POPULATION ECOLOGY Variation in Winter Survival of the Invasive Hemlock Woolly Adelgid (Hemiptera: Adelgidae) Across the Eastern United States

Thehemlockwoolly adelgid (Adelges tsugaeAnnand) is a small, aphid-like insect native to East Asia and western North America. First documented in the eastern United States in Richmond, VA, in 1951, it has spread to at least 17 states, where it causes increased mortality among both eastern and Carolina hemlocks (Tsuga canadensis Carrière and T. caroliniana Engelmann., respectively). Previousworkhas suggested lowtemperaturesmay limitnorthward spreadof theadelgid.Using recent surveys of A. tsugaemortality across the infested latitudinal gradient of the eastern United States, we show there is a signiÞcant positive relationship between minimum winter temperatures and winter survival at the landscape scale. The strength and nature of this relationship, however, varies through time,with absoluteminimumwinter temperatures explaining almost onehalf of the tree-level variance in survival in the spring of 2004 but only 9% in 2003. Post hoc analyses of the data suggest the explanatory power of temperature can be improved in ongoing studies by examining seasonal temperature proÞles. Previous studies have also suggested adelgid survival may be density dependent, and although these data support this observation, contemporary density is a poor predictor of adelgid survival at the landscape scale. Using landscape estimates of minimum winter temperature, we show two simple methods of estimating landscape-scale adelgid survival rates. Both methods suggest much of the range of T. canadensis in the eastern United States, and the entire range of T. caroliniana falls in areas where winter temperatures will not impose critical limits on A. tsugae populations.