Temporal and spatial variation of terpenoids in eastern hemlock (Tsuga canadensis) in relation to feeding by Adelges tsugae

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 over a 1-yr period to examine the variation of terpenoid composition in space and time. Terpenoids were quantified by using headspace solid-phase microextraction/gas chromatography/mass spectrometry (SPME/GC/MS). New growth needles and leaf cushions do not resemble the previous year's growth either visually or in chemical composition until October/November, when the adelgid breaks estivation and begins feeding. Nearly all of the 23 terpenoids present exceeding 0.1% varied significantly either temporally or spatially, usually with complex interactions. Ordination and factor analysis revealed that terpenoids are less variable in mature leaf cushions than in young tissue. By entering a nonfeeding diapause during the late spring and summer, HWA avoids the unstable, variable levels of terpenoids in the immature leaf cushion and needles.