Extreme Supercooling as an Overwintering Strategy in Three Species of Willow Gall Insects from Interior Alaska

(1) Seasonal cold-hardiness was studied in insect larvae associated with three different types of willow galls. (2) Mean supercooling points in summer were unusually low, ranging from -25.5°C to -29.4°C. Winter supercooling points were the lowest that have been recorded for any insects and ranged from -56°C to -58°C, with a number of individuals of each larval species exhibiting supercooling points of -61° to -64°C. (3) Low supercooling points in winter were associated with accumulation of glycerol averaging between 4 and 6 molal in the three species. Melting points of body fluids were greatly depressed and averaged -14° and -18°C in two species. (4) None of the willow gall larvae were freezing-tolerant, and all relied on freezing avoidance by extreme supercooling in order to survive low winter habitat temperatures. (5) Injection of winter larvae with efficient ice-nucleators raised the supercooling point approximately 25°C but did not confer freezing-tolerance.