As cold weather moves into Colorado, insects and mites undergo major changes to survive. Some species die out over the winter--those that annually re-colonize the state with spring and summer migrations (aster leafhoppers, potato psyllids, most corn earworms, black cutworms, etc.). Other insects prepare in several different ways for the winter. Caterpillars and beetles tend to burrow into soil or other protective cover. Aphids produce cold-resistant eggs that are attached to buds and needles. Several species like the protection that buildings and homes provide and cause seasonal nuisance problems. Regardless, most species that successfully overwinter, undergo physiological changes as well. Insects become "cold-hardy," which involves chemical changes, including the production of antifreeze that protects their cells from lethal freezing. During cold weather, most insects are in a condition known as diapause, a semi-dormant state where reproduction, development, and most feeding ceases. Diapause persists for months, and is only ended when certain environmental triggers are passed. Day length is sometimes used to determine when diapause occurs; a critical exposure to chilling temperatures may also be required to end diapause. The following summary explains how many arthropods survive winter in Colorado:
--ANTS: Ants, like honeybees and termites, have persisting perennial colonies that remain intact with wingless workers (infertile females) comprising almost all members. A single fertile female--the queen--or sometimes a few queens are also present. Winged forms (reproductive females and males) are usually absent or present in greatly reduced numbers during winter. Some visible foraging of the insects may occcur during warm periods, but the colony is essentially dormant during winter. In spring, activity renews and some species (notably field ants, Formica spp.) are particularly visible in homes.
--APHIDS: Most aphids overwinter as eggs on tree or shrubs. A typical Colorado aphid lifecycle involves feeding on an herbaceous summer host plant followed by return of the aphid to a perennial plant in late summer and early fall. Some aphids, notably the Russian wheat aphid, overwinter on the plant on which they feed, continuing to feed and develop throughout winter as long as temperatures permit. Other aphids, such as the cotton aphid and greenbug, rarely survive Colorado winters and most insects present in the summer originate as annual migrants from southern areas.
--BORERS, GRUBS: Beetles that feed underground or within plants (e.g. borers, grubs, bark beetles) typically spend the winter as larvae that resume feeding in spring after temperatures warm.
--BUMBLEBEES: Although bumblebees are social insects they make a new colony each year. The only stage that overwinters are large, fertilized queens that hide in protected areas, then emerge to reestablish a colony in abandoned rodent or bird nests or in hollows that have insulating material nearby. As the new colony is produced solely by the efforts of a single queen, the first workers produced are small and malnourished; however, as these and more workers are produced to help with colony chores, the colony becomes full-sized in late summer, before it is abandoned again.
--BUTTERFLIES: Most Colorado butterflies spend the winter as pupae, in sheltered corners often several yards from the plants on which the caterpillar stage earlier fed. A few manage to tough it out as adult butterflies--notably the mourning cloak that may even be seen flying around during warm days in winter. The monarch overwinters in the butterfly stage in a fairly restricted area in a mountainous region outside of Mexico City.
--CUTWORMS: Cutworms, including the infamous army cutworm (the "Colorado miller"), typically spend the winter as small caterpillars; they resume feeding and growing on young plants in spring. A few cutworms overwinter as eggs that hatch in early spring; others such as tobacco/geranium budworms winter as pupae within the soil.
A few of the "climbing cutworms" such as corn earworms, black cutworms and armyworms can not usually survive Colorado winters, although some corn earworms do appear to survive mild winters in parts of western Colorado. Each year, new infestations arise from migrants flying into the state from southern areas.
--DRAGONFLIES/DAMSELFLIES: Dragonflies overwinter in water as nymphs. Most damselflies spend the winter as eggs inserted into shoreline plants.
--EARWIGS: Earwigs spend the winter in the adult stage. In spring the male and female dig out an egg chamber and eggs are laid. The female tends the eggs and for several weeks cares for the newly hatched nymphs.
--FLIES: Many of the most common flies spend the winter in the adult stage. Often these flies enter buildings after they emerge from winter shelter. During the winter these flies are in a semi-dormant state known as diapause when they do not feed or reproduce. Increasing day length and temperatures "reawaken" these flies to pursue normal activities. Other flies spend the winter as pupae in soil.
--GRASSHOPPERS/CRICKETS: There are more than 60 species of grasshoppers in Colorado all of which have various ways of making it through the winter. Most of the damaging grasshoppers (Melanoplus species, Mormon cricket, etc.) and crickets overwinter as eggs, in an egg pod inserted into soil. Other species overwinter as adults and nymphs.
--HONEYBEES: Honeybees are social insects that maintain a colony from year to year--usually a hive, although a few wild colonies live in hollow trees, hollow walls, and other protected sites. Overwintering stages are workers and a single queen. Egg laying is suspended in fall and begins again during late winter.
--HORNWORMS: Hornworms spend the winter in the pupal stage in soil buried several inches deep. In parts of the state some species of hornworms may be killed by winter temperatures. Infestations return from spring migration of adult moths that originate in southern states.
--LACEWINGS: Most lacewings overwinter as pupae in a silken case usually attached to trees or other plants. A few overwinter in the adult stage, some of which use buildings or homes as overwintering shelter.
--LADY BEETLES: Almost all lady beetles overwinter in the adult stage, typically seeking protected locations (under clapboards, leaves, bark flaps, etc.) in the vicinity of where they spent the summer. In particularly good sites, they often winter in clusters. One species of lady beetle--the multicolored Asian lady beetle--often finds its way indoors and may winter within the living area of the home. A few of the lady beetles may migrate long distances seeking winter shelter, including at least two species that fly to the mountains and spend the winter under the snow at elevations typically above 9,000 feet. These beetles often occur in spectacular groups literally covering the ground that are most commonly observed during fall. The beetles then fly to lower elevations in late spring as snow melts. Mass winter aggregations occur all along the Front Range but apparently do not occur in western Colorado.
--LEAF BEETLES: Beetles that feed on leaves (potato beetles, asparagus beetles, elm leaf beetles, flea beetles) overwinter in the adult stage, often in the general vicinity of the plants on which they earlier fed.
--MOSQUITOES: Most Colorado mosquitoes (floodwater types such as Aedes species) spend the winter as eggs that were laid earlier around the edge of receding water sources.
Others spend the winter as adults (Culex species) seeking protected sites such as caves, wall voids, animal burrows, etc.
--POTATO PSYLLID: Potato psyllid overwinters on native plants along the U.S.-Mexico border. Its occurrence in Colorado is based on annual migrations northward from these southern areas.
--SAWFLIES: Almost all sawflies overwinter within silken cocoons buried shallowly in soil. Winter is spent as full-grown larvae which transform to the pupal stage in late winter. One species, known as the bull pine sawfly, is an actively-feeding larva throughout winter.
--SPIDER MITES: Spider mites survive winter in various forms. The most common are dormant, mature females that typically turn reddish or orange as they undergo winter diapause. Two-spotted spider mites and honeylocust spider mites are examples. Overwintering mites spend the winter in scattered, protected locations around previously infested plants or around buds and bark scales of trees and shrubs. Other spider mites survive as eggs. For some of the cooler-season species (spruce spider mites) peak populations may occur in fall or spring but extreme cold forces them into more resistant eggs. However, clover mites on lawns appear capable of continuously developing on lawn grasses throughout winter as long as daily temperatures allow.
--SPIDERS: The overwintering habits of spiders varies greatly; however, most of the orb-weaving spiders and other species that act as "passive hunters" and use a web to ensnare prey, winter as eggs that are produced at the end of the growing season. Conversely, many of the more active "hunting spiders," such as wolf spiders, may winter as nymphs, becoming full-grown in spring. A few spiders are quite long-lived. Giant wolf spiders may survive two to three years, and tarantulas a decade or more. They spend winter in protected retreats, such as underground burrows or hollows. Widow spiders also may live a second season in a protected location, although they usually die off in fall. Spiders that move indoors and adapt to indoor conditions with available food, may survive and reproduce year-round.
--TRUE BUGS: 'True bugs' (Hemiptera), such as the squash bug, boxelder bug, conifer seed bug and various stink bugs, typically spend the winter in the adult stage. Immature nymphs that get caught at season end by a lack of food, can't survive the rigors of winter. Surviving adults hide in protected locations often around the plants on which they earlier fed. Some commonly seek shelter in homes and often become nuisance pests. A few plant bugs that feed on trees (honeylocust plant bug, ash plant bug) lay eggs on those trees during the growing season, which then hatch in spring.
--WHITEFLIES: Whiteflies are subtropical insects that can not survive Colorado outdoor winter conditions. Populations of whiteflies are maintained season-to-season on indoor plants and in greenhouses. Infested plants are the source of most whitefly populations in gardens.
--WOOLLYBEARS: Overwintering habits differ among the various fuzzy caterpillars known as "woollybears." Most will pupate in a silken cocoon in late fall; however the banded woollybear survives as a caterpillar.
--YELLOWJACKETS/HORNETS: Yellowjackets and hornets are social insects that abandon the nest at the end of the season and start a new colony each spring. The overwintering stage is a fertilized queen that hides in protected sites such as under bark, in building cracks or other locations. In spring, surviving females attempt to individually establish a new colony--a project that is rarely successful. Colonies that do become established grow slowly during the early season when the queen and a few worker wasps are available for colony chores.As the season progresses, colonies expand rapidly. At the end of the season, queens and males are produced. By early fall, the colony is abandoned and the workers and males die.

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