Hemlock woolly adelgid, Adelges tsugae Annand, is an exotic forest pest threatening the health of New England’s eastern hemlock, Tsuga canadensis (L.) Carrière. Research indicates that the northern spread of this insect may be slowed or prevented by cold temperature. We conducted laboratory studies to assess the coldhardiness of A. tsugae, collected at sites within plant hardiness zones 5a (Northern), 6a (Central), and 6b (Southern) in January, February, and March. Adelgids were exposed to −15, −20, −25, −30, and −35°C for 2, 4, or 8 h, and comparisons in survival were made with insects held at 0°C. For all months and sites, mortality increased as temperature decreased, and no survival occurred among those from the Central and Southern sites exposed to −30 and −35°C. In January and February, ≤3% of the adelgids collected from the Northern site survived −30°C, and none survived −35°C in January or March. Adelgids from all sites entered the winter with similar levels of coldhardiness, but those from the Central and Southern sites lost their tolerance to cold earlier in the season than those from the Northern site. In January and February, −25°C was a critical temperature at which significant reduction in coldhardiness was observed. Despite the evidence that adelgids in the Northern site possessed greater tolerance to cold temperatures than the other sites, only 14% survived exposure to −15°C in March. In all sites the actual percentage of adelgids that survived after exposure to −15°C decreased 50–60% from January to March. Our results show that adelgid coldhardiness differs depending on geographical location and time of year, and further confirm that cold temperature has a significant impact on their survival.
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