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Browntail moth caterpillars, the invasive insects carrying rash-causing hairs, are continuing to die off across Maine.
A state monitoring program with sites in Bangor, Bridgton, Newport and Turner has found visible differences in development among caterpillars at the same sites within the last month, a trend similar to last year. That can be a sign of parasitism or infection by pathogens, according to state officials.
At three of the four state monitoring sites, caterpillars have been found dead from viral and fungal pathogens. It comes in the second consecutive year showing signs of relief from the itchy hairs they shed that can make outdoor activities unpleasant, though their populations seem to follow natural boom-and-bust cycles, according to experts.
The caterpillars affected more than 150,000 recorded acres across Maine at their 2022 peak, according to state data, an estimate that dropped by more than half the next year. Their microscopic hairs can cause long-lasting itchy rashes and respiratory issues for some people.
Fewer than 5,000 acres were known to be affected in 2024 and 2025, a crash helped along by rainy conditions that boosted the prevalence of a fungus and virus that target the caterpillars. People have been encouraged to cut the caterpillars’ winter webs, and tree treatments are also available.
Webs decreased across much of the state this winter, according to the Maine Forest Service.
The state is now seeking reports of diseased caterpillars with photos attached to help show what pathogens they may have.
Signs of a fungus include white or yellow fungal spores on the caterpillars, or caterpillars hanging from branches by their rear “prolegs,” which look like legs but extend from their exoskeleton. If a dead caterpillar is liquified, that’s a sign of a virus, and they often rest in a V-shape upside down, according to the forest service.
A swollen, puffy caterpillar can be a sign of both virus and fungus presence.
Native fungal pathogens that reduce the moth can be bolstered by prolonged rainy conditions, the forest service noted last year, when caterpillars were also being found dead and populations declined at monitoring sites.
At this time of year, older caterpillars are commonly wandering farther from their nests looking for food or escaping predators, according to the service. If they’re wandering near your house, you can knock them down with a strong jet of water or use a wet vacuum with several inches of soapy water and a HEPA filter, letting them soak for several days.
The adult moths generally become active later this month and stay out until early August. Turning off outdoor lights at night might attract fewer of them to your yard.
