How insects survive cold: The potential effect of a mild winter

Large insect populations are possible this year, but many factors determine overwintering survival and success.

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).

To avoid exposure to severe cold and or fluctuating temperature, many insects overwinter under plant debris or burrow into the soil. As air temperature changes, the temperature under the cover rises and falls slowly (especially when insulated by snow cover), giving insects a far more stable environment.

Some examples: A first generation corn borer larvae collected in June is easily killed by cold. However, a second generation corn borer collected in December is freeze tolerant, and can survive for months at -4°F, even with ice crystals in its tissue. Overwintering eggs of many aphid species contain protectants like glycerol and mannitol to avoid freezing. In the case of soybean aphids, which spend the winter in the egg stage on exposed branches of buckthorn, eggs can be super-cooled to -29°F. Bean leaf beetles overwinter as adults, and typically survive temperatures only into the 20s°F. However, beetles overwinter in protected areas in woodlots or under leaf litter to avoid colder temperatures. In general, milder winter temperatures put less stress on these and other overwintering insects, and likely increase overall survival into the spring.

Once an insect successfully overwinters by avoiding freezing, it must successfully emerge, perhaps feed, colonize a crop, and eventually reproduce. A mild spring can help or hurt this process. For many adult insects (and some larvae) emerging from winter sleep, often the first task is to find food. Until food is available, they must live off of fat reserves stored in the body from the previous year. For other insects that overwinter as late-stage larvae, feeding is not an option; the fat reserves have to last through pupation, and even into the adult stage. If insects do not find food or complete development before energy reserves run out, the result is lower fitness, less reproduction, or even starvation. Thus being active too early or out of synch with a host crop can lead to reduced overall fitness. For example, alfalfa weevils emerging now in southern Michigan will likely find legumes to eat. But ladybird beetles that emerge early may not find enough prey to survive.

Early insect emergence often times coincides with earlier green-up of perennial crops or bud break on overwintering hosts, giving the insect population a head start and leading to larger pest populations. However, a cold snap can still kill spring vegetation and set the population back. For example, in 2007 a hard freeze damaged emerging leaves of buckthorn. This reduced the feeding sites for soybean aphids that had just emerged on these leaves, and 2007 ended up as a low aphid year in the state, although initial spring populations were high. Likewise, early pest emergence may coincide with earlier planting of the host crop (based on degree days), again leading to larger pest populations. However, a cold or wet period can suddenly set planting or emergence back, so that the insect life cycle and crop are out of synch. For example, in some years with delayed planting, corn rootworm larvae emerged into bare field or corn borer moths did not find tall enough corn to produce a large first generation.

So the bottom line is to be observant as the spring progresses. Chances are that we will see a few unusually large insect populations, or some population peaks occurring earlier than expected. But, there could be weather events in April and early May that kill insects, or create synchrony problems between insect life cycles and crops. From the perspective of many insects, this is just another year in a bug’s life.

For another extension article on this subject, see “Mild Winter, Record-Breaking March Temperatures: How Will Field Crop Insects Respond?” in the March 22 edition of The Bulletin from the University of Illinois.

Gluttons for punishment on this subject can read “Insect overwintering in a changing climate” from the Journal of Experimental Biology.