Examining the Role of Foliar Chemistry in Hemlock Woolly Adelgid Infestation and Hemlock Decline

Abstract Preliminary data from 45 hemlock woolly adelgid monitoring plots across the Northeast indicate that a suite of macronutrients is strongly associated with adelgid infestation and hemlock decline. Although it is unclear whether specific chemical characteristics cause (palatability effect) or result (defensive strategy) from infestation, N, Ca, K, and Mn were all significantly (p < 0.01) higher on infested trees. By adding Al, lignin, and cellulose, infestation was predicted with 89% accuracy. A logistic model indicates that very small degradations in hemlock health also are detectible using foliar chemical concentrations. Aluminum, calcium, phosphorus, and nitrogen all increased with severity of decline, predicting a 10-class hemlock health rating (to within one class) with 89% accuracy. A comparison of eastern hemlock to resistant species indicated that resistant species had significantly lower levels of nitrogen, and higher concentrations of P, lignin, and Al. These same elements were significant when examining intraspecies susceptibility. However, a predictive model was only 68% accurate. Although this data is not sufficient to explain why these relationships exist, it is clear that the examination of foliar chemistry (in addition to landscape variables) may help explain some of the variability in hemlock decline and infestation patterns. Keywords: