Joshua Pezet, a masters student in OEB, studies an invasive insect pest that threatens one of our important native trees. The hemlock woolly adelgid (HWA) is a small but destructive invasive insect that was introduced to eastern North American hemlock trees in the 1950s. Infestation and feeding by HWA results in needle loss, branch die-off and tree death, often in as little as 2-4 years. The pest has already spread to hemlocks throughout a large expanse of the eastern United States and threatens to extirpate the trees from the region. Cold winter temperatures are believed to have checked the spread of HWA in central New England, and to be a major source of mortality for the insect at higher altitudes and elevations. However, recent studies tracking densities of adelgid on infested hemlocks throughout the year have shown that although substantial adelgid mortality does occur during the winter, even greater numbers of the insect appear to be succumbing even before cold winter temperatures set in.
The reason for this high adelgid mortality during during mid-summer and fall is unclear, but it has been hypothesized it may be due to chemical defenses mounted by the hemlocks themselves. Terpenes, a class of compounds comprising a major component of conifer foliage, have often been implicated in important plant-herbivore interactions, particularly resistance against herbivort. There is preliminary evidence that certain terpenes may have a toxin effect on the adelgid, but careful studies relating seasonal and annual fluctuations in hemlock foliar terpene levels to adelgid mortality and fecundity are lacking. By manipulating hemlock foliar chemistry through controlled soil fertilization treatments, laboratory analysis of foliage of both manipulated and wild, control trees, and careful tracking of seasonal and annual population fluctuations of the adelgid, Joshua hopes to elucidate the relationship between foliar terpenes and the susceptibility of eastern hemlocks to the hemlock woolly adelgid.