The rainy, unseasonably cool days in May and June triggered die-offs of some browntail moth caterpillar populations, scientists say, providing temporary relief to Mainers who battle the itchy rash and respiratory problems caused by the caterpillars.

A browntail moth caterpillar Maine Department of Agriculture, Conservation and Forestry

If there’s frequent damp weather in late spring, that can lead to the proliferation of a fungus that kills the caterpillars. In some areas this year, weather conditions were just right for the fungus to attack the caterpillars.

“In a few locations, we are seeing collapses of browntail moth due to the fungus entomophaga aulicae and possibly other pathogens,” said Thomas Schmeelk, an entomologist with the Maine Forest Service.

In May, scientists had predicted in that browntail moths would be prolific this year, based on the large number of webbed nests observed at treetops.

But Schmeelk said Tuesday that field surveys in June revealed localized collapses caused by the fungus in Bristol, Arrowsic, Whitefield, Jefferson, Camden, Turner, Harpswell, Brunswick and Wiscasset.

The high season for the caterpillars is typically mid-May through June, and by early July they start forming into pupae. The nearly microscopic-sized caterpillar hairs that are toxic to human skin get caught up in the wind and can cause health problems to nearly anyone near trees, especially oaks. While a bad rash that’s similar in intensity to poison ivy is most common, some can suffer respiratory problems as well.

The rash is hard to avoid as the hairs are difficult to see and can penetrate clothing.

Typically, browntail moths are most prevalent in Freeport, Brunswick, Yarmouth, Falmouth and areas up through Wiscasset, but the moths have been expanding their range up and down the coast and inland. Portland reported a mild browntail moth infestation at Deering Oaks park last week.

In Freeport, Town Manager Peter Joseph said in his “unscientific” observations, there are fewer caterpillars near his home, office and other areas of Freeport.

“The numbers I’ve seen are really reduced,” Joseph said. “Last year they were everywhere, crawling all over the sides of buildings. People were stepping on them on the sidewalks.”

At  Coastal Pharmacy in Portland, marketing director Kim Crabb said they’ve had fewer requests this year compared to 2018 for the specialized lotion and spray that the pharmacy makes to help those suffering from the rash. Several pharmacies in the area create their own rash lotions for their customers.

“Demand (for the products) has been really slack this year compared to last year,” Crabb said. “We have definitely seen the effects.” Crabb said they have seen more lotion orders recently from the Rockport and Rockland areas.

Those who develop a rash should wash it immediately with cold, soapy water, and avoid hot water.

Schmeelk, the entomologist, said scientists don’t have the resources to conduct widespread field surveys, so it’s difficult to say exactly where and to what extent the population collapses have occurred. Variations in rainfall and temperature in coastal areas can make a difference in how the fungus affects the caterpillars.

“It’s a localized population collapse, not a statewide population collapse,” said Schmeelk, adding that the effects are likely to last one season and not cut into browntail moth populations in 2020. The exception would be if there were a few years in a row of similar rainy, cool weather in May and June.

“Population trends in general last one year and do not carry over to the next, but if it happens two to three years in a row, it can have a cumulative effect,” he said.

The last large-scale collapse of browntail moth populations in Maine occurred in 2004 and 2005.