By Bonnie Ennis, Colorado State University Cooperative Extension agent, horticulture
You and I undoubtedly have turned up the heat more than once this winter to survive the cold. Or we've bundled up in coats, hats, mittens and heavy socks to stay warm.
Insects also devise ways to stay warm in winter. Some go dormant, into a state called "diapause," while others remain active. Insects that spend the winter in diapause withstand a greater range of low temperatures than those that remain active -- some to temperatures as low as -94o F, which is about the lowest reported temperature for natural environments.
With few exceptions, insects that go dormant for the winter fit into two classes: freeze-susceptible and freeze tolerant.
A freeze-susceptible insect avoids freezing altogether by depending heavily on antifreeze compounds, called cryoprotectants, to supercool body fluids and tissues above their freezing point. Ethylene glycol, the same compound found in antifreeze for cars, is the most common cryoprotectant.
Freeze-tolerant insects do not really freeze, at least not totally. Just the fluid, which bathes their living cells, freezes. This freezing process forces water out of the living cells thus lowering the freezing point even further. (Smaller amounts of water freeze at lower temperatures than larger amounts of water.)
Freeze-tolerant insects aren't necessarily more cold tolerant than freeze-susceptible insects. Each class of insects just has its own unique survival strategy.
Both types of insects also are affected by other conditions that affect their rate of winter survival: size, moisture, nutrition, temperature, stage of growth and species.
Insects essentially are little bags of water. Their ability to supercool depends, in part, on how much water they contain. The smaller the amount of water, the greater their ability to supercool without freezing. Smaller arthropods (ants, insect eggs, tiny spider mites) can supercool to colder temperatures than larger species, such as grasshoppers. This phenomenon also is true between individuals within a species. A smaller housefly may be more cold hardy than a larger housefly.
Partial drying can be beneficial for some insects. The freezing point of a housefly is lowered by 50o F after dehydration. For other insects, such as boxelder bugs, dehydration has no cold weather benefits.
An insect with an empty gut will survive lower subfreezing temperatures than an insect with a full gut. This is because food in the gut attracts water that can freeze and promote growth of ice crystals.
The intensity and duration of cold and freezing, as well as exposure time to cold is critical to insect survival. If exposed long enough, an insect will die at moderately cold temperature.
Stage of Growth
Insects pass through several stages of growth before beginning a new generation. For the most part, each insect species overwinters in a stage of growth best adapted to cold temperatures.
For example, non-feeding growth stages (eggs, pupae) survive colder temperatures than feeding growth stages (larvae and adults, although not all larvae and adult stages feed actively.) Some insects improve chances of winter survival by building protective structures, such as cocoons or pupae, at a specific growth stage. This reduces surface moisture contact that could cause freezing.
Species and Migration
Insects are adapted to different climates. Whiteflies evolved in tropical and subtropical climates and perish in Colorado unless they can overwinter in a heated building. Tomato hornworms spend the Colorado winter in the pupal stage (inside a hardened case) and buried several inches deep in the soil.
Other insect species migrate to warmer climates for the winter. The Monarch and Painted Lady butterflies migrate south in late summer, then return in spring with warmer weather.
Most species, however, move only to a different and generally nearby habitat, such as from fields to woodlands, and in agricultural areas from fields to groves, hedges or shelterbelts. Other insects overwinter in their summer sites.
Winter conditions and suitably protected habitats can be critical to an insect's winter survival. Snow cover offers good insulation from winter cold. Light, fluffy snow provides more insulation than packed snow. The north side of a tree maintains the same temperature as the air, but the south side of a tree may be several degrees warmer on a sunny day. Wet substrates (soil, leaves) cool slower than dry substrates; saturated substrates may suffocate an overwintering insect.
With so many variables affecting winter insect survival, it is difficult to predict such rates from one year to the next. Just the same, we can use winter conditions to monitor the development of certain agricultural pests.
Mild conditions, for example, will produce an earlier spring migration of green peach aphids (you may see this little pest on your lettuce and spinach leaves in summer). Long periods of sub-zero temperatures result in winter mortality of some grasshoppers. By understanding these factors, we can more accurately time pest control procedures for some insects.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010