Freezing temperatures are detrimental to many forms of life, including most insects. Insect are exothermic (cold-blooded), which means they cannot produce their own body heat. So to survive and thrive in climates such as ours, insects have developed several ways to deal with cold weather.
The first strategy is to avoid freezing conditions altogether. The classic example of this is the monarch butterfly, which migrates south in the fall to overwintering sites in Mexico. In the spring, the monarch population makes its way back north. Eventually the children or grandchildren of last year’s monarchs return to Michigan. Pest insects such as armyworms, earworms, potato leafhoppers, and some grain aphids do not survive the winter in Michigan either. Instead, populations continuously reproduce in southern states, and insects move north with spring weather fronts to recolonize northern states. The mild winter of 2011, and above normal temperatures this spring, did not allow these insects to survive in Michigan, but much of the central United States has been above normal as well, giving some migratory insects a head start. For example, on March 22, the University of Kentucky reported armyworm moth catches in their pheromone traps at levels that are at least two weeks ahead of normal.
Insects that do overwinter in Michigan have ways to survive typical winter weather. Death by freezing isn’t so much related to low temperature itself as it is the result of ice crystals forming in the body. Rapid formation and expansion of ice crystals cause cells to burst, resulting in organ and gut damage. Some insects are freeze-tolerant – they actually survive the formation of ice crystals in their body by producing ice nucleating proteins that “control” the freezing process.
Other insects are freeze avoidant – they accumulate antifreeze in their cells prior to the winter. The antifreeze is composed of specialized carbohydrates (in a fancy term, “cryoprotectants”) that lower the freezing point of the body fluid, preventing the formation of ice crystals. Examples of cryoprotectants are the sugars trehalose and mannitol, or the sugar alcohol glycerol (we humans use glycerol as an antifreeze in industrial processes). These cryoprotectants are effective as long as the insect body cools gradually (i.e., the insect acclimates to the cold, as in the fall, triggering the production of the compounds) and until temperatures get really cold (beyond the freezing point of the antifreeze).