The influence of host plants on parasitism of the invasive Light Brown Apple Moth, Epiphyas postvittana in California

Tritrophic interactions between host plants, herbivores, sand their natural enemies are frequently mediated by the physical architecture of the host plant as well as by volatile signals from herbivore damage. We studied the influence of nine different host plant species on parasitism by indigenous parasitoids in California of the Light Brown Apple Moth (Epiphyas postvittana), an invasive herbivore from Australia. We conducted experimental field and laboratory studies using both eggs and larvae of E.postvittana to determine the importance of plant effects in this tritrophic system. In the field, we found that larval parasitoids were not influenced by the architecture of the host plant species, in contrast to the egg parasitoid, Trichogramma fasciatum, that was better represented on taller plants with fewer and larger leaves and less branching. We found similar results to the field study using laboratory cage studies with T. fasciatum. For individual leaves, we found that T. fasciatum had shorter egg location times on non-hairy leaves compared to leaves with a high density of trichomes. These results demonstrate that host plant effects on parasitoids can vary with the life stage of the herbivore, in this case being significant for egg parasitism but not for larval parasitism. As the egg stage of E. postvittana does not damage host plant tissue, host plant architecture and leaf structure are more likely to account for the differential success of T. fasciatum on different host plant species than host plant volatile signals.