Hemlock Wooly Adelgid Case

Eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) appear in mesic flats, draws, ravines, coves, and canyons of the Southern Appalachian Mountains (Whittaker 1956). Although once more abundant in the forest, hemlock populations declined dramatically approximately 5,500 years ago due to climatic shift resulting in summer droughts that weakened the hemlocks and left them vulnerable to a subsequent widespread insect outbreak (Allison and others 1986, Davis 1981, Haas and McAndrews 2000). In its northern range, canopy gaps were filled by Acer, Betula, Fagus, Pinus, Quercus, and Ulmus (Fuller 1998). Although hemlock did re-establish itself, its recovery may have taken up to 2,000 years and, in many sites, is still not as prominent as it was before the decline (Fuller 1998, Haas and McAndrews 2000). Now, hemlocks are at risk from the invasive exotic insect pest HWA .

In its native Japan, HWA populations are maintained at low densities on hemlocks (Tsuga diversifolia and T. sieboldii) by a combination of host resistance and natural enemies (McClure 1992, 1995a, b; McClure and others 2000). The first report of HWA in North America was in the Pacific Northwest in the 1920s; however, western hemlocks were resistant to the adelgid. In the Eastern United States, the first reports of HWA were in 1951 in Richmond, Virginia (Gouger 1971; McClure 1989, 1991). With no natural resistance or natural predators, HWA slowly made its way northeast and has subsequently been moving southwest along the eastern side of the Appalachian Mountains. Little is known about stand-level characteristics that influence HWA susceptibility in the Southeastern United States. However, studies on HWA infestation levels in the northeastern range of this insect noted only latitudinal effects on infestation severity (Orwig and Foster 1998, Orwig and others 2002). This would seem to suggest that all hemlock stands have the potential of being infested and killed, regardless of site and stand factors.