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.

The hemlock woolly adelgid (HWA), Adelges tsugae Annand, is an invasive pest of eastern hemlock, Tsuga canadensis (L.) Carriere and Carolina hemlock Tsuga caroliniana Englem. in the eastern United States. A newly reported beetle predator for HWA, Scymnus (Pullus) coniferarum Crotch (Coleoptera: Cocinellidae) preys on the pest in the western United States, and was approved for release in the eastern United States for the control of HWA. This research investigated the viability of S. coniferarum as a biological control agent of A. tsugae in the eastern United States, as well as the ecological dynamics between S. coniferarum and host prey species in its native range of western North America. In objective one, S. coniferarum predation, reproductive potential, and survival were evaluated in field-cages on adelgid infested T. canadensis in southwestern Virginia. Adult S. coniferarum fed on both generations and all life stages of A. tsugae at rates comparable to other adelgid-specific predators, and survived for extended periods of time in the field. In objective two, host-range tests for S. coniferarum were conducted in a series of no-choice and paired-choice feeding, oviposition, and development studies. Scymnus coniferarum adults fed on all adelgid species, and completed development on HWA and Adelges piceae Ratz. Scymmnus coniferarum oviposition was extremely low. In the final objective, Douglas-fir, Pseudotusga menziesii Mirb., Shore pine, Pinus contorta Dougl., western white pine, Pinus monticola Dougl., and western hemlock, Tsuga heterophylla (Raf.) Sarg. host tree species were sampled in Tacoma, Washington to investigate the life history of S. coniferarum and associated adelgid prey species in the western United States. Scymnus coniferarum adults were found on both pine species, Douglas fir, and western hemlock, and seemed to move between host tree species seasonally. Each host tree supports a different adelgid species, and a limited diet of strictly HWA in host-range tests could have contributed to low oviposition rates. This study suggested that S. coniferarum is a voracious predator of HWA in the field and laboratory. However, S. coniferarum laid very few eggs in laboratory studies, and zero eggs were recovered in field-cage analyses. This suggested that S. coniferarum may rely on multiple adelgid species to reproduce and establish in the eastern United States. / Ph. D. / Biological control is an economically and environmentally logical approach to pest management, through the introduction of a natural enemy or predator. This dissertation investigated the potential of Scymnus (Pullus) coniferarum as a biological control agent of the invasive pest species, hemlock woolly adelgid, otherwise known as HWA. The hemlock woolly adelgid is an invasive pest insect in the eastern United States, responsible for causing widespread death of eastern hemlock trees from southern Maine to Georgia. Scymnus coniferarum is a lady beetle that preys on HWA in the western United States, part of HWA’s native range. In objective one, S. coniferarum predation, reproductive potential, and survival were evaluated in field-cages on HWA-infested eastern hemlock trees in southwestern Virginia. Adult S. coniferarum beetles fed on HWA year-round, at a rate comparable to other successful biological control agents. In objective two, S. coniferarum beetles were confined to a selection of insect species similar to HWA and native to the eastern United States. Scymnus coniferarum predation, oviposition and development was observed in these tests in order to assess the potential impact of S. coniferarum on non-target species in the eastern United States. Scymnus coniferarum beetles fed on all adelgid species, and successfully laid eggs and developed to adults while feeding on the balsam woolly adelgid, another pest species in the eastern United States. Adult beetles laid very few eggs, and were very difficult to rear in a laboratory setting. In the final objective, Douglas-fir, Shore pine, western white pine, and western hemlock trees were sampled in Tacoma, WA to investigate the life history of S. coniferarum and other adelgid prey species in the western United States. Scymnus coniferarum adults were found on every species of sample tree except for Douglas-fir, and seemed to move among host trees seasonally. Host tree species support different adelgid species, and a limited diet of strictly HWA in host-range tests could have contributed to low oviposition rates. The lady beetle S. coniferarum was a voracious predator of HWA in field and laboratory studies. However, S. coniferarum laid very few eggs in laboratory studies, and zero eggs were recovered in field-cage analyses. This suggested that S. coniferarum may rely on multiple adelgid species to reproduce and establish in the eastern United States. This had either positive or negative implications regarding its viability to establish, flourish, and impact HWA in the eastern United States.