Those flashy fall caterpillars aren’t as dangerous as you might think — well, most of them anyway

Editors note: This story was originally published in October 2019.

During autumn in Maine, the leaves aren’t the only colorful things on the landscape. Crawling through the cool grass, caterpillars of all shapes and shades are on the move, and many of them are quite flashy.

“There’s the spotted yellow tussock and the milkweed tussock, which I think looks like it’s had a bad hair day,” said Charlene Donahue, president of the Maine Entomological Society.

With long orange, black and white hairs sticking out in all directions, it does appear that the milkweed tussock just rolled out of bed. But actually, it’s the other way around. The caterpillars that are wandering around Maine in the fall are heading to bed, in a way.

As the weather cools and the days shorten, these small creatures are searching for shelter, and perhaps a final meal, before spending several cold months in hibernation.

“During the growing season, they’re usually on their host plants and kind of hidden away,” Donahue said. “In the fall, they’re on the move, so people are more apt to notice them.”

The majority of the fancy-looking caterpillars seen in the fall in Maine are the larvae of moths, Donahue said. Some will spend the winter in their caterpillar form, while others will spin a cocoon, moving into their next life stage before being buried under snow.

The woolly bear, one of Maine’s most well-known fall caterpillars, is one that spends the winter as a caterpillar.

Covered in thick, fluffy-looking hair, the woolly bear sports bands of black and reddish-brown. There’s an age-old belief that the amount of brown on this critter can predict the severity of the coming winter. The more brown, the milder winter will be.

“The woolly bear caterpillar — they’re showing up now in pretty good numbers,” said Herb Wilson, biology professor emeritus at Colby College. “And then there are the tent caterpillars and fall webworms. Those are pretty common.”

Weaving nests in trees in the late summer and fall, the fall webworm is a yellow caterpillar that has long, spiky-looking hair. Its body is dotted with black and orange spots. And before winter sets in, it retreats into a brown cocoon, often nestled under bark or leaf litter.

The smeared dagger caterpillar is another bright fall wanderer, Wilson said. It has a black and yellow body covered with clusters of spiky, rust-colored hair.

There are two ways that caterpillars get through Maine’s harsh winter, Wilson said. One way is the “freeze avoidance strategy,” in which the animal produces a kind of antifreeze solution that prevents ice from forming inside their body. The other way is called the “freeze tolerant strategy,” in which ice actually forms inside certain parts of the animal’s body.

“So you can pick up a caterpillar and it seems to be a block of ice, but then once spring rolls around, they’re good to go,” Wilson said.

Next year, these caterpillars will transform into moths.

“Some of them have some pretty nice patterns on them,” Donahue said, “while others are not so exciting because they fly at night and don’t need to be flashy [to deter predators].”

The strange-looking milkweed tussock caterpillar turns into a moth with drab gray wings, though it does retain some color on its orange-tan body, marked with black spots. The bright fall webworm becomes a mostly white moth, though if you look closely, you may spy some orange or yellow patches on its front legs. And the woolly bear transforms into the Isabella tiger moth, which is orange-yellow, with black spots on its wings and body.

Are fall caterpillars dangerous?

Most of these colorful, hairy caterpillars are harmless to people. However, if touched, a few have irritating hairs that can cause people to develop skin rashes.

“It’s really if you handle them,” Donahue said, “and most of them still aren’t a problem. Wooly bears are fine [to handle]. Some tussocks are not.”

In particular, the hickory tussock can cause quite a severe rash, according to the University of Maine Cooperative Extension . However, it’s important to note that you actually have to touch this caterpillar to develop the rash.

“The only moth that you don’t have to come into contact with to cause problems is the browntail moth,” Donahue said.

Also a member of the tussock family, the browntail moth caterpillar is covered in tiny hairs that can cause a rash similar to poison ivy. Furthermore, their microscopic hairs can float through the air, causing some people to experience respiratory distress.

In Maine, this moth is considered to be an invasive species and a pest. In recent years, their numbers have increased in certain areas of Maine, especially in the midcoast region .

While browntail moth caterpillars are building nests in the fall, the greatest risk for exposure to their toxic hairs is between April and July, according to the Maine Division of Disease Surveillance . Late October is actually a good time to clip their nests out of trees and burn them or douse them in soapy water, lowering their population before next spring.