Yearly Variations in the Overwintering Mechanisms of the Cold-Hardy Beetle Dendroides canadensis

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Abstract
Successful overwintering by insects is dependent primarily on one of two modes of adaptation: the ability to survive freezing (freezing tolerance) and the ability to avoid freezing by supercooling (freezing susceptibility). Studies on the larvae of the beetle Dendroides canadensis from northern Indiana during the 197 7-1978 and 1978-1979 winters revealed that this species was freeze tolerant, exhibiting supercooling points (SCP) between -8.0 to - 12.0 C with lower lethal temperatures (LLTs) of - 28.0 C. We report here that D. canadensis have switched from a freeze-tolerant to a freeze-susceptible mechanism of overwintering. During the winter of 1981-1982, larvae exhibited extensive supercooling (averaging approximately -26.0 C). The LLTs corresponded to their SCP temperatures; therefore, they could not tolerate freezing at that time. Evidence accumulated during 1979-1980 and 1980-1981 suggests that the switch in overwintering adaptations occurred between these winters. The only notable change in the cold-hardening parameters of D. canadensis throughout this time involves the apparent loss of ice nucleating proteins. Regardless of the overwintering strategy employed, LLTs are similar from year to year. However, this is the first known instance where the overwintering stage of a species has been observed to display both types of adaptations.