Post by Allison
Earlier this week, my husband came in from the yard with a mosquito on his forehead. Had it been summer, that little tag-along would never have made it so far—but not in December. In the colder months, critters that are commonplace during the Midwestern summer are often the farthest things from our minds. It always amazes me when the weather has been cold for an extended period, then, at the first sign of warmth, insects seem to magically reappear. Where have they been hiding? How did they survive the frigid air that makes me shiver in my sweater when I’m outdoors longer than a few minutes?
Where do insects go in the winter?
Perhaps it is easiest to break up the answer to that question into large categories, beginning with overwintering as eggs. Some insects lay eggs that survive the winter. The most prominent insects in this category are praying mantids (Family Mantidae). A female mantid lays an egg mass in a protective case, called an “ootheca,” on a plant stalk or leaf. These eggs survive the winter, and 100-200 tiny praying mantids will hatch from the egg case the following spring! Look for these egg masses in open areas. For an even bigger challenge, try to find a female mantid laying her eggs in late summer or fall—it’s an amazing process.
In addition to those species waiting out the cold weather inside a cozy egg, many other insects successfully pass the winter as immature larvae. The protective cover of heavy leaf litter or similar shelter protects some larvae such as woolly bear caterpillars (Pyrrharctia isabella), while some other insects replace the water in their bodies with glycerol—a type of antifreeze! Some larvae, such as beetle “grubs” simply burrow deeper into the soil to escape the cold. (The same is true for some local frogs and salamanders!)
Other insects successfully pass the winter as nymphs. Few insects are active in the winter. However, the nymphs of dragonflies, damselflies, mayflies and stoneflies live in the water of small streams and ponds—even beneath ice. They actively feed, growing all winter and emerge as adults in the spring or summer. Many larger dragonfly species survive multiple winters underwater, living two to seven years as aquatic nymphs before emerging from the depths for a (much shorter) winged adult stage.
Other insects survive the winter as pupae, emerging as adults the following spring. Some moth species, such as Cecropia moths (Hyalophora cecropia), spend the winter in a pupal case, or cocoon, attached to a part of the their larval food source. More commonly, many moth caterpillars sense the change to cooler weather in the autumn and burrow into the soil where they will pupate, sometimes incorporating leaf litter into their cocoon as added insulation.
There are also insects that hibernate, passing through winter as adults. Some beetles prepare for hibernation by storing up fat, like groundhogs and bats, while others burrow into the soil below the frostline. Some insects can eliminate water from their bodies, allowing them to endure freezing temperatures for weeks at a time. Tree holes, leaf litter, logs and rocks are common shelters for overwintering adult insects. Ladybird beetles, or “lady bugs,” hibernate in big colonies underneath loose tree bark and inside tree cavities (or within the walls of buildings). On warm winter days they may emerge to crawl around. Many bees stay in their hives or burrows during the winter, and form clusters when temperatures fall. Honeybees stay semi-active in hollow trees by generating body heat, which is made possible by the consumption of up to 30 pounds of stored honey created during the summer months. Oxidation of the honey produces heat energy, which is circulated throughout the hive by the worker bees as they vibrate their wing muscles. The mourning cloak butterfly (Nymphalis antiopa) survives winter without the advantage of hive-mates. These butterflies spend the winter in hollow trees, emerging to fly about on unusually warm winter days. In Lake County, Illinois, the mourning cloak butterfly is usually the first butterfly seen in the spring, sometimes when snow is still on the ground. Just like the insect larvae mentioned earlier, mourning cloak butterflies spend the winter in “cryo-preservation,” having reduced the water content of their bodies and built up glycerol, which acts as an antifreeze.
Finally, a few insects escape the freezing temperatures by migrating to warmer areas. The monarch butterfly (Danaus plexippus) is Lake County’s best example of this strategy, migrating up to 2,500 miles away to warm weather!
In general, no matter their current stage of life, uniformly cold winters with plenty of snowfall are easiest on insects. Blankets of snow benefit insects by insulating the ground and keeping the temperature surprisingly constant. Next best for insects are mild winters, as we saw last winter (2011/12) in Lake County. Winters that fluctuate between warm thawing days and cold spells can be disastrous to many kinds of hibernating insects. Watching the temperatures fall from 50 degrees Fahrenheit to below 30 degrees in the Chicago area yesterday turned my mind to all those insects out there. I hope they have found snug homes for the coming months.