Browntail moth populations have been declining across Maine over the past few years, giving the state some relief from their rash-causing hairs and their signature tree damage.
In 2024, they dropped sharply, affecting 148,000 fewer acres statewide than two years previously.
But they haven’t gone away, and their winter webs show signs of larger populations surviving in a few pockets around the state, including one near Bangor. To stop the caterpillars from emerging in mid-April, now is the time to clip and destroy their winter webs.
“Time is running out,” said Brittany Schappach, an entomologist with the Maine Forest Service.
Browntail moths, an invasive species, have been established in Maine for decades and seen population surges throughout the years. In addition to the rashes they cause people, the caterpillars feed on trees, chewing through their leaves and sometimes causing lasting damage.
More than 150,000 acres across Maine were reportedly defoliated in 2022, a number that dropped down to roughly 46,000 acres the following year. In 2024, that number dropped to just about 2,000, according to Schappach.
Experts have said that’s likely due to the spread of a fungus and other pathogens that can kill the caterpillars, helped along by weather conditions. It’s also possible there were so many they out-competed each other for food, or that some just didn’t make it through the winter.
But some high-activity areas remain around the state, identified by higher numbers of webs in the trees over winter. Those include towns around Sebago Lake, a swath of southern Maine from Livermore Falls to Falmouth and the Hancock County town of Dedham.
The state does its winter web survey on primary roads through town, according to Schappach, and in Dedham saw webs on Route 46 around Philips Lake on Route 1A. The area isn’t as severe as in southern Maine, but there were also notable populations on MDI, in Eddington, Orland, Penobscot and down to Sedgwick.
City staff in Bangor reported far fewer sightings last summer through the local monitoring program than in the years before, though the moths remained a problem in some spots including Fairmount Park despite chemical treatments.
But overall, there seemed to be a marked decline, and reports from residents had also dropped sharply, staff wrote in July. That meant fewer caterpillars to survive through the winter.
Experts recommend destroying the webs before the caterpillars emerge in mid-April. They appeared on April 15 last year, but warmer temperatures this year mean they could come out earlier, Schappach said.
Once clipped from the tree, webs can be soaked in soapy water overnight or burned to kill the moths.
Scientists can’t predict the future, but it’s possible numbers will remain at their current low level in 2025 without dropping much more, Schappach said. Each web can contain up to several hundred caterpillars, and once they emerge they could spread and hitchhike to new locations.
“I don’t want people to think it’s something they don’t need to worry about anymore,” she said.
