Endophyte-mediated tritrophic interactions between a grass-feeding caterpillar and two parasitoid species with different life histories

Plant secondary chemicals can alter herbivore suitability for parasitoids by weakening or stunting the host, delaying its development, or when larval parasitoids encounter ingested phytotoxins in the body of their host. Experiments with different parasitoids that exploit the same host species feeding on the same plant may provide insight about how parasitoid life history affects the strength of such interactions. The encyrtid wasp Copidosoma bakeri, a slow-developing polyembryonic egg-larval parasitoid, and the tachinid fly Linnaemya comta, a fast-developing solitary species, both parasitize Agrotis ipsilon, a generalist noctuid. We tested the hypothesis that of the two parasitoid species, the encyrtid, because of its more prolonged developmental association with the host, would suffer greater fitness costs when A. ipsilon feeds on perennial ryegrass containing an alkaloid-producing fungal endophyte. Indeed, fewer parasitized cutworms yielded C. bakeri broods, and those host mummies were smaller, formed more slowly, and contained fewer adults when the hosts fed on endophytic as opposed to endophyte-free grass. In contrast, L. comta fitness parameters were similar regardless of the type of grass upon which their host fed. Our results highlight that the outcome of endophyte-mediated tritrophic interactions may differ for different parasitoid species. Implications for integrating the use of endophytic grasses and biological control are discussed.