Rebound of Adelges tsugae spring generation following predation on overwintering generation ovisacs by the introduced predator Laricobius nigrinus in the eastern United States

Hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), has devastated eastern hemlock (Tsuga canadensis [L.] Carriere) in a major portion of its native range in eastern North America. Population dynamics of HWA in the absence of predators have been studied for decades. After many years and much effort directed towards rearing and releasing biological control agents to manage HWA, one of these agents, Laricobius nigrinus Fender (Coleoptera: Derodontidae), is now successfully established at significant densities at sites from the southern to the mid-Atlantic states of the eastern U.S. However, high densities of HWA still persist at many locations throughout the region and spread of HWA and associated damage to hemlock continues. Population models for HWA have suggested that even upwards of 90% predation on eggs laid by the overwintering sistens generation will have minimal effect in reducing the population densities of HWA, if HWA are at high density. In this study, we tested the ability of L. nigrinus to reduce HWA densities, and experimentally tested these model predictions to better understand what impact, if any, L. nigrinus has on HWA densities. By using predator exclusion cages at field sites with well-established populations of L. nigrinus, we were able to record HWA densities, fecundity, overwintering mortality, and predation by L. nigrinus, as well as the proportion of branch tips producing new growth on study trees. Using our field-collected data, we refitted the model in ways that allowed us to predict what population densities we could expect for the following summertime progrediens generation given previous HWA density and levels of L. nigrinus. In both years, we found that despite high rates (greater than 80% ovisac predation) of predation by L. nigrinus on uncaged branches compared to caged branches, there were no significant differences in subsequent densities of the HWA spring generation between caged and uncaged treatments, as predicted by our model. In 2018, our field-collected densities of the summer progrediens generation were lower than what was predicted by the model in both predator exclusion treatments, possibly due to the model not incorporating tree health and climatic factors. Simulation models of pest insect populations based on field-collected data such as fecundity, density, overwintering mortality, and predation, could prove to be important in informing researchers and managers about the role of the biological control agent in the population dynamics of the target host.