Ecology

Where do insects go in winter?

Anne Gageby
Strawberry Hill Nature Preserve

(12/2022) South Central Pennsylvania and Northern Maryland have already experienced the first hard frost of the year and with that, a significant decrease in biting insects. This is a welcome shift for anyone who enjoys the outdoors. Hikers no longer have to contend with mosquitos or no-see-ums and can simply enjoy their time outside. As cold sneaks into the spaces between the days and settles in, a question naturally comes to mind: "where do insects go in winter?"

The question is multifaceted, and the answer depends on the species. Many insects prefer a migratory lifestyle to aid in escaping plummeting temperatures. Monarch butterflies are probably the first insect to come to mind when we think of migrating insects. However, there are an estimated 71 different species of migratory insects including dragonflies, beetles, butterflies and moths, grasshoppers, and more. The milkweed bug, like the monarch butterfly, undertakes long migrations that begin in spring. They follow an emerging path which starts in southern states and Mexico and continues up to Southern Canada where they remain for the summer. As fall reclaims the land, the milkweed bug and monarch butterfly turn south toward their winter homes.

Dragonflies have about sixteen different migratory species. Green darners, like monarch butterflies, are multi-generational migrators. The first generation emerges in the Caribbean, parts of Mexico, and deep southern areas of the United States. By May, this first generation will have traveled hundreds of miles north where they will eventually settle and produce the next generation of migratory dragonflies. This second generation will reach maturity sometime between July and October. With the coming of fall, the second generation reverses its course and heads south to lay eggs. The third generation is non-migratory and will spend its life in the comfort of the southern climate. This generation’s offspring will become the first generation in the next cycle.

Not all insects’ life cycles depend on migratory patterns. Some tough out the cold by depending on their colonies. Honey bees are famous for surviving deep freezes by huddling together and vibrating their wings to raise the temperature inside their hives. Unlike other bee and wasp species, honey bees stay active all winter despite a lack of available pollen and flowers. Ants, like honey bees, cluster together inside their colonies to maintain body heat and keep their queen warm. Ants don’t remain active during the winter, though. As the temperature drops, they seal off the entrance to their colonies and enter a dormant state until spring.

Other insects spend the winter months as immature larvae and nymphs. Some bark beetles, like the mountain pine beetle and wood borers, spend the winter nestled under the bark of infested trees. Even the emerald ash borer has been known to winter in a pre-pupal stage despite being susceptible to extreme cold.

Enduring winter’s harsh conditions is more difficult for early-stage insects. Few species can tolerate the chill as immature larvae and nymphs. Fewer still survive as eggs. Field crickets, praying mantis, mosquitoes, and aphids are some of the few insects that can spend the winter as eggs, waiting to emerge as new adults when the days grow longer. Male mosquitoes die off during the fall yet their counterparts, the frustratingly hardy female mosquitoes, burrow into the ground, in logs, trees, or any other safe place where they enter diapause, a state of suspended animation. Diapause is often used interchangeably with hibernation though they are somewhat different. Hibernation is a deep winter sleep while diapause is a state of delayed development. Female mosquitoes can remain in diapause for months until the temperatures rise, or enough water makes their environment more hospitable.

Mosquitoes aren’t the only insect adapted to survive long, enduring winters. Mayfly and stonefly nymphs are well-suited to the cold. Both actively grow and feed throughout winter despite often living under a thick layer of pond ice.

Stink bugs are becoming infamous for invading homes during cold snaps. Despite being agricultural pests, they have taken to finding warm spaces in attics, crawl spaces, and inside walls. For many homeowners they have become an unwelcome sight, especially if the temperature rises slightly. Ladybird beetles, known locally as ladybugs, are another example of hardy insects that have adapted to endure winter’s firm grip by seeking out warm and well-regulated environments such as attics and the hollow spaces inside walls. Homeowners in Southern Pennsylvania and Northern Maryland are likely used to seeing large numbers of ladybugs in the fall as they congregate indoors. Ladybugs enter homes via tiny cracks and open window spaces. Once inside, they gather by the hundreds with other beetles before entering diapause.

Not every insect takes shelter in our homes, however. Rocks, tree holes, and under logs are common shelters for overwintering adult insects. Various beetles, including leaf beetles, most leafhoppers, and some aphids overwinter inside the loose bark of trees, under fallen leaves, and within debris on the ground. In general, insects are more likely to survive the winter as long as the temperature remains stable. Problems arise when the season has alternating thaws and freezes. Insects that burrow under leaves and within fallen logs do surprisingly well as long as they have enough snow cover. Snow is a fantastic insulator that can maintain a consistent temperature for insects wintering beneath its sparkling canopy.

This doesn’t mean every insect can spend the winter in comfort. For Japanese beetles, ambient temperatures can dramatically decrease their chance of survival. Studies have shown that cold snaps with temperatures in the low-mid-twenties Fahrenheit for more than fourteen days can be lethal to Japanese beetle eggs. Despite being able to produce glycerol, the insect version of antifreeze, extended periods of extreme cold can devastate egg populations.

Insects, overall, are some of the most important pieces in the web of life. The value of pollinators such as honeybees, for example, is nearly incalculable. And while some insects such as stink bugs are seen as nuisance, many more are invaluable to our ecosystem. We might not be able to see them in the wintertime, whether hidden or on temporary hiatus, but we know that we will soon be seeing them once again during the first hint of spring.