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The terpenoid content of eastern hemlock (Tsuga canadensis) foliage was measured over an annual cycle of development from bud opening, shoot elongation, shoot maturation, to bud-break at the start of the next growing season. The objective was to determine if variation in terpenoid composition is linked with spatial and temporal feeding preferences of the hemlock woolly adelgid (HWA; Adelges tsugae). The HWA has two periods of feeding over the course of 1 yr spanning two complete generations. There are two periods of feeding separated by a nonfeeding period where the adelgid estivates. HWA prefers to feed on mature, rather than young, expanding tissue. Feeding occurs in the leaf cushion at the base of the needle. The needle is the only tissue in hemlock with resin canals that store terpenoids. The needle and leaf cushion of both the current and previous years’ growth were analyzed separately …