Jan. 4—Proliferation of a fungus caused browntail moth populations to crater in 2024, which likely means that Maine residents will get relief from the forest pest this spring and summer, scientists said.
“I am cautiously optimistic about a less itchy summer,” said Angela Mech, a University of Maine assistant professor of forest entomology who studies the browntail moth.
The microscopic hairs of the browntail moth caterpillars, when they come into contact with people, cause a red, itchy rash that can last for weeks. The hairs can also cause respiratory problems, especially in people with chronic conditions like asthma.
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In areas of high infestations, which in past years included Yarmouth, Brunswick, Freeport and cities and towns further up the Midcoast, the hairs, which can penetrate clothing, were difficult to avoid, as the wind dispersed them widely. Deering Oaks park in Portland also experienced heavy infestations in the early 2020s.
Typically, the worst part of the year for contracting the rash is mid-May to late June. Maine pharmacies make homemade anti-itch compounds to try to provide relief.
Thomas Schmeelk, Maine Forest Service entomologist, said that the state tracks defoliation caused by the caterpillars, and saw it plummet from 46,000 acres in 2023 to 2,000 acres in 2024.
Schmeelk said weather conditions in both those years — primarily rainy springs — helped boost the population of the fungus — entomophaga aulicae — and also a virus that attacks the caterpillars.
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Schmeelk said the extent of the population plunge was a “little bit surprising,” but the pathogens had ideal weather conditions to attack the caterpillars over the past two years.
“Sometimes it can take multiple years in order to crash the population,” Schmeelk said.
Mech said faculty and students have been monitoring and researching browntail moth populations in Orono for years, and this summer they saw the fungus devastate local populations.
“The fungus ruined our experiments because we ended up with a lot of dead caterpillars,” Mech said.
UMaine was trying to study how long the caterpillars could go without feeding. Mech said there was some evidence that caterpillars avoid eating leaves from trees treated with BTK, a biological, environmentally-friendly insecticide. If they avoid eating the leaves, would they die of starvation? Or could they survive and go back to eating the leaves once the BTK had dissipated from the leaves?
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Browntail moth caterpillars tend to feed on leaves of fruit, oak, birch, elm and poplar trees.
Mech had also previously pointed to how “zombie caterpillars” from fall 2023 could have helped reduce populations in spring 2024. Caterpillars infected with the fungus in the fall become carriers, and it “zombifies” them. The “crawling dead” will then carry the fungus to winter nests that are being built. When a new population of caterpillars emerges in the spring, those that come into contact with the dead caterpillars may pick up the fungus.
The university is also researching whether a pheromone can be used to disrupt the mating patterns of browntail moths, another way to control the population.
Browntail moth populations typically go through a boom-bust cycle that lasts 10-12 years, and 2024 was considered the ninth year of the current outbreak. So in year 10, 2025, it appears likely that Maine will enter a “bust” cycle for the browntail moth.
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