Fitness and physiology of the hemlock woolly adelgid, Adelges tsugae Annand, in relation to the health of the Eastern hemlock, Tsuga canadensis Carriere

Both population and physiological interactions between phytophagous insects and their host plants can determine the insect’s success. The hemlock woolly adelgid (HWA) is a small invasive insect that settles at the base of hemlock needles (Tsugae spp.). While infested eastern hemlock (T. canadensis Carriere) can die within as little as four years, many hemlocks survive for ten years or more in poor health. Healthy trees that become infested by the adelgid are depleted of nutrients, leading to a reduction of the adelgid population. It has been observed that hemlocks will make a partial health recovery, only to undergo re-colonization by the adelgid the following season. Since the adelgid is dependent on hemlock for nutrients, feeding on trees in poor health may affect the insect’s ability to obtain necessary nutrients for growth and reproduction. We assessed adelgid populations by examining their density, survival from aestivation, and fecundity in comparison to host tree health. We also assessed adelgid nutritional status by examining total carbon and nitrogen, amino nitrogen and total sugar in comparison to host health. Preliminary results on adelgid fitness indicate that adelgid fecundity is greater and survival is less on lightly impacted (healthy) hemlocks compared to less healthy trees. Adelgid nutritional uptake from hemlock increases as the insect matures and less nutrient-rich adelgids appear on moderately impacted hemlocks compared to healthy trees. These data will lead to greater understanding of the density dependent relationship between the adelgid and its host.