Top-down regulation of hemlock woolly adelgid (HWA), Adelges tsugae, in the Pacific Northwest of North America

Hemlock woolly adelgid (HWA) is a destructive forest pest in eastern North America, native to Asia and the Pacific Northwest (PNW) of North America. Since its accidental introduction to the eastern United States in the early 1900s, HWA has killed millions of eastern hemlock trees throughout much of their range. For the last 20 years, the biological control effort for HWA has focused on derodontid beetle, Laricobius nigrinus. However, research suggests that despite high levels of predation by La. nigrunus on HWA winter generation ovisacs, HWA fecundity is such that there is no impact on the resulting HWA spring generation population. To test the effects of bottom-up and top-down pressures on HWA in its native range, we established a predator exclusion experiment in Seattle, Washington using mature native western hemlock and mature plantings of eastern hemlock in Washington Park Arboretum. We found that HWA survival and proportion of branches infested with HWA, post-inoculation with HWA ovisacs, was higher on western hemlock than eastern hemlock suggesting that bottom-up effects didn’t influence HWA establishment on western hemlock. When comparing pairs of branches with and without predator exclusion bags, we found that predators, particularly summer-active predators, were responsible for significantly lowering HWA densities on non-bagged branches. Our results suggest that top-down forces are responsible for reducing HWA population densities in its native range in the PNW. These results support the addition of summer-active predators, Leucotaraxis argenticollis and Le. piniperda, from the PNW to the HWA biological control program in the eastern U.S.