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Impact of Predators on Hemlock Woolly Adelgid (Hemiptera: Adelgidae) in the Eastern and Western United States

Abstract
Hemlock woolly adelgid, Adelges tsugae, native to Asia and the Pacific Northwest of North America (Pacific Northwest), has devastated eastern hemlock (Tsuga canadensis) in a major portion of its range in the eastern U.S. After many years and much effort directed towards rearing and releasing biological control agents to manage HWA, one of these agents, Laricobius nigrinus, native to the Pacific Northwest, is now well-established in sites from the southern to the mid-Atlantic states of the eastern U.S. However, there have yet to be studies of its efficacy in lowering A. tsugae densities, and there has been no noticeable drop in A. tsugae densities. Population models for A. tsugae have suggested that even upwards of 90% predation on eggs laid by the overwintering generation will have minimal effect in reducing the population densities of A. tsugae, if A. tsugae are at high density, due to compensatory density-dependent survival in the progrediens generation. Additionally, no studies showing insect predators are indeed what regulate A. tsugae in its native range exist. We established predator exclusion experiments, and recorded A. tsugae densities, mortality factors, and fecundity for multiple generations in both the native and invaded ranges. In the invaded range, we studied A. tsugae populations in sites with well-established populations of L. nigrinus to test its efficacy in reducing A. tsugae and tested model predictions of minimal difference in A. tsugae densities between treatments with and without predators. In the Pacific Northwest we tested the effect of insect predators and tree species, western (Tsuga heterophylla) and eastern hemlock, on populations of A. tsugae. In the invaded range we found that L. nigrinus predation was significantly higher in unbagged branches, however, model predictions were validated, and there was no effect of predation by treatment on the A. tsugae summer generation. In our plots in the Pacific Northwest we found that tree effects were not significant, but that summer-active predators were significantly lowering levels A. tsugae densities on unbagged branches. Our study demonstrates the importance of summer-active predators in reducing A. tsugae and suggest that summer- and winter-active predators are needed to suppress A. tsugae to innocuous densities.
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