Eastern hemlock resistance to the woolly adelgid: Progress and prospects
The hemlock woolly adelgid (Adelges tsugae, ‘HWA’) is an invasive insect that attacks and kills eastern hemlock (Tsuga canadensis, ‘hemlock’) on the east coast of the U.S. Research has shown that other Tsuga species are not similarly affected by HWA; plant chemistry has been suggested as the basis for this resistance. Because hemlock has a large population size and reproduces sexually, rare individuals may exist that possess some degree of defense against HWA. We worked with the public, state and local organizations, and concerned researchers to identify potentially HWA-resistant trees growing in areas where HWA had caused large-scale tree mortality. We identified and tested potentially-resistant trees growing in two small stands, one in NJ and one in CT. Although neither stand had been treated with pesticides, each contained several mature trees (>10 m) that appeared healthy despite close proximity to large numbers of HWA-killed hemlocks. We conducted common-garden experiments with both grafted and propagated cuttings from putatively-resistant and known-susceptible trees. After inoculating all of the cuttings with adelgids, we assessed adelgid settlement and survival over time.
Both rooted and grafted cuttings from putatively-resistant trees had lower HWA settlement and survival than rooted and grafted cuttings from known-susceptible trees. In addition, twigs from resistant trees (sampled in situ and growing in a common-garden environment) had much higher terpenoid concentrations in both locations and over time, indicative of high constitutive defenses. These findings confirm naturally-occurring HWA resistance in eastern hemlock; the use of adelgid-resistant hemlocks in reforestation efforts might provide a useful complement to existing chemical and biological control efforts.