Bennett et al. 2005

Bennett VA, Sformo T, Walters K, Toien O, Jeannet K, Hochstrasser R, Pan Q, Serianni AS, Barnes BM, Duman JG. 2005. Comparative overwintering physiology of Alaska and Indiana populations of the beetle Cucujus clavipes (Fabricius): roles of antifreeze proteins, polyols, dehydration and daipause. Journal of Experimental Biology 208: 4467-4477.

These authors measured various aspects of winter survival in the larvae of a bark-dwelling beetle, in two locations (Alaska and Indiana) over three years. These larvae produce antifreeze proteins (AFP) and polyols such as glycerol that lower their supercooling points considerably. Dehydration in Alaskan insects increases the haemolymph concentration of these AFPs and further depresses the supercooling point of body tissues. Previous reports of this species’ overwintering abilities suggested they were freeze tolerant, but this paper strongly suggests they are freeze avoiding, and may be capable of body water vitrification under some extreme circumstances.

To examine the distinction between freeze tolerance and freeze avoidance, these authors measured supercooling points, thermal hysteresis activity, body water content, polyols content, and respiration rates in larvae. Supercooling points were taken as the temperature at which exotherms were recorded, when the heat of fusion of water was released by ice formation. All larvae that produced exotherms died, while all larvae that were cooled to just above expected supercooling points but were not frozen survived. No larvae produced exotherms colder than -58°C, and about half of those cooled lower than this survived, which was taken by these authors as indirect evidence of body water vitrification. Further analyses would be necessary to confirm this suggestion.

The differences in survivability of winter temperatures shown by Alaskan and Indianan larvae may relate primarily to the timing of AFP production. Alaskan larvae synthesize AFPs much earlier in the season than do Indianan, and are capable of survival in Indiana while Indianan larvae all died when overwintered in Alaska. The extreme dehydration of Alaskan larvae prevented measurement of body water content and AFP concentration, thus the relative importance of amounts produced, timing of production, and duration and severity of winter could not be determined.

In summary, there appears to be a genetic component to differences in larval overwintering capabilities across this species’ very broad latitudinal range. The physiological mechanism of this difference, either timing, magnitude, or composition of the production of antifreeze molecules, is not clear.