonstressful conditions. The number of ascospores collected was highly variable among days, stromata, and sites. Yet, ascospores were not liberated until at least 88% of total larval feeding had occurred. Larvae consumed at least some perithecia on 28% of stromata, removing an average of 22% of the surface area of each stroma. Because larvae consume perithecia before most ascospores are released, P. phrenione should substantially reduce ascospore release by the fungus.