Cercropia moth caterpillar feeding on cherry leaves.
People don’t often think about insects in winter. Frankly, most people don’t think about insects at all except when they are being tormented by mosquitoes in the summer. As gardeners, though, we tend to consider insects and the natural world more frequently than other people. We notice tomato hornworms nibbling tomato leaves and spiders scurrying from under squash leaves. In the flower garden we notice the praying mantis waiting patiently for prey and butterflies visiting flowers we planted just for them. In winter though, insects seem to disappear. What happens to the pests that drive us crazy and the other bugs that fascinate us during warmer months?
Contrary to popular belief, a cold winter won’t eradicate garden pests. Insects didn’t become so successful by dying out each winter. They have evolved many different ways to cope with the lethal cold that befalls much of the country each year. Many insects, like birds, just get out of town and migrate south to Florida or Central America. A well-known example of this is monarch butterflies, which migrate thousands of miles each fall to southern California and Mexico, only to return each spring. This is also the strategy of potato leafhoppers, which migrate north from the Gulf Coast each spring to feed on squash, maples and many other plants.
The majority of insects have found ways to survive the winter without transcontinental migration. Scarab beetles such as Japanese and June beetles, spend the winter underground where temperatures remain fairly constant. To achieve this, adult beetles lay eggs in the summer from which grubs hatch. The grubs burrow into the ground, feeding on plant roots. As they become larger they burrow deeper and spend the winter 12 inches or more below the surface.
Japanese beetle grub with parasitic Tiphia wasp larvae attached to thorax (note lump behind legs), which feeds on grub fluids.
Perhaps more interesting than the grubs are the parasitoid wasps that use the grubs as food and a free ride below ground for the winter. Adult wasps in the genus Tiphia are 1/2 an inch long and feed on nectar. Female wasps burrow into the ground following the scent of their preferred grub species. They sting the grub to paralyze it long enough to attach an egg to the grub’s body. The wasp larva is a parasite of the live grub piercing its skin and drinking its juices. While the grub moves down the wasp larva is carried deep in the ground where, in its final stage, the wasp larva eats the entire grub and spins a silk cocoon in which to spend the winter.
Other insects, which spend the winter exposed to freezing temperatures, have physiological ways to cope. Spending the winter in a non-feeding life stage is one way to cope with extended periods of cold weather and inactivity. Eggs are a non-feeding life stage of most insects and a common way to spend the winter.
Praying mantids exemplify this behavior. The insect has one generation per year that begins and ends with a large brown egg mass. In the fall, adult female mantids release brown foam that hardens around a twig or a stalk of grass. Each foamy mass contains hundreds of eggs that will remain dormant all winter and hatch in the spring. Look for these when cleaning up the garden in fall. Although the pest control benefit of mantids is mixed – they consume pests but also spiders, butterflies, bees and other beneficials – they are one of the most interesting insects to watch and a favorite for classroom show-and-tell.
Praying mantis egg case deposited on the dried stalk of ornamental switchgrass.
Another insect that spends the winter as eggs in a foam-like mass is the eastern tent caterpillar. You can easily spot the oval masses on cherry twigs in winter when leaves have fallen. Each contains hundreds of caterpillar eggs. Prune them off to avoid dealing with big silk tents in the spring.
Butterflies and moths often spend the winter as a pupa, commonly known as a cocoon. You may find silken cocoons stuck in sheltered locations such as under tree bark or on the ground under leaf litter or mulch. The cecropia moth caterpillar spins its cocoon attached to a tree branch. Caterpillars are 3 to 4 inches long with spiny red, blue and yellow tubercles. Caterpillars feed on cherry, maple, apple and other trees all summer. In the fall they spin a large brown cocoon with a tough outer layer to protect them from predators.
Most insects and spiders will remain in your yard and garden throughout the winter. Research has found that leaf litter and mulch will help preserve predatory insect abundance in winter, and reduce pest problems in spring.
Many gardeners work hard to plant the correct flowers to attract and feed butterflies and other beneficial insects in the summer. Allow stalks of perennial flowers and grasses to remain in the garden through the winter to provide shelter for many insects and preserve the eggs and pupae of butterflies and other insects that may be attached to them. Considering the needs of butterflies and other insects when winterizing your garden could increase their abundance in spring and help conserve some threatened insect species.
A version of this article appeared in Carolina Gardener Volume 22, Number 9.
Photography courtesy of Steve Frank.