INTRASPECIFIC VARIATION IN ASPEN PHYTOCHEMISTRY - EFFECTS ON PERFORMANCE OF GYPSY MOTHS AND FOREST TENT CATERPILLARS

Individual quaking aspen trees vary greatly in foliar chemistry and su sceptibility to defoliation by gypsy moths and forest tent caterpillar s. To relate performance of these insects to differences in foliar che mistry, we reared larvae from egg hatch to pupation on leaves from dif ferent aspen trees and analyzed leaf samples for water, nitrogen, tota l nonstructural carbohydrates, phenolic glycosides, and condensed tann ins. Larval performance varied markedly among trees. Pupal weights of both species were strongly and inversely related to phenolic glycoside concentrations. In addition, gypsy moth performance was positively re lated to condensed tannin concentrations, whereas forest tent caterpil lar pupal weights were positively associated with leaf nitrogen concen trations. A subsequent study with larvae fed aspen leaves supplemented with the phenolic glycoside tremulacin confirmed that the compound re duces larval performance. Larvae exhibited increased stadium durations and decreased relative growth rates and food conversion efficiencies as dietary levels of tremulacin increased. Differences in performance were more pronounced for gypsy moths than for forest tent caterpillars . These results suggest that intraspecific variation in defensive chem istry may strongly mediate interactions between aspen, gypsy moths and forest tent caterpillars in the Great Lakes region, and may account f or differential defoliation of aspen by these two insect species.