One frosty winter’s day, while tracking a mink along the edge of a stream, I discovered some tiny winter stoneflies creeping on top of the snow. Unaccountably, the stream-dwelling larvae of these insects metamorphose into adults and emerge in the depths of winter. Feeding on algae, they move along the rocks, snow, and tree bark. After mating, females lay eggs back in the stream. While we bundle up to ward off Jack Frost, these stark-naked little creatures remain unperturbed by winter’s chill.
Insects employ a variety of strategies for coping with winter. Like the winter stonefly, when the thermometer rises above freezing, winter craneflies and the snow scorpionfly crawl about atop the snow. Their bodies can absorb the sun’s heat and warm up to a tad above the ambient temperature.
Some insects, like the milkweed bug and potato leafhopper, migrate to balmier climes. Monarchs are famous for their epic journeys – populations from eastern and midwestern North America fly south each fall and roost on trees in the Sierra Madre Mountains of Mexico, Honduras, and Guatemala.
But many insects remain in the North Country and survive by entering a period of diapause – which is like hibernation but more complete. During diapause insects live off fat stored during the growing season and their metabolism drops to one-tenth of normal.
Some insects overwinter beneath the snow or under leaf litter, which provides a measure of protection from extreme weather. Others, however, spend the winter above the snow – in leaves, stems, and galls – and are exposed to the brunt of arctic blasts. The goldenrod gall fly can survive temperatures below -58°F.
How do they endure such extremes? Many overwintering insects — including some butterflies, moths, bees, wasps, ants, beetles, flies and midges — increase their cold tolerance as the temperature drops. Water migrates out of their body’s cells and into the spaces between. When the water surrounding the cells freezes, it is not as likely to burst cell membranes and damage tissues. At the same time, concentrated sugars and alcohols form within the cells which lower their freezing point and keep cellular fluids from icing up. This biological antifreeze enables the overwintering larvae of the Arctic willow gall insect – whose winter weight is 20 percent glycerol – to survive polar air down to -90°F.
Some insects elude freezing by lowering the volume of water within and between their cells. Many avoid the “seeding” of ice by overwintering in a dry site and by expelling the contents of their guts so that no solid particles remain on which ice can crystallize. Their pupal cases and exoskeletons are waterproof. Using these adaptations, ticks, mites, spiders, and many insects won’t freeze if the temperature stays above -4°F.
Each kind of insect survives winter in a particular life stage. Some species enter diapause as adults, including mourning cloak butterflies, squash bugs, and ladybird beetles or “ladybugs,” which cluster under leaves at the base of a tree or rock. This is also true of bumblebee, yellow jacket, and bald-faced hornet queens, which are the only individuals that live past autumn. Cricket and grasshopper eggs rest quietly in the soil. Walking sticks, most aphids, and eastern tent caterpillars also overwinter as eggs. The pupal stage of the tiger swallowtail makes it through the cold season, as well as those of the noctuids and sphinx moths. Cicadas, dragonflies, and damselflies pass the winter as nymphs.
Fritillary butterflies and Isabella tiger moths winter as larvae; the latter are the “woolly bears” that crawl about each autumn in search of a sheltered site to hibernate, sporting felt-like bands of black on each end and reddish-brown in the middle. These endearing caterpillars curl up for the winter but don’t spin a cocoon and pupate until springtime.
When trying to maintain warmth and activity during the winter there is strength in numbers. Ants, honeybees, and termites overwinter in nests. Honeybees, which evolved in the tropics, must store enough honey to provide the heat and energy needed to see the hive through to springtime. By clustering, a bee colony can maintain 48°F (9°C) at the edge of the mass, and 86-95°F (30-35°C) in the center. Termites and ants cope by moving deeper into the soil as the frost penetrates.
Meanwhile, winter stoneflies and others remain active in the frigid air. These fascinating insects possess a low thermostupor point – the temperature at which an animal becomes immobile. I can relate. After enduring three decades of Vermont winters, I’ve finally adapted: my own thermostupor point has fallen by a few degrees. Unfortunately, my general level of stupor has risen noticeably.