Defense chemistry of eastern hemlock (Tsuga canadensis) putatively resistant to hemlock woolly adelgid (Adelges tsugae)

Recent studies have identified a small number of individual eastern hemlock trees (Tsuga canadensis) that show relative resistance to the introduced pest, hemlock woolly adelgid (Adelges tsugae, abbreviated HWA). This study investigates one potential mechanism for such resistance: the profile of volatile compounds released by hemlocks in response to herbivory. Eastern hemlock primarily releases terpene and terpenoid class volatiles, but its limited set of phenolic class volatiles have previously been implicated in the defense response to HWA. Using gas chromatography and mass spectrometry, we compare the volatile profile of susceptible and relatively resistant trees, both in the forest and in propagated clonal saplings in a common garden experiment. Preliminary results suggest resistant status may be characterized by elevated levels of a small set of compounds including phenolics and several terpenoids, accompanied by an overall decrease in terpenoid compounds. Ongoing analysis across seasons will explore whether the timing of changes in the volatile profile are relevant to resistant status.