Insect cold hardiness: A matter of life and death

It has often been stated that insects survive at low temperature by one of two main strategies, freeze tolerance and freeze avoidance by supercooling. Apart from the observation that the risk of freezing and its consequences is not the only injurious and possibly lethal effect of low temperature, it is interesting to consider a closely related question. If insects survive at low temperature by tolerating or avoiding freezing, under what circumstances do they die? A survey of the literature suggests there are five reasonably distinct situations representing decreasing levels of cold tolerance where death occurs (1) at some temperature below the freezing temperature or supercooling point (SCP), (2) when the organism freezes, (3) after prolonged chilling at moderate to low sub-zero temperatures, (4) after brief chilling at moderate to high sub-zero temperatures and (5) when temperatures are too low to maintain normal metabolism and the species is unable to enter any form of dormant state. This paper examines how the principles of freeze tolerance and freeze avoidance, derived primarily from studies on species from extreme environments, can be combined with observations on the cold tolerance of insects from less severe climatic zones, to produce a system of classification which allocates species to particular categories, using specific, ecologically-relevant criteria. The proposed new classes of cold hardy insects - freeze tolerant, freeze avoiding, chill tolerant, chill susceptible and opportunistic survival - thus form a continuum of distinct groups, from the most to least cold hardy, in a system of classifying insect cold hardiness that can be applied world-wide.