Naturally-occurring resistance and a defensive hypersensitive response in the eastern hemlock (Tsuga canadensis (L.) Carriere) to the invasive hemlock woolly adelgid (Adelges tsugae Annand)
The hemlock woolly adelgid (Adelges tsugae Annand; ‘HWA’) is an invasive insect that is killing eastern hemlock trees (Tsuga canadensis) throughout the eastern United States. Although HWA and elongate hemlock scale (Fiorinia externa Ferris; ‘EHS’) are both sessile sap feeders that feed on eastern hemlocks, HWA causes much more damage than EHS. For the first part of my master’s research, I assessed one of the mechanisms potentially responsible for the deadly effect of HWA feeding on eastern hemlock. The rapid rate of tree death following HWA infestation has led to the suggestion that HWA feeding induces a defensive hypersensitive response (HR) in hemlock trees. I assessed the potential for an herbivore-induced HR in eastern hemlocks by measuring H2O 2 levels in foliage from HWA-infested, EHS-infested, and uninfested trees. Both insects induced a localized response at the site of infestation, suggesting the presence of a localized hypersensitive response. Additionally, an HWA-induced hypersensitive response was detectable in undamaged tissue, suggesting that HWA infestation induces a ‘systemic’ response in eastern hemlocks. There was no systemic defense response in EHS-infested trees. This strong hypersensitive response in HWA-infested trees may be one reason why the hemlock woolly adelgid is so harmful to eastern hemlock. ^ Preliminary results suggest that some rare eastern hemlocks possess naturally-occurring resistance to this pest. During landscape-level forest surveys, rare surviving eastern hemlocks were found in otherwise adelgid-devastated forests. The second part of my master’s research involved the evaluation of grafted cuttings from these putatively resistant trees. In March 2010, cuttings were taken from mature putatively resistant forest trees in CT and NJ and brought back to the University of Rhode Island where they were grafted according to standard protocols. Putatively resistant scions were grafted onto adelgid-susceptible hemlock rootstock from a Michigan nursery, and scions from Michigan trees were grafted back onto Michigan rootstock as a control. Ungrafted trees from the Michigan nursery were included to assess the effects of grafting on adelgid survival. In order to remove environmental differences and assess genetic variation in HWA-resistance, resistant plants were grown alongside controls in a common garden experiment. All trees were inoculated with HWA in April 2011, and adelgid density (# HWA/cm hemlock growth) was monitored from May through October on old and new growth foliage. Using repeated-measures ANOVA, we found no significant differences in HWA densities between putatively resistant and grafted control trees on both old and new growth. However, we found that ungrafted controls had higher HWA densities than all grafted trees. By the end of the season, the second generation of adelgids on ungrafted control trees had between three and four times higher HWA densities than on grafted trees. This suggests that the process (and, presumably, associated stress) of grafting generally reduces hemlock susceptibility to HWA, rendering both control and resistant trees poor hosts for at least 18 months. Although this study demonstrates reduced adelgid survival on grafted plants, our plants apparently had not recovered from the stress of propagation and this test cannot be used to assess grafting as a means of propagating and evaluating putatively resistant trees. ^
Biology, Ecology|Agriculture, Forestry and Wildlife
"Naturally-occurring resistance and a defensive hypersensitive response in the eastern hemlock (Tsuga canadensis (L.) Carriere) to the invasive hemlock woolly adelgid (Adelges tsugae Annand)"
Dissertations and Master's Theses (Campus Access).