Hemlock woolly adelgid (Adelges tsugae, 'HWA'), an invasive insect pest of eastern hemlock (Tsuga canadensis), causes near-100% mortality of this tree species within a few years of infestation. Occasionally, however, single hemlock trees appear resistant to HWA and persist following infestation despite mortality of adjacent individuals. We report on an effort to experimentally evaluate these rare individuals. We rooted cuttings taken from 18 potentially-resistant eastern hemlocks and grew them under controlled conditions. Prior to inoculating the cuttings with HWA, we examined the chemical profiles of compounds which have been suggested to be related to HWA resistance. We used standard techniques to compare the terpenoid profiles of four contrasting groups: (1) rooted cuttings from the potentially-resistant parent trees; (2) foliage from the potentially-resistant parent trees from which the rooted cuttings were taken; (3) an uninfested control group of eastern hemlocks growing north of the current northern limit of HWA; and (4) a known HWA-resistant congener, the western hemlock (Tsuga heterophylla). We used classification trees to discriminate between individuals based on their chemical profile.
Results/Conclusions
Of the seven terpenoids considered, the relative abundance of alpha-pinene, beta-caryophyllene, and myrcene discriminated resistant T. heterophylla from all other groups with an accuracy rate of 0.75. Parent T. canadensis trees were often misclassified as members of the control group, suggesting these two groups possessed broadly similar terpenoid profiles. In contrast, cuttings were strongly discriminated from parent, control, and resistant groups with an accuracy rate of 0.92, suggesting that the chemical makeup of these individuals is similar to one another and distinctly different from their parent plant, perhaps due to the rooting and potting process. These results suggest a high degree of intraspecific diversity of terpenoids and imply that a response to the propagation method may alter the chemical composition of the cuttings relative to their parent tree. Ongoing work will determine how terpenoid and cation profiles are related to variations in levels of resistance within eastern hemlock to HWA infestations.