An insect doesn’t have the benefit of body fat, like bears and groundhogs, to survive freezing temperatures and keep internal fluids from turning to ice. Like all ectotherms, insects need a way to cope with fluctuating temperatures in their environment. So how do insects survive the cold winter months?
Some insects head to warmer climes, or at least better conditions, when winter weather approaches. The most famous migrating insect is the Monarch butterfly. Monarchs in the eastern U.S. and Canada fly up to 2,000 miles to spend their winter in Mexico. Many other butterflies and moths also migrate seasonally, including the gulf fritillary, the painted lady, the black cutworm, and fall armyworm. Common green darners, dragonflies that inhabit ponds and lakes as far north as Canada, migrate as well.
There’s warmth in numbers for some insects. Honey bees cluster together as the temperatures drop, and use their collective body heat to keep themselves and the brood warm. Ants and termites head below the frost line, where their large numbers and stored food keep them comfortable until spring arrives.
Certain insects, particularly ones that live in higher altitudes or near the Earth’s poles, use a state of torpor to survive drops in temperature. Torpor is a temporary state of suspension or sleep, during which the insect is completely immobile. The New Zealand weta, for example, is a flightless cricket that lives in high altitudes. When temperatures drop in the evening, the cricket freezes solid. As daylight warms the weta, it comes out of the torpid state and resumes activity.
Unlike torpor, diapause is a long-term state of suspension. Diapause synchronizes the insect’s life cycle with seasonal changes in its environment, including winter conditions. Put simply, if it’s too cold to fly and there’s nothing to eat, you might as well take a break (or pause). Insect diapause may occur in any stage of development:
- Eggs – Praying mantids survive the winter as eggs, which emerge in spring.
- Larvae – Woolly bear caterpillars curl up in thick layers of leaf litter for winter. In spring, they spin their cocoons.
- Pupa – Black swallowtails spend winter as chrysalids, emerging as butterflies when warm weather returns.
- Adults – Mourning cloak butterflies hibernate as adults for the winter, tucking themselves behind loose bark or in tree cavities.
Many insects prepare for the cold by making their own antifreeze. During the fall, insects produce glycerol, which increases in the hemolymph. Glycerol gives the insect body “supercooling” ability, allowing body fluids to drop below freezing points without causing ice damage. Glycerol also lowers the freezing point, making insects more cold-tolerant, and protects tissues and cells from damage during icy conditions in the environment. In spring, glycerol levels drop again.