A browntail moth caterpillar sits on a green leaf
Browntail moth caterpillars, like those seen here, are coming from the trees and bringing with them health risks, according to the Maine Centers for Disease Control. Credit: Courtesy of Maine CDC

Bad news for Mainers: It’s likely to be another bad season for browntail moths.

While Mainers have been enjoying picturesque, sunny spring weather in recent weeks, it’s not the type of weather that will help reduce browntail moth caterpillar populations ― and the tree defoliation and rashes they cause ― in coming months.

Forest insect experts predict that this year’s browntail moth conditions will be as bad as last year ― the worst Maine has ever experienced ― unless more rain falls in the next month and a half to help bolster the spread of diseases that kill the caterpillars.

Without wet weather, folks in areas that have had high populations of browntail moths in recent years ―  especially along the coast and adjacent inland counties ― will likely not get a reprieve from these irritating caterpillars.

“A lot of the areas that have [had] really high populations of browntail will likely have high populations of browntail again this year, unless we do get some rain a little bit later in May here,” Maine Forest Service entomologist Tom Schmeelk said. “[Improvement is] not likely, unless we get that weather.”

For the Knox County area, the recent sunny weather is predicted to stay through Saturday before transitioning into a several day stretch that carries a chance of showers, according to the National Weather Service.

Maine has been experiencing a yearly browntail moth outbreak since 2015, when the invasive species was responsible for more than 10,000 acres of tree defoliation in the state. Over the last seven years, populations have rapidly grown, with almost 200,000 acres damaged mapped in aerial surveys last year, according to the Maine Forest Service.

In 2021, browntail moth populations were detected in every county, with the highest populations in Androscoggin, Cumberland, Kennebec, Knox, Sagadahoc and Waldo counties.

The moths create the biggest nuisance when they are in their caterpillar stage. The caterpillars have toxic hairs that cause a blistery rash when a person comes in contact with them. The hairs can also cause respiratory distress if they are breathed in.

In April, browntail moth caterpillars emerge from their winter webs that are spun in trees. The Maine Forest Service confirmed observance of caterpillar emergence at some of its monitoring sites, including in Brunswick, by late April after receiving reports of emergence elsewhere as early as April 11.

“They are definitely awake right now. If you have nests on your trees and you don’t see caterpillars that’s a good sign. It probably means that they died last fall. But definitely they are awake and feeding in all parts of the state,” said Angela Mech, an assistant professor of forest entomology at the University of Maine.

Once they emerge, the caterpillars will feast on the buds and leaves that are unfurling from the host trees where their webs are located. As the caterpillars grow, they shed their skin and toxic hairs into the environment.

By late May, Schmeelk said people typically start to notice the defoliation of trees in their yards because they have reached a size where they’re consuming a lot of foliage. As the caterpillars exhaust their food source, they’ll drop out of the trees and wander around.

Caterpillars will continue shedding their toxic hairs until they pupate and emerge from their cocoon as moths around early to mid-July. The moths do not shed the toxic hairs.

In recent years, warm and dry weather have helped browntail moth caterpillar populations increase because fungal and viral diseases that can kill the caterpillars need cool, wet weather to thrive.

Late last summer, these diseases were detected in some areas ranging from Blue Hill down through the midcoast, Schmeelk said, but the prevalence has been isolated to individual trees and is not yet widespread enough to cause population collapses.

April provided some cool and rainy weather, but the crucial time for wet weather to hurt caterpillar populations is when they are most active in May and June, Schmeelk said.

“In order for the fungus to kind of have the right conditions for it to impact the browntail moth, it needs to be a little cool and wet,” Mech said. “That’s what we haven’t had in some of these previous springs. So every time you see rain in the forecast here this month, be a little excited about the chance.”

Mech said it would likely take at least two to three consecutive years of cool, damp spring weather to cause enough of a widespread population decline to get Maine out of an outbreak stage.

Until then, Mainers in areas with high caterpillar populations will have to be proactive to help mitigate the populations in their yards and also know how to be prepared for the potential interactions with the caterpillar’s toxic hairs.

While it’s past the time of year when folks can clip the overwintering nest from trees, licensed pesticide applicators and arborists can use pesticides to kill the caterpillars through the end of May, according to the Forest Service. However, with the high demand for this service, it may be difficult to access this service in time if it hasn’t already been scheduled, according to Schmeelk.

To limit exposure to the caterpillars’ hairs when they are most prevalent between April and mid-July, Schmeelk recommends doing yard work on days after it has rained, or wetting down an area first, to help prevent the hairs from becoming airborne while you are working.

The Maine Center for Disease Control and Prevent recently released information on other precautions to take to protect yourself from exposure to browntail moth hairs when outside.

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