May 28—The browntail moth population collapse that began in 2024 is continuing this year, which means a less itchy spring for Maine residents.
Browntail moth caterpillars shed microscopic hairs that drift through the air and can provoke an allergic reaction when they come into contact with human skin, often resulting in red, itchy rashes that can last for several weeks. The hairs can also cause respiratory problems for people with chronic conditions, like asthma.
The caterpillar hairs have been most plentiful, and most annoying, between mid-May and the end of June.
But this year, a combination of factors is providing relief from the usual itchy conditions in much of the state.
The moths have a natural boom and bust cycle, and also can be killed in large numbers by pathogens, including a fungus that proliferates during rainy springs like the ones Maine experienced this year.
The state tracks tree defoliation caused by the browntail moth while they are in their caterpillar stage and munching on leaves, especially the leaves of fruit, oak, birch, elm and poplar trees. The area of defoliation peaked in 2022 at about 200,000 acres, but crashed to 2,000 acres last year.
There are still pockets of Maine that have enough browntail moth caterpillars to create some distress, including the Sebago Lake region, Lewiston-Auburn, Orono, Old Town and parts of Waldo and Knox counties, said Thomas Schmeelk, Maine Forest Service entomologist. But, he said, there is no indication that populations will come back in most areas in 2025 based on the numbers of webs where the caterpillars overwinter.
“I’m happy to report that the populations should remain down this year,” Schmeelk said.
The rainy weather, interspersed with a few sunny days, has been good for the fungus that kills the caterpillars.
“Even though we hate the cloudy and rainy weather, it’s really good for the pathogens,” Schmeelk said.
The natural boom and bust cycle lasts about 10-15 years, so Maine could be in for an extended period of relief from the forest pests, Schmeelk said. This year is considered the 10th year after the start of the boom cycle.
Longer-term trends favor the browntail moth, however. Climate change is extending the moth’s range and making it easier for the moths to survive winter.
“Drier springs hurts the pathogens and help the browntail moth. And extended summers that go deeper into fall give the caterpillars more time to feed, so they are healthier going into their winter webs,” Schmeelk said.
In past years in areas of high infestations — including Yarmouth, Brunswick, Freeport, Deering Oaks Park in Portland and other cities and towns in the Midcoast — the hairs became difficult to avoid, and local pharmacies made homemade compounds to ease symptoms from the rash.
The rash results from both a chemical reaction to a toxin in the hairs and a physical irritation as the barbed hairs become embedded in the skin.
If you are in an area with a browntail moth infestation, the Maine Centers for Disease Control and Prevention recommends covering your face and exposed skin. Wear a hat, long-sleeved shirts and pants and cover your face with a dust mask. Also wear a disposable coverall.
Try to avoid areas with high infestations. If there is an infestation on your property, wait for a damp day and spray vegetation on your property with water to help reduce the microscopic hairs from becoming airborne.
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