Direct effect of high temperatures on Adelges tsugae mortality

The hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), is an exotic sap-sucking insect that is causing widespread mortality of both eastern and Carolina hemlocks in North America. The life cycle of HWA includes two parthenogenetic generations on hemlock, sisten and progredien, with summer dormancy (aestivation) lasting up to five months. Unexpected trends were observed at low elevation sites (< 500 m) in Georgia where hemlock trees were in low decline following 6-7 years of HWA infestation instead of anticipated mortality 3-5 years post infestation. A controlled experiment was designed to test if high temperatures and lengths of heat waves experienced during summer months in Georgia may affect aestivating HWA sisten survivorship. During the summer of 2011, twigs with aestivating HWA sistens were collected from eastern hemlock trees in northern Georgia. Live sistens were selected (n = 4,279) and twigs were then randomly placed in one category for each treatment: temperature (20°, 25°, 30°, 35°, or 40°C) and length of time it would remain in that temperature chamber (2, 4, 6, or 8 days). Logistic regression showed a significant (p < 0.001) positive correlation between increasing temperature/duration and percent HWA mortality. Results from this experiment will contribute to future climatic models that will more accurately predict ecological impacts of exotic insects, and further aid in the management of HWA by predicting locations where biological control may have more time to become effective.