Often when we think of different organisms in the winter, we think of bears, squirrels, or other charismatic animals. Rarely do we think of the secret life of insects.
Most insects are poikilotherms, meaning that they rely on behavior and the environment to adjust body temperature. In the winter, ice can become a major problem for poikilotherms. To deal with ice, insects either adopt freeze tolerance or freeze avoidance strategies.
Insects that employ a freeze avoidance strategy are sometimes described as dry hibernators. These insects enter a supercooled state. Supercooled liquids have a depressed freezing point, which can be created by dissolving different solutes in the liquid.
In order to enter a supercooled state, insects convert glycogen to glycerol, which acts as an antifreeze. The insects also stop eating and evacuate the gut cavity, which eliminates potential for seeding ice. Insects also produce thermal hysteresis proteins to block ice formation.
Supercooled insects are in a very precarious position. If the supercooled liquid comes into contact with ice, the liquid will rapidly turn into ice. To prevent this from happening, insects have a waxy outer integument barrier which protects the supercooled liquids inside the insect from coming into contact with environmental ice.
The insect with the lowest supercooling point is the arctic willow gall insect, which can be supercooled to negative 66 degrees Celsius.
Freeze tolerating insects adopt quite a different strategy, which looks similar to the frog’s strategy. The freeze tolerators promote ice formation when the temperature dips below zero degrees Celsius. The ice is restricted to extracellular spaces in order to prevent ice formation within cells, and thus cell death. To promote ice growth in extracellular space, freeze tolerating insects produce ice nucleating proteins that allow ice to form more readily.
Some insects have the ability to switch between freeze tolerance and avoidance from year to year, and others blend the two strategies. Generally, freeze avoidance is more favorable in mild conditions or when the insect is active during the winter, while freeze tolerance is more favorable in extreme cold conditions.
References: Kevin Taylor, December 19, 2013