Food plant legacy and tri-trophic interactions in the context of novel invasive species associations

Background/Question/Methods

Positive interactions between exotic species could enhance one another’s invasion potential and accelerate the invasion process. Although, invasive species may experience an advantage, how resistance by natural enemies is influenced is unknown. The polyphagous light brown apple moth, Epiphyas postvittana, is a recent invader in California, and continues to expand its range into climatic suitable regions. Previous work has shown that E. postvittana performance was better on invasive host plants compared to native plants, implying accelerated spread using abundant invasive plants. Recently, 18 parasitoid species utilizing different life stages of E. postvittana have been collected of which some reach high rates of parasitism. Thus, whether the ongoing invasion of E. postvittana will maintain to be successful depends on both host plant effects and the efficacy of the parasitoids.

In a series of greenhouse experiments we investigated how the performance of three of the most significant parasitoids in the field are influenced by host plant origin (invasive or native) mediated through their invasive host. We tested egg to adult development time, sex ratio and adult body size of the egg parasitoid Trichogramma fasciatum, the larval parasitoid Meteorus icteris and the pupal parasitoid Pediobius ni. The parasitoids were offered E. postvittana with feeding histories on 3 host plants invasive to California and 3 native congeners.

Results/Conclusions

Only development time of P. ni showed a difference in that it was faster in hosts from invasive plants, however, depending on host size. Although highly variable among plant species, there was no plant origin effect on development time and sex ratio of T. fasciatum. We also show that the larval development time of M. ictericus was shorter in hosts that fed on invasive plants compared to native plants and that this was positively related to plant nitrogen levels, however, this was not reflected in their body size. Our previous work suggests that prior invasion of exotic plants could function as a catalyst for the subsequent invasion of an exotic insect herbivore, thereby enhancing the invasion process and accelerating expansion of its novel geographic range. However, our results on the parasitoid performance suggest that the advantage of positive interactions between invasive species can be hindered by natural enemies when their performance is not different between hosts from invasive and native plants or also enhanced in hosts with an invasive host plant legacy.