Several caterpillar species are active during winter months, and the term “winter caterpillar” can refer to any of them. Some survive freezing temperatures by hiding under leaf litter and producing their own antifreeze. Others stay perched on branches in plain sight all winter long. And one famous species, the winter moth caterpillar, gets its name not from overwintering as a larva but from its adults emerging in the coldest months. Here’s what you need to know about each.
Caterpillars That Stay Active in Winter
The most familiar winter caterpillar is the woolly bear, the fuzzy black-and-brown larva of the Isabella tiger moth. In autumn, woolly bears leave their food plants and wander across roads and sidewalks looking for sheltered spots to spend the winter: leaf litter, rock crevices, fallen logs, or the inside of barns and sheds. They curl up and enter a dormant state, but they can tolerate astonishingly cold temperatures, surviving conditions as low as -90°F by producing compounds in their body fluids that work like antifreeze. On warm or sunny winter days, they sometimes emerge and wander before tucking back in.
Woolly bears aren’t alone. A whole group of inchworm caterpillars in the geometrid moth family spend winter exposed on branches or pressed against foliage rather than hiding. These include the larvae of the porcelain gray moth, the esther moth, the one-spotted variant, several euchlaena species, the common lytrosis, and the crocus geometer. They’re twig-shaped and well camouflaged, so most people walk right past them. The porcelain gray moth caterpillar is one of the most abundant, found every month from September through May in parts of New England. Euchlaena caterpillars, colored in patches of brown, red, and gray, often turn up on blueberry bushes, especially at night.
A few butterfly caterpillars also overwinter. The viceroy and the red-spotted purple hibernate as half-grown larvae, tucked away until the first warm days of April push them out onto their host plants to search for fresh leaves. Various noctuid moth caterpillars do the same. The white pine dart, for instance, can be spotted nibbling pine needles on warmer winter and early spring days.
The Winter Moth Caterpillar
The species most commonly called the “winter moth” is Operophtera brumata, a pest whose adult moths emerge in late autumn and winter. The females are wingless and climb tree trunks to lay eggs, which overwinter on the bark and hatch in early spring once temperatures reach about 55°F. The tiny caterpillars then crawl upward and burrow into leaf and flower buds, feeding on expanding foliage.
Winter moth caterpillars are generalist feeders. Their preferred hosts include oak, maple, apple, elm, ash, crabapple, cherry, and blueberry, but they’ll eat many other plants too. Young caterpillars produce silk threads they use to “balloon” to new trees and to tie buds and leaves together for shelter while feeding. A single year of heavy defoliation is usually tolerable for a healthy tree, but three or more consecutive years of severe feeding can cause branch dieback and even kill the tree.
Managing Winter Moth on Fruit Trees
The most effective low-tech defense is a grease band or sticky barrier wrapped around the trunk. Because the female moths are wingless and must walk up the trunk to lay eggs, a sticky band intercepts them before they reach the canopy. The Royal Horticultural Society recommends placing bands about 45 cm (18 inches) above soil level in late October, before adults begin emerging in November. The band needs to stay sticky and free of debris from late October through April, since some related species with wingless females are active that late.
For smooth-barked trees, pre-made sticky paper bands work well. For trees with rough, fissured bark, applying horticultural grease directly to the bark creates a better seal. The glue should not be strong enough to trap birds, bats, or mice. On ornamental trees, the RHS advises tolerating the damage rather than treating it, since winter moth caterpillars are an important food source for nesting birds in spring and the defoliation won’t threaten the tree’s long-term health.
Caterpillar Fungus: “Winter Worm, Summer Grass”
If your search brought you here through traditional Chinese medicine, you may be looking for something entirely different. The Chinese name “dong chong xia cao” translates to “winter worm, summer grass,” and it refers to a parasitic fungus called Ophiocordyceps sinensis that infects ghost moth larvae on the Tibetan Plateau. The fungus invades the larva underground during winter, eventually killing it. In summer, a slender fungal stalk sprouts from the mummified body, poking above the grassline. Despite the common English name “caterpillar fungus,” the host is technically a larva (hairless) rather than a caterpillar.
This organism grows only at high altitudes, between about 3,900 and 4,900 meters above sea level, in grasslands above the tree line. The largest specimens, harvested at the highest elevations, are considered the highest quality. In Tibetan it’s called “yartsa gunbu,” meaning “summer herb, winter worm,” and sometimes simply “bu” (worm). It has been used in traditional medicine for centuries and remains one of the most expensive biological products in the world by weight.
Safety Around Winter Caterpillars
Most overwintering caterpillars in North America are harmless to handle. Woolly bears are famously docile, and the twig-like geometrid larvae pose no risk at all. In parts of Europe and expanding regions, however, the pine processionary moth is a winter and early spring concern. Its caterpillars carry thousands of tiny hairs containing an irritating protein called thaumetopoein. Contact with these hairs causes painful skin rashes, eye irritation, and throat inflammation. In rare cases, it triggers a serious allergic reaction. Dogs are especially vulnerable because they tend to sniff or mouth the caterpillars.
If you encounter processionary caterpillars or their white silk nests in pine trees, keep your distance. Don’t try to remove nests yourself. A pharmacist can provide relief for mild skin or eye irritation after contact, but a more severe reaction warrants medical attention. Pets showing symptoms like excessive drooling or a swollen tongue after contact should see a vet promptly.