Winter moth, the polyphagous caterpillar Operophtera brumata, was accidently introduced to the northeastern United States in the 1990s. Previous invasions of winter moth in Canada were successfully suppressed by the introduction of a parasitic fly Cyzenis albicans from winter moth’s native range. Following these successes, C. albicans has now been established at sites across the northeastern U.S. where establishment has coincided with a dramatic decrease in winter moth density. However, this success likely depends on additional mortality from native natural enemies including predators and parasitoids. In the native range of Europe, pupal predators were found to regulate winter moth densities. Further, in the two invasive populations of winter moth in Canada, predation was found to increase following the introduction of the biocontrol agents. Our study builds on this earlier research to evaluate the role of pupal predators and parasitoids. Over five field seasons, we deployed winter moth sentinel pupae in the field to determine rates of predation and parasitism. We deployed pupae across 14 sites in the northeastern U.S. across a range of winter moth pupae and C. albicans puparia densities.
Results/Conclusions
Overall, mortality on the pupae was high across sites and years, ranging from 85 to 95% and was primary caused by a diverse community of generalist ground predators including carabid beetles, staphylinid beetles, and shrews. In years when winter moth densities were high, we did not observe density dependent mortality; however, in the most recent year as densities have decreased to a level comparable to what was found it its native range, mortality on the pupae was density dependent (pseudoR2=0.39; DF=19; p=0.036) and has the potential to stabilize the population at low density. In contrast to the Canadian introductions, we found that pupal mortality has not increased with C. albicans establishment but has been high throughout the years of study. However, in the last three years, we have detected high parasitism rates by an Ichenumonid wasp Pimpla spp., which is causing additional mortality on the pupae (15 to 40% after predation). Overall, our research suggests that mortality on winter moth pupae was already high in the northeast, but that the introduced biocontrol agent provides enough additional mortality to render winter moth a non-pest.