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Where do all the Insects go in the Winter?

We are often asked where all the insects go after harvest and whether a cold winter will lead to fewer pests next year. Like most answers to broad questions involving insects, we have to first mention that insects are a huge group of animals - the most diverse in terms of species on the planet. With so many kinds of insects, there are bound to be many ways to avoid the problems of winter. Insects have adapted strategies to survive some very harsh environmental conditions. Basic survival techniques include:

1. Choose to live in the tropics. One way to beat winter is to completely avoid it. In fact, the majority of insects never experience cold weather because they do not live in our part of the world at all!
2. Leave the country. Much like the fabled "snow birds" - retirees who flee from Canada each winter - there are some kinds of insects that migrate. The most notable are monarch butterflies, giant swallowtails and a few other species of butterflies that perform remarkably long journeys to avoid hockey season. Most interestingly, it is the offspring of these southern fliers that eventually make their way back north in the following year.
3. "Hibernate". This is the strategy employed by most of our northern species to avoid winter. However, "hibernation" is really a concept that describes mammal wintering. Insects that go dormant in winter enter a state called "diapause". Their bodies respond to changes in daylength, temperature, food quality and other environmental cues. We've had so many issues with multi-coloured Asian ladybird beetles in the last few years that we know you're all aware these insects overwinter as adults en masse. Many people are also familiar with cluster flies which can wander into the house at any time in the winter. These insects have winkled their way into the house in the fall and follow the source of warmth from within. Barring the availability of a structure, they would have congregated in a cave, crack in a cliff, hollow tree, or other protected place until spring. Some butterflies even overwinter as adults; a frequently "first seen" butterfly in spring is the mourning cloak. Leek moth, a pest of Allium species, overwinters as an adult. And so do some species of mosquitoes (maybe in your basement right now).
4. Pick the least vulnerable life stage. Depending on the species and which growth stage is best suited to surviving the cold, insects (and let's not forget mites) will overwinter as eggs, larvae or nymphs, pupae, or adults. Insects that overwinter as eggs are common and can be found in a wide range of locations including under tree bark, in turf, underground, or in other sheltered places. Many overwintering caterpillars spin a lovely silken cocoon to reduce the chances of freezing.
5. Tough it out. Numerous special physiological adaptations take place in insects to allow them to survive freezing temperatures. Basic physical processes are slowed by the cold but some insects have remarkable adaptations to survive actually freezing. Freezing usually disrupts cell functions and may cause cells to burst as water expands when frozen. Some insects can replace the water molecules in their cells and cell contents with glycerol, which acts as an anti-freeze. Others effectively squeeze water from their living cells to lower the freezing point. These processes become extremely important for highly exposed insects and mites.
6. Slow down and relax. Below the ice of ponds, lakes and streams in winter is a huge aquatic world where many insects go about their normal business at a slightly more relaxed, cold-induced slow pace. Many of our peskiest insect friends such as blackflies and mosquitoes are aquatic in the larval stage. But then so are some important beneficial insects like dragonflies.
7. Stick close to warm friends. Many youth today want to spend all of their time in the protective environment of "The Mall." Insects figured this strategy for easy living out many millions of years ago - both ectoparasites (parasites outside of the host) and endoparasites (parasites inside the host) avoid winter by staying on (or in) their protective hosts. As long as the hosts survives the cruel winter, the parasites will as well. Bees huddle together for warmth; the colder it gets, the tighter the cluster.
8. Get out and enjoy it. There are a few kinds of insects that you may see around in the winter. Keep your eyes open on cool days when you're outside and you may see "snow fleas" (spring tails or collembola) - small jumping insects - on the surface of the snow. You may also see winter crane flies and some moths flying even at 0 °C.
9. Stay under a warm blanket. There are lots of warm places to hide: hidden in the soil, plant debris, crevices in bark, under blankets of snow and other protected locations. One of the most productive areas for winter insect activity is right under a thick warm blanket of snow. At ground level under a decent depth of snow, the temperature won't vary much from freezing. Many ground beetles and other insects are still active below the snow, even if they are a little slower than normal. Location of overwintering does make a big difference - the more sheltered, the better chance of survival.

Each type of insect has its own particular strategy to avoid the worst of winter. For organisms that generally can't regulate their body heat, winter can be a dangerous time. The incredible adaptations to avoid this danger exhibited by Class Insecta are truly remarkable.

But to answer the question of whether a cold winter will lower insect numbers - well, yes and no. To species that have specialized physiological or behavioural adaptations to handle winter temperatures, the cold means very little. Instability - fluctuations of freezing and thawing similar to what we've had the last few weeks - can be problematic to overwintering insects. For some insects, if their overwintering sites get too cold (e.g. if there is little snow cover and very cold temperatures), they may have higher mortality but won't be eradicated by the cold by any means. They usually catch up in numbers later in the summer anyway.

But in the end, let's put it this way: Have you ever known a summer without mosquitoes?