Evaluating Seedlings of Eastern Hemlock Resistant to Hemlock Woolly Adelgid (\u3cem\u3eAdelges tsugae\u3c/em\u3e)

Hemlock woolly adelgid (HWA), Adelges tsugae Annand has invaded the North American range of the eastern hemlock, Tsuga canadensis (L.) Carriere, and Carolina hemlock T. caroliniana Englemann, causing widespread mortality. Of the nine Tsuga species known worldwide, only T. canadensis and T. caroliniana experience mortality from HWA. Researchers at the University of Rhode Island and their cooperators have identified stands of HWA-resistant eastern hemlocks and confirmed this resistance through studies of vegetatively propagated progeny. This study investigates HWA resistance of plants grown from field-collected open-pollinated seed from these parent plants with known resistance. Trees were evaluated under standardized conditions, artificially inoculated with HWA and measurements of tree growth and HWA density were taken during the settlement phase of each HWA generation. Based upon published literature, HWA densities achieved in this study should have inhibited new growth and eventually killed at least some of the test plants. However, in this study, heavily infested seedlings continued to grow throughout the three years of observation and there is as yet, no HWA-induced tree mortality. It is possible that the HWA population used in this experiment is relatively non-virulent, not inducing the hypersensitive reaction of infested trees which causes declining health of both the trees and the HWA population