September 15, 2020
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.
