Insect Winter Ecology - Freeze Tolerance

Freeze Tolerance

Freeze tolerance in insects refers to the ability of some insect species to survive ice formation within their tissues. All insects are ectothermic, which can make them vulnerable to freezing. In most animals, intra- and extracellular freezing causes severe tissue damage, resulting in death. Insects that have evolved freeze-tolerance strategies manage to avoid tissue damage by controlling where, when, and to what extent ice forms. In contrast to freeze avoiding insects that are able to exist in cold conditions by supercooling, freeze tolerant organisms limit supercooling and initiate the freezing of their body fluids at relatively high temperatures. Physiologically, this is accomplished through inoculative freezing, the production of ice nucleating proteins, crystalloid compounds, and/or microbes.

Although freeze-avoidance strategies predominate in the insects, freeze tolerance has evolved at least six times within this group (in the Lepidoptera, Blattaria, Diptera, Orthoptera, Coleoptera, and Hymenoptera). Freeze tolerance is also more prevalent in insects from the Southern Hemisphere (reported in 85% of species studied) than it is in insects from the Northern Hemisphere (reported in 29% of species studied). It has been suggested that this may be due to the Southern Hemisphere's greater climate variability, where insects must be able to survive sudden cold snaps yet take advantage of unseasonably warm weather as well. This is in contrast to the Northern Hemisphere, where predictable weather makes it more advantageous to overwinter after extensive seasonal cold hardening.

Examples of freeze tolerant insects include: the woolly bear Pyrrharctia isabella, the flightless midge Belgica antarctica and the alpine cockroach Celatoblatta quinquemaculata.