The Relative Abundance and Diversity of Parasitoids of the Browntail Moth (Euproctis chrysorrhoea L.) and Factors that Influence Their Population Dynamics

The browntail moth (Euproctis chrysorrhea) is an invasive forest pest that has been present in the Northeast since it was first introduced from Europe in 1897. Originally, its range expanded very rapidly until it reached its peak invasion of 150,000 km2, which included most of New England and parts of Southern Canada and Long Island, NY, in 1915. After this point, its range collapsed until only relic populations remained on islands in the Casco Bay Region of Maine and outer Cape Cod in Massachusetts. In 2016, a large population outbreak occurred that expanded its range into inland Central Maine and appears to be continuing to expand north and east today. Our research aims to assess the relative abundance and diversity of parasitoid natural enemies present within the browntail moth population and the factors that influence browntail moth population dynamics over this outbreak period. We sampled browntail moth in infested areas across mid-coast and central areas of Maine to assess overwintering survival, larval and pupal density, healthy moth emergence, and parasitoid diversity and abundance. We estimated densities using timed 10-minute density counts, while survival and emergence was estimated by rearing out samples. Models to test important factors for both hosts and parasitoids were conducted, where year, distance to coast, age of infestation, habitat, and annual climate variables were tested. Nine parasitoid species were recovered from browntail moth pupation nests, three of which were hyperparasitoids. The highest parasitism occurred from Townsendiellomyia nidicola, a primary parasitoid accounting for 24 percent and Monodontomerus aerus, a hyperparasitoid accounting for 36 percent parasitism across all years. Between 2016 and 2018, hyperparasitoids increased in their proportion and mean percent parasitism while primary parasitoid decreased in these regards. Negative binomial results indicated habitat, year, and total annual precipitation were the most significant factors for the abundance of parasitoids, where distance to coast and age of the infestation were not significant. The mean number of pupation nests per 10-minute density count increased slightly in 2017 (28 ± 6) but decreased in 2018 (20 ± 4). In comparison, the mean rank of MFS winter hibernacula per tree decreased across all years. Browntail moth post-diapausing larvae, late-stage larvae, and pupation nests significant decreased at coastal sites in 2017 while inland sites significantly increased in 2018. Moth survival, however, significantly increased between 2017 and 2018. Negative binomial results indicated that habitat was an important factor across post-diapausing larvae, late-stage larvae, and pupation nests while post-diapausing larvae was the only significant factor for moth abundance. Abundance significantly decreased in 2017 across all browntail moth life stages, likely due to an epizootic outbreak of an entomopathic fungi. This study presents data that indicates a higher incidence of hyperparasitoids that may negatively impact primary parasitism, which could positively impact browntail moth survival. Both parasitoids and hosts were negatively impacted in 2017, likely due to an epizootic outbreak, another factor that may drastically impact population dynamics. The data presented gives new insight to the current population dynamics of browntail moth and their parasitoids