This browntail moth caterpillar found in Augusta was killed by a fungus. It's producing spores that will kill other caterpillars. Credit: Courtesy of Maine Forestry Service

Researchers with the Maine Forest Service have noticed something welcome at some of their monitoring sites in recent days — browntail moth caterpillars that have been killed by a fungus that thrives in wet, cool conditions.

The die-off, most noticeable in the midcoast and parts of the capital area, presents a glimmer of hope for Mainers contending with the itchy rash, respiratory issues and other problems caused by the caterpillars.  

But it really is just a glimmer, officials said.

This 2017 photo by Holland Haverkamp shows a browntail moth caterpillar in Maine. Credit: Holland Haverkamp / UMaine via AP

“We’re really pretty cautiously optimistic that there’s going to be some relief in parts of the outbreak area,” Allison Kanoti, the director of forest health and monitoring at the Maine Forest Service, said Friday. “But we don’t want to be overly optimistic. We do expect that if there’s a decent amount of fungus, the relief could last a couple of years.”

Earlier this year, experts had predicted that the warm, sunny conditions in early May would lead to high populations of browntail moth caterpillars. That’s not good for both the trees and the people living in affected areas.  

But the rain that has fallen on and off in the past two weeks has changed the equation.

The precipitation has helped the fungal diseases develop in browntail caterpillars. In a recent bulletin, the Maine Forest Service said that it had found at least some fungus-caused mortality at all of its monitoring sites, and at a few sites, it wasn’t easy to find live caterpillars at all.

Some areas where trees had many webs this past winter, researchers haven’t found the severe canopy defoliation they expected to find.

The monitoring areas include Cumberland, Brunswick, Turner, Manchester, Chelsea, Whitefield, Jefferson, Belfast, Lincolnville and Liberty.

“It’s not been a wet spring in particular, but there appears to have been enough moisture, enough fungus around in the environment and enough caterpillars to trigger a disease in the caterpillar population,” Kanoti said. “That is good news for those who live in that area and hopeful news for those who live outside it.”

Still, the reprieve is found in pockets, she said, and the state hasn’t done enough surveying to definitively know the areas affected by the fungal disease, she said.

Outside of those pockets, the impact of the caterpillars can still be severe. And even if there are no signs of caterpillars in a specific location, the toxic microscopic hairs from last year’s caterpillars can still cause problems.

The browntail moth caterpillar outbreak in 2021 was a bad one.

“We’re not out of the woods yet,” the forest service bulletin said.

Maine has been experiencing a yearly browntail moth outbreak since 2015, when the invasive species was responsible for more than 10,000 acres of tree defoliation in the state. Over the last seven years, populations have rapidly grown, with almost 200,000 damaged acres mapped in aerial surveys last year, according to the Maine Forest Service.

In 2021, browntail moth populations were detected in every county, with the highest populations in Androscoggin, Cumberland, Kennebec, Knox, Sagadahoc and Waldo counties.

The last outbreak in Maine, which was centered on the Casco Bay region, lasted from 1999 to 2005. It ended due to a combination of a cool, wet spring and a fungal disease that infected the moths.

coping with browntail moth caterpillars

Mainers affected by browntail moths are encouraged to use an integrated approach to mitigation, Kanoti said. Useful steps include clipping the webs beginning in November, limiting the use of pesticides and turning off outside lights from late June to late July, when the caterpillars have emerged as moths and are on the move.

People are also asked to keep an eye out for diseased browntail caterpillars. Infected caterpillars have a puffy, swollen appearance and are covered in fungus spores, which look like white or yellowish dust. When caterpillars die from the fungal disease, they hold tight to the branches they die on and help to spread the caterpillar-killing spores.

The Maine Forest Service would like to hear about infected caterpillars, and asks that reports be sent to foresthealth@maine.gov.

And ultimately, it will get better, Kanoti said.

“An outbreak doesn’t last forever,” she said. “These things do pass.”