With winter approaching, you may wonder if or how pests survive the months of brutal cold and food shortage ahead. Insects and rodents have a variety of strategies for coping with the seasons, but because they have fundamentally different physiology, those strategies are quite different.
Collectively, rodents have two options for surviving the winter; hibernate or remain active. Groundhogs are one of the best-known hibernating rodents; in autumn, they fatten themselves up and dig a hibernation burrow, and during hibernation their body temperature, heart rate, and breathing rate all greatly decrease. Mice and voles are excellent examples of rodents that remain active; they maintain networks of tunnels under the snow that they use to navigate and continue foraging for food such as , with the snow insulating them against the most extreme cold and providing some measure of protection against predators. Mice also commonly move indoors during the winter, though it is unclear whether warmer conditions or more food access are the greater attractants for them. Squirrels also remain active throughout the winter, but because they build up large fat reserves and hoard food before winter, they can spend relatively little time outside of their dens. Spending more time resting in their dens allows the squirrels to conserve energy and body heat.
Being more diverse, insects have a greater variety of tactics for surviving the winter. Some migrate south for the winter. The monarch butterfly is almost certainly the most famous insect migrator in America, but other butterflies, moths, and dragonflies use the migration tactic to avoid winter altogether. Still other insects simply freeze to death, with northward migrants from farther south re-colonizing every year. The Asian tiger mosquito is one such pest that is not known to survive the winter as far north as Minnesota, but they’re such efficient hitchhikers that they make their way back into the state most summers. Tomato and tobacco hornworms are also unable to survive the extreme winters of the upper Midwest, but the widely-roaming, strong-flying adults often make their way as far as North Dakota and Minnesota over the summer.
Other insects enter a form of dormancy known as diapause, which is triggered by environmental cues such as decreasing temperatures, shortening day length, changes in the chemical makeup of food plants, or the insects’ own biological clock. In many insects, diapause may only occur during a certain life stage; for example, praying mantises only diapause as eggs, goldenrod gall midges only diapause as larvae, alfalfa leafcutter bees only diapause as prepupae, boxelder bugs only diapause as adults, and queen yellowjackets and paper wasps diapause as adults. These insects typically synchronize their life cycle so that only the stage that is capable of diapause is present at the onset of winter, though in some cases other life stages are simply killed off by freezing temperatures. Diapause often comes with major physiological changes, such as dehydration of the body and/or production of cryoprotectant substances that protect the body from the formation of ice crystals. In many insects, diapause ends well before weather is reliably favorable for the insect to resume activity and they transition into post-diapause quiescence, which is easier to terminate so the insect can resume activity. For example, cluster flies, Asian lady beetles, and queen paper wasps typically are in diapause from roughly September/October to February or March, and will not resume activity even if weather warms during that period. However, these same insects in post-diapause quiescence will awaken on any warm, sunny day from when diapause ends until spring. As a result, if these insects overwinter indoors, they may be apparent in fall as they enter and begin diapause, seemingly vanish through November, December, and January regardless of weather, than become active on warm, sunny days in February or March.
Some pest insects, such as certain ants, cockroaches, and bed bugs, avoid winter entirely by living indoors alongside humans. Thanks to heated buildings and steam tunnels, many cockroaches live far beyond where they otherwise could. For example, German cockroaches (native to tropical southeast Asia) and American cockroaches (native to tropical Africa) cannot survive long at temperatures below 50°F, but access adequate shelter in buildings and sewers to survive alongside humans throughout the upper Midwest. Many aquatic insects also remain active throughout the winter, living beneath the ice in ponds and lakes, though the low temperatures greatly slow their metabolic rates.
Finally, a small handful of insects are highly specialized to take advantage of the relative absence of predators that winter provides and are active in or on the snow. Fall cankerworm moths are typically active in late October or early November, finding mates and laying eggs right around the first snow. Snow crane flies, snow scorpionflies, and snow springtails are active slightly later, scuttling or hopping about on the surface of snow on relatively warm winter days to seek mates or feed on moss, algae, and lichen. Spring stoneflies are often the first harbinger of spring, emerging from the first ice-cracks to form on rivers and streams in early spring and mating on the ice.
So while you’re piling on layers, staying indoors, and sipping hot chocolate, the insects and rodents are using a variety of strategies to handle the winter chill.