A fungus that kills browntail moth caterpillars appears to have knocked back populations of the perennial pest in portions of Maine this spring, providing a reprieve from the annual suffering caused by the tiny hairs they release into the air.

But it’s too early to know whether the fungus will help spur a statewide population collapse of the forest pest or if this is the start of a prolonged break in the yearslong infestation that has spread along the coast and inland.

Microscopic hairs from the caterpillars can cause an extremely itchy, lingering rash in people, typically from May to mid- to late June, before the adult caterpillars pupate and transform into moths. In addition to a rash, symptoms can include respiratory problems, including difficulty breathing and irritation of the lungs and throat.

There is no data that would show how many people experience these symptoms, but potential exposure to the microscopic hairs is linked to how robust the population of browntail moth caterpillars is each year.

Thomas Schmeelk, an entomologist with the Maine Forest Service, said the weather this spring created “Goldilocks” conditions for the fungus to proliferate and kill the caterpillars – not too dry and not too rainy. A virus that kills the caterpillars – also tied to weather conditions – has also contributed to reducing browntail moth populations.

“We have many reports of the fungus and the virus throughout the infested areas of the state, and it’s a pretty wide swath. It’s a very good sign. We are hopeful this is the beginning of the end for this outbreak (of browntail moths),” Schmeelk said.

While rain helps grow the fungus, too much rain, like the consistent rainy conditions in spring 2023, suppresses the proliferation of the fungus, which made it less effective in controlling browntail moth populations last spring, he said.

But this year, periodic heavy rainstorms have often been followed by several days of dry weather, which has helped to disperse the fungus, Schmeelk said. Spores from the fungus – entomophaga aulicae – need a break in the wet weather so that wind can carry them to other locations to infect more caterpillars.

Schmeelk said an aerial surveillance of leaf canopy this August and September will help to quantify the range and density of a new crop of browntail moth caterpillars that will be emerging from eggs this summer and later overwinter in their winter webs.

The amount that the newly emerged caterpillars eat this summer should help determine how effective the fungus was in killing the adult caterpillars this spring. Browntail moth caterpillars tend to feed on leaves of fruit, oak, birch, elm and poplar trees.

The smaller population of adult caterpillars this spring is attributed to fungal and viral infections this year. But a separate fungal outbreak last fall also may have helped control populations by making some caterpillars carriers of the fungus.

The “zombie caterpillars” infected by a fall fungus often live long enough to carry it back to the exterior of the winter webs before dying.

When the surviving caterpillars emerge from their winter webs, they can become infected by the fungus by crawling over the infected caterpillars that died in the winter webs.

Schmeelk said there are encouraging indications that the fungus outbreaks helped control populations this spring, with significant outbreaks observed in Brunswick, Freeport, Durham, Waterville, Richmond, Old Town and other areas.

“Certainly some regions experienced relief this year,” Schmeelk said.

Separate from the year-to-year weather conditions that can help the fungus curtail browntail moth populations, the moths also tend to be in a “boom-bust” cycle, with a “boom” typically lasting 10-12 years. Maine is currently in the ninth year of the “boom” cycle.

Angela Mech, assistant professor of forest entomology for the University of Maine, said she also is seeing some reports that the fungus has been effective this year. Mech and a team of students have been studying the browntail moth for years, including whether environmentally friendly pheromone treatments can work to control populations.

“There were a number of areas out by us in Orono that definitely saw the effects of a spring caterpillar mortality event, in many cases, by the fungus. Trees that had dozens of winter webs on them at the start of spring had barely any defoliation by early June, when we would have expected to see a lot of feeding,” Mech said.

Schmeelk said to minimize browntail moths from laying eggs on your property, minimize the amount of outdoor lights you put on at night, as the moths are attracted to the lights.

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