Recruitment of native parasitic wasps to populations of the invasive winter moth in the northeastern United States

Ecological communities may be resistant to invasive species through a combination of top-down and bottom-up mechanisms, including predation, competition, parasitism, and disease. In particular, natural enemies that cross over from native species to use newly introduced non-native species as hosts can influence invasive species population dynamics and may slow down invasions. However, research on parasitism in biological invasions is lagging behind research on biological invasions in general. We used the model species winter moth (Operophtera brumata) to study the effect of recruitment of native parasitoids on an invasive population of winter moth in the northeastern United States. We deployed sentinel pupae over 4 years across this population’s range, identified recovered parasitoids, and measured the rate of parasitism by native sources across years, seasons, invasion history, and host densities. Native Pimpla wasps inflicted 98% of the parasitism detected, resulting in an annual average of 15–40% mortality on pupae not depredated. Pimpla were present across all years, seasons, and sites. Where winter moth has invaded, parasitism was greatest when winter moth pupal density was high (i.e., positive density-dependent mortality) suggesting that Pimpla is helping to regulate the population. The wasps were morphologically identified as Pimpla aequalis Provancher; however, using a multilocus genetic comparison approach, they were determined to comprise two cryptic species. Overall, this study shows that recruitment of these native wasps to the invasive winter moth population is likely playing a significant role in regulating population outbreaks and is aiding in biological control of winter moth.