Insects Over Winter

It is starting to get chilly in the Midwest as we head toward winter.  Insects also recognize the shorter daylight hours and dropping temperatures.  They use two main strategies to cope with winters: freeze avoidance and freeze tolerance.  Avoidance may require travel or a dry place to stay warm.  Tolerant insects can stay here and control their body’s response to freezing temperatures.  Staying in place has several advantages including being able to emerge early in spring, allowing those insects to feed before predators are out.

^{Clockwise from upper left: Monarch butterflies on migration by Dopeyden, Getty Images; Cecropia moth cocoon by Sylvie Bouchard, Getty Images; Milkweed bugs by Rick Wood, Rick Wood’s Images; Four-lined plant bug, Poecilocapsus lineatus, by Heather Broccard-Bell, Getty Images Signature; Common pill-bug by Ines Carrara, Getty Images}

Those insect groups that opt for traveling to warmer climates often make a one-way trip.  Monarchs are one of the best known insect migrators.  Monarchs east of the Rocky Mountains leave in early fall to arrive at overwintering sites in the mountains of Mexico.  Their offspring return to the southern U.S. in  spring to breed, and this second generation returns to the East and Midwest over the following summer.  Monarchs west of the Rockies overwinter in Baja, California, and many of the same individuals that fly south do return, but their trip is considerably shorter allowing time for breeding during summer months in the northwest U.S.

Many different species of insects can control or stop ice formation in their bodies.  Honeybees cluster together and shiver, generating enough body heat to keep the air above freezing in their hive. Other insects get rid of all the food and water in their bodies, becoming dehydrated and unable to freeze.  Some species can control where ice crystals may form in their bodies, and prevent their formation inside cells that would cause harm to the insect.  Insect species  that retain some water use cryoprotectants, chemicals resulting from large amounts of sugar.  One such chemical is glycerol, which lowers the freezing temperature of water and other liquids.

Some insects remain active throughout the winter.  Aquatic insects in immature stages can easily spend the winter in fast moving or deeper water that does not freeze.  Some insects move inside warm and protected places like attics, sheds, and gaps in house walls.  They also overwinter in tree cavities, under bark, inside dead plants, and in old burrows.  Allowing vegetation with hollow stems to stand through the winter will provide many populations a safe place to spend the season.

The insect version of hibernation is called diapause, when growth and development is halted until longer daylight hours prevail.  Late winter weather patterns with warm periods interspersed with cold periods endanger many species.  Another change we have experienced in recent years involves warm temperatures lasting later in fall and starting earlier in spring.  These changes interrupt the historical natural cycles, causing insects and other animals to abort hibernation and actively look for food or mates.  Food plants do not produce enough to support animal populations, or another cold period returns, and species are not able to adapt, and often perish.

^{A garden in winter by Vermontalm, Getty Images; Snag and deadwood in winter by BayDavn0211, Getty Images; Spring garden and returning insects by Anthony Lerma, Getty Images}

Insects are a vital part of healthy habitats, providing pollination services and food to many species.  We can help overwintering insects by leaving dead stalks standing until late spring to provide safe habitat.  Leaves left piled up provide nutrients, insulation, and a place of shelter.  Uncovered soil can also host ground nesters such as native bees.  Snags and logs provide lots of cavities protected from the weather.  Selecting some plant species that bloom early or late in the year will provide additional food at those times.  Growing plants in clusters enables insects to find all they need in one place.  With a little planning, everyone’s backyard can be an inviting habitat for insects year-round.