Low Lethal Temperature for Hemlock Woolly Adelgid (Homoptera: Adelgidae)

Abstract

The hemlock woolly adelgid, Adelges tsugae Annand, is not currently found in the U.S. Department of Agriculture plant coldhardiness zones colder than 5a in New England. Cold winter temperatures may be a factor limiting the northern expansion of this pest. Laboratory studies were conducted to determine the low lethal temperature of A. tsugae collected in January, February, and March 1997 from Holyoke, MA, located in plant coldhardiness zone 5a. Adelgids were exposed to temperatures of −20, −25, −30, −35, and −40°C for 2, 4, and 8 h. To correct for experimental mortality, comparisons in survival were made with untreated controls held at 0°C. For samples collected in January and February, a gradual decline in survival occurred as temperatures decreased. In March, differences in adelgid survival were not significant below −20°C. In this month, the percentage of live adelgids after cold treatment was ≤11% for −20°C and <2% for −25 and −30°C. In all months, no adelgids survived after exposure to −35 or −40°C. The minimum low temperature range for plant coldhardiness zone 4b, the next coldest zone north of the Mt. Tom infestation, is −28.9 to −31.6°C and further north in zone 4a it is −31.7 to −34.4°C. Therefore, the northerly spread and establishment of A. tsugae may be impeded or perhaps prevented by the cold climate of northern New England. However, our studies are laboratory based and may not completely reflect what occurs in nature and adelgids along the leading edge of the infestation may develop an ability to tolerate colder temperatures in the future.