The browntail moth caterpillar, an invasive species that has proven harmful to humans, has a higher population than usual in Maine this spring, and the Maine Forest Service is warning residents to stay clear of them.

The microscopic hairs on the caterpillars can cause “a blistery, oozy rash and respiratory distress,” according to a recent press release issued by the Maine Forest Service.

“It’s like poison ivy, where some people react and some people don’t,” said Maine Forest Service entomologist Charlene Donahue. “And the rash is actually similar to a poison ivy rash.”

Direct contact with a caterpillar isn’t necessary for ill effects. When the caterpillar molts, the barbed hairs break-off and become airborne. These airborne hairs can lodge into people’s skin or be inhaled, and the hairs remain toxic for a year or more after they break away from the insect.

“You can get a rash from just being outside in an infested area,” Donahue said. “The other thing is, you can develop a sensitivity to the caterpillar over time, if you’re exposed to the hairs over and over again.”

Browntail moth caterpillars overwinter in webs that usually are at the top of oak trees but also in apple trees and sometimes birch. Surveys conducted by the Maine Forest Service this past winter identified extremely high winter web counts, according to the press release.

Winter web counts were highest in parts of Bowdoinham, Bath, Topsham, West Bath, Brunswick, Freeport and Harpswell this year. Browntail moth webs also were seen in more southerly places, including Cape Elizabeth, Scarborough, Westbrook, Windham, New Gloucester and Yarmouth.

Pockets of infestation also were found in coastal locations, from Lincoln County south, as well as inland towns such as Augusta, China, Vassalboro, Waterville, Lewiston, Turner, Whitefield and Gardiner.

“Gardiner High School is having a problem with them,” Donahue said. “They completely stripped every single leaf and flower from their crab apple trees, and they moved all over the building, surrounding trees and benches.

“It took a while for them to figure out what the problem was because Gardiner was outside of where we usually see browntail moths,” she said.

Donahue isn’t sure whether the school plans to use pesticides to control the infestation, but she does know the school has stopped using certain doors to avoid contact with the noxious insects.

“There aren’t really any good natural controls for it,” Donahue said. “The birds don’t seem to like them, and they don’t have many insect predators or parasites. There is a fungal disease that gets into the population once the numbers get really high in an area, but it doesn’t seem to be effective.”

An invasive species, the browntail moth arrived in the U.S. around 1910 on nursery stock coming from Europe, according to the Maine Forest Service. They spread through Massachusetts, Maine, New Hampshire and Nova Scotia, Canada, before the population collapsed — the specific reason for which is unclear. The only place where the browntail moth is found in North America is the coast of Maine and Cape Cod.

Since then, the population has fluctuated, Donahue said.

The Maine Department of Agriculture, Conservation and Forestry has posted a list of recommended precautions for people visiting areas infested with browntail moth caterpillars from June to August, when the toxic hairs are most prevalent in the atmosphere, at

The list includes performing outdoor tasks on damp days or wetting down material with a hose, as moisture helps prevent the hairs from becoming airborne. The chances of coming into contact with the toxic hairs increases during dry, windy days.

“It’s kind of a nasty little bug,” Donahue said. “We don’t have very many things in Maine that cause problems in the natural world. This is from away, and it definitely causes problems.”

If you develop a severe reaction to the browntail moth, it’s important to consult your physician.

Certain pesticides can be used to control a population of these caterpillars. For the list of licensed pesticide companies dealing with browntail moth caterpillar, call the Maine Forest Service entomology lab at 287-2431 or email Donahue at

For more information about the browntail moth caterpillar, visit

Aislinn Sarnacki

Aislinn Sarnacki is a Maine outdoors writer and the author of three Maine hiking guidebooks including “Family Friendly Hikes in Maine.” Find her on Twitter and Facebook @1minhikegirl. You can also...