Cold Temperatures . Will They Limit Range Expansion ?

The hemlock woolly adelgid (HWA), Adelges tsugae Annand, primarily infests trees along the eastern seaboard in USDA plant hardiness zones where the minimum low temperature does not exceed –28.8°C. Laboratory studies were done to determine the low lethal temperature of HWA collected in January, February, and March from locations within different USDA plant hardiness zones. The Mt. Tom, Massachusetts HWA population, which exists in the coldest location, does not survive under laboratory conditions (at –35 to –40°C). It was evident that HWA cold hardiness declined as the winter progressed. HWA that has an ability to tolerate short periods of exposure to low temperatures also were able to tolerate longer periods. This suggests that time of exposure is not a major factor in survival. HWA cold hardiness differs depending on the geographical location of the population and the time of year. Most sensitive were populations collected from a site with a low minimum temperature not exceeding –20.5°C. In March only about 5% of the HWA survived an exposure of –20°C. Only HWA collected from Mt. Tom were able to survive an exposure of – 30°C. The northerly spread of this pest may be impeded or even prevented by the cold winter temperatures in more northerly locations.