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Northern Research Station
One Gifford Pinchot Drive
Madison, WI 53726
(608) 231-9318
(608) 231-9544 TTY/TDD

Hemlock Woolly Adelgid

Host Plant Resistance

Although western hemlocks appear more resistant to hemlock woolly adelgid (HWA), hemlocks native to eastern North America (T. canadensis and T. caroliniana) are highly susceptible to injury and have an increased probability of mortality if infested by the insect. The more common eastern hemlock is found throughout New England, New York, Pennsylvania, and the middle Atlantic states, and populations extend southward in the Appalachian Mountains to northern Georgia and Alabama and westward to northeastern Minnesota.  Carolina hemlock is found in the southern Appalachians in southwest Virginia, northeastern Tennessee, western North Carolina, northwest South Carolina, and extreme northeast Georgia.  Because of the widespread geographic range of eastern hemlocks, their visibility, and the ecological niches they occupy on Federal, State, and private lands, the HWA has the potential to make large-scale changes to the landscape.

Although HWA has been in the western United States for more than 80 years and in the East for at least 50, little is known about the mechanisms that drive differences in hemlock resistance to this insect at either large continental scales (i.e., differences in susceptibility between western and eastern hemlock species) or at a stand-scale or inter-tree scale.  As management efforts are designed to mitigate the impacts of this invasive insect, information on the biotic and abiotic factors affecting resistance will prove crucial in adopting effective strategies.  Currently, the operating assumption is that natural enemies in its native Asian range as well as in western North America play a role in controlling HWA populations below damaging levels, but host plant resistance has not been adequately investigated and may play a large role in moderating adelgid populations. There are currently more than 200 cultivars of eastern hemlock; it has been assumed that all are equally susceptible to HWA. However, no evaluations have been conducted to determine if a range of resistance exists in T. canadensis or other Tsuga species.

Montgomery, M.E., Gottschalk, K. W. 2009. Research and development program on host plant resistance to hemlock woolly adelgid. In: McManus, Katherine A; Gottschalk, Kurt W., eds. Proceedings. 19th U.S. Department of Agriculture interagency research forum on invasive species 2008; 2008 Jan. 8-11; Annapolis, MD. Gen. Tech. Rep. NRS-P-36. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 52-53.

Selected Research Studies

Last Modified: 09/28/2010