Where do insects go in the winter?
Editor's note: This article by Allison Frederick, public affairs assistant manager, is reprinted from the current issue of "Horizons," the quarterly publication of the Lake County Forest Preserves.
In the colder months, creatures that are commonplace during the Midwestern summer are often the furthest things from our minds. It's always amazing when the weather has been cold for an extended period, then, at the first sign of warmth, insects seem to magically reappear.
Suddenly ecologists are walking through clouds of midges again. Where have these delicate critters been hiding? How did they survive the frigid air that makes us shiver inside a sweater when outdoors for longer than a few minutes?
Perhaps it is easiest to break up the answer into large categories, starting with overwintering as eggs. Some insects lay eggs that survive the winter. The most prominent insects in this category are praying mantids (family Mantidae). In late summer or early fall, a female mantid lays an egg mass in a protective case called an "ootheca" on a plant stalk or leaf.
Many of those eggs will 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 on twigs, gardening stakes and similar spots. For an even bigger challenge, try to find a female mantid laying her eggs in late summer or fall -- it's an amazing process to see.
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 ride out 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 emerging 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 leaflike pupal case, or cocoon, attached to a part of their larval food source. Alien-like at first glance, the delicate features of this gorgeous moth can be seen from the pupa, including the folded antennae and wings.
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 for 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 frost line. 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, 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 plummet. Honeybees stay semiactive 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, the mourning cloak butterfly is usually the first butterfly seen each spring, sometimes when snow is still on the ground. Just like the insect larvae mentioned earlier, mourning cloak butterflies spend the winter in "cryopreservation," having educed 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. Each fall, millions of monarch butterflies migrate up to 2,500 miles away to warmer weather in Mexico.
In all the world, no butterflies migrate as far as the monarchs of North America. And they have fellow flyers. Some larger dragonfly species also migrate south in the fall. Green darners (Anax junius), for example, have been spotted in such massive groups that the swarms could be seen on Doppler radar like a pulsating storm cloud.
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 temperatures constant. Next best are mild winters for insects 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 this time of year turns our thoughts to all those insects out there. We hope they have found snug homes in the preserves for the coming months.
• Kim Mikus is a communications specialist for the Lake County Forest Preserves. She supplies a bimonthly column about various aspects of the preserves. Contact her with ideas or questions at kmikuscroke@LCFPD.org. Connect with the Lake County Forest Preserves on social media @LCFPD.