Extension Educator, Horticulture
I suspect few people worry about how insects survive winter. Every spring, however, I get questions from gardeners wondering if maybe, just maybe, winter has killed off all the Japanese beetles, Asian lady beetles, or whatever their garden nemesis might be.
If I had to use one word to describe insects, it would be "adaptive". Our resident insects are very well adapted to survive winter. Some people may see this as bad news. If you have ever spent time picking off Japanese beetles from your favorite rose, total annihilation of all insects might mean party time. However, very few insect species are actually pests. Many are beneficial and we couldn't live without them or at least it would be a very different, very uncomfortable, very boring world.
In order to survive winter, insects push the pause button, actually the diapause button. The definition of diapause (and coincidently also the definition of an evening spent watching TV) is "an inactive state of arrested development". The shorter daylight lengths of fall trigger insects to enter diapause. During diapause an insect's metabolic rate drops to one tenth or less, so it can use stored body fat to survive winter. Also many insects produce alcohols for antifreeze. Their bodies can supercool (reach temperatures below freezing) without forming cell-damaging ice.
Insects spend the winter in various life stages: egg, nymph, larvae, pupae or adult. Many overwinter as eggs. Aphid eggs can be found in the bud scales of woody plants. Bagworms hang out as eggs inside this year's bags. Tent caterpillars spend the winter as egg masses on branches.
Many insects such as mourning cloak butterflies and bean leaf beetles spend the winter as adults in protected areas such as under loose tree bark and in fallen leaves. Native Lady bugs overwinter in herds under fallen tree bark or firewood. Asian multicolored lady beetles look for a warm spot in our homes to wait for spring.
Other insects overwinter in the larval or immature stage. Turf feeding grubs overwinter deep in the soil as beetle larvae. European corn borers survive as full grown larvae. Others such as cecropia moths and swallow tail butterflies overwinter as pupae in cocoons or chrysalis.
In order for insects to continue to the next life stage, diapause has to be terminated. The "play button" is generally warm temperatures. However it would be a deadly mistake for an insect to "wake up" too soon. Therefore, most insects do not come out of diapause unless a long period of cold precedes the warm temperatures.
Insects are certainly adaptive, but winter conditions can effect their survival. Cold temperatures, fluctuations in temperatures, how long cold temperatures continue, how protected the overwintering location is, and if any snow cover is available all effect an insect's survival.
For example Fred Miller reported in UI Extension Home, Yard, and Garden Pest newsletter about an Iowa study on mimosa webworm, a common defoliator of honey locust. These webworms overwinter as pupae in silken cocoons in many protected areas including under house shutters, in mailboxes and under tree bark. In the study two to three weeks of consistently low temperatures of 20 to minus 20 degrees F produced close to 100% mortality in webworm populations. The study also showed that larvae located in especially protected areas such on houses, where the temperature may be 5 to 10 degrees warmer, were more likely to survive than their open area relatives under tree bark.
We still have a lot to discover about how insects spend the winter.