As we shiver through winter’s freezing temperatures and heavy snow, many gardeners hope the severe weather translates to fewer garden insects in the coming growing season. But, unfortunately, insects have evolved many coping mechanisms enabling them to tolerate winter conditions just fine. Of course, the insects don't just 'disappear' and magically reappear the next year. Each species has developed some way of dealing with the cold weather.
Most of the creatures mentioned in this article are true insects, but a few, like ticks and mites, are scientifically classified as arachnids - not true insects. To entomololgists, insects are small arthropods (invertebrate animals with an exoskeleton, segmented body and paired jointed legs) with six legs. However, arachnids are also mentioned, which arthropods with eight legs.
Most commonly, each insect species overwinters in a specific developmental stage – egg, larva, nymph or adult – which is characteristic of that species, but some insects can overwinter at various stages of development. Many insects spend the winter in immature stages - as eggs (the bagworm is a good example), as larvae underground (cicadas and June beetles) or as pupae (the large silkworm moths such as the Cecropia).
Yellowjacket and paper wasp queens, some mosquitoes and the mourning cloak butterfly are other examples of local insects that overwinter as adults. Like the ladybird beetle, they seek out a protected spot and become dormant until warm weather activates them again.
Freeze avoidance and freeze tolerance are two major insect strategies. Freeze avoidant insects may migrate, have a way to keep themselves warm or find a relatively warm place to overwinter. Freeze tolerant insects can either survive being frozen or control the formation of ice crystals in their body.
Freeze Avoidant Insects
Some insects migrate, moving long distances, but these migrations are different from those of birds or animal. For most insect species it’s a one-way trip. Monarch butterflies are a classic example, although they are not a garden pest! We enjoy their annual migration flight every fall, as summer’s butterflies move to southern United States or northern Mexico. In most cases, the monarchs that headed south are not the same insects returning the following spring.
Monarch butterflies originating east of the Rocky Mountains eventually find their way to central Mexico, where they overwinter in the mountain highlands near Mexico City. One or two areas are protected as monarch refuges. Monarchs from west of the Rockies travel to a spot near Santa Barbara, California. These are true migrating insects because the same individuals that go south for the winter come back the next year.
Other insect species, such as spotted cucumber beetle and black cutworm, typically die out in fall and new insects fly in or are blown north with spring storm fronts each year to recolonize the area. Bean leaf beetles are another example; most adults present in late fall die when air temperatures drop below 14 F, but a few find enough protection under plant debris and loose soil to survive.
Honeybees are one of the few insects adapted to survive winter without becoming completely dormant; instead they change their behavior and physiology to increase survival. These changes include increased bee life span (up to 8 months for winter bees, compared to approximately 30 days for summer bees), no rearing of new bees during winter and formation of bee clusters devoted to keeping the hive warm. They do this by vibrating their flight muscles, which generates heat, and can maintain temperatures at the outer edges of the hive between approximately and 42-53 F.
Fall Invaders
Most insects stay here year round. They employ a variety of tactics for survival. One is simply to move in with humans. Insects such as ladybird beetles (ladybugs), cluster flies, elm leaf beetles and boxelder bugs overwinter as adults in protected locations. Before humans started building insect hotels, they probably found shelter in hollow logs and other natural cavities.
Common household invaders, like multicolored Asian lady beetle and boxelder bugs, often utilize warm protected places like attics, sheds, areas under porches or decks, gaps under home siding or inside wall voids to escape winter’s coldest temperatures. Fallen trees or limbs, holes in trees, leaf litter, under or inside dead plants, in burrows and under tree bark are all common insect overwintering sites. A good reason to clean up your vegetable and fruit production areas in fall to minimize insect pest overwintering sites.
Many insects burrow into the soil where temperatures are warmer than winter air temperature. A good blanket of snow moderates soil temperature even further, making it easier for insects to survive. Japanese beetle grubs are one example of a soil-overwintering insect which benefits from snow cover's good insulating properties.
Freeze Tolerant Insects
Other insects are able to stop ice formation from occurring within or between their cells. They do this in a variety of ways, including finding a very dry place to hibernate where ice crystal formation cannot occur. Insects are generally small and their bodies contain little water making this hibernation state easier to achieve.
Some insects can control where ice crystals form in their bodies, ensuring their organs and tissues are not damaged. When warm weather arrives in spring, the ice melts and the insect becomes active again.
Finally, some insects can change their body chemistry as winter approaches, producing glycerol. Glycerol is the same thing we spray on our car windshield to prevent freezing, so essentially these insects make their own antifreeze, which ensures they survival throughout the winter. Butterflies that overwinter as adults, such as the question mark butterfly, produce glycerol/antifreeze enabling them to overwinter in rock piles, cracks and crevices in trees, and under house eaves.
Finally, some insects do remain active all year round. These are primarily aquatic insects that spend the winter as immatures in rapidly flowing streams that don't freeze all the way to the bottom.
Common Garden Insects: Overwinter Stage & Sites
- American dog tick (technically not an insect, but an arachnid with eight legs) – adult, in soil
- Bean leaf beetle - adult, in soil or under plant debris
- Codling moth – larva, silken cocoon attached to rough bark areas or protected locations near tree
- Ground beetle – adult (most species), in soil or sheltered areas
- Iris borer – egg, old iris leaves or other crop debris
- Japanese beetles – mature grub (larva), in soil
- Mexican bean beetle - adult, protected areas near the garden
- Spruce spider mite (technically not an insect, but an arachnid with eight legs) – egg, attached to bark of branches, near base of needles, around buds
- Squash bug - adults, protected sites near areas of previous infestation
- Squash vine borer – larva or pupa, cocoon in soil
- Two-spotted spider mite (technically not an insect, but an arachnid with eight legs) – semi dormant adult female, under bark cracks and other protected locations
Whatever insect you're looking at or whatever its winter survival may be, the pattern of development doesn't leave much room for variation. For example, moths that form cocoons in the fall must have that winter cold period. If you bring them indoors and keep them warm, chances are that they won't complete their development. Or, if they do hatch, it will be well in advance of the proper time so they have no hope of surviving outdoors.
Though some insects may become unseasonably active during an extended midwinter thaw, the true hibernators and the majority of those that rest in the adult stage will not be tricked to become active early. Of course, that's not much consolation to the homeowner who has to deal with a midwinter wasp buzzing the family or an "invasion" of slow, stupid flies bumbling into windows and walls during a midwinter mild spell.
With or without antifreeze, most insects simply can not function at temperatures below 40 degrees F. Because they rely entirely on the world around them for the warmth they need to function, they've developed this wide range of techniques for surviving cold weather and assuring the survival of their species.
Images
- Feature image - Question Mark butterfly adult. Image from Wikipedia.
- Image of monarch butterfly adult. Image from Pixabay.com.
- Image of lady beetle adult. Image from Pixabay.com
- Image of May/June beetle larva, one of Nebraska's most common white grub species. Image from Clemson University - USDA Cooperative Extension Slide Series , Bugwood.org