SURVIVAL OF HEMLOCK WOOLLY ADELGID (HOMOPTERA, ADELGIDAE) AT LOW-TEMPERATURES

The hemlock woolly adelgid (Adelges tsugae Annand [Homoptera: Adelgida e]) (HWA), a nonnative insect, is a major threat to New England forest and suburban environments, where one of its hosts, eastern hemlock, T suga canadensis (L.) Carriere, is a dominant native tree species. To d etermine the potential spread of HWA into northern New England, we stu died the winter coldhardiness of a northerly population. Sistens were collected at three times during the winter and subjected to temperatur es typical of those during winter in the next coldest USDA plant coldh ardiness zone. Samples collected in January and February 1996 were sub jected to -20, -25, and -30 degrees C; those collected in March to -15 , -20, and -25 degrees C; all date-temperature treatments were tested at exposure times of 1, 2, 4, 8, and 24 hr. When all exposure times we re combined, the percentage of live adelgids after subzero treatment ( adjusted for control mortality at 0 degrees C) declined significantly as temperature decreased. In most cases, fewer HWA were alive after tr eatment as exposure time increased, Significantly more HWA collected i n January were alive after treatment than those collected in February and March. This suggests that HWA coldhardiness declines over this tim e period. When com paring the percentage of live HWA from February and March for -20 and -25 degrees C, a noticeable decline occurred betwee n the February and March sample dates. Although mortality was as high as 95% in one of the most severe treatments (February collection expos ed to -30 degrees C for 24 hr), the fact that complete mortality did n ot occur suggests that HWA may be able to continue to expand its range into areas with colder winters.