Maine Forest Service: Browntail moth caterpillar infestation collapses

Posted Tuesday, June 28, 2011 in Sustainable Maine

Maine Forest Service: Browntail moth caterpillar infestation collapses

Browntail moth caterpillar cocoon.

AUGUSTA – There’s good news on the bug front: The population of a noxious caterpillar infesting the Brunswick area appears to have been killed off by a fungal disease brought on by May’s rainy weather, according to the Maine Forest Service.

A naturally occurring fungus, Entomophaga aulicae, which affects only browntail moth caterpillars, appears to have wiped out the infestation in the Brunswick, Bath, Topsham and Bowdoinham area, according to MFS Forest Entomologist Charlene Donahue.

Winter web surveys had indicated that the caterpillar population, which can harm human beings, was going to be extremely high this year. Twice as many webs were seen in comparison to the previous year, particularly in the Brunswick-Bowdoinham area.

The caterpillars came out on schedule in May and started feeding, but 11 straight days of rain put a stop to that, Donahue said. The rainy weather forced the caterpillars to return to their webs, and “like people in the winter, they hang out together, and if one gets sick, they all get sick,” Donahue explained.

“The silver lining to those clouds is that they caused an epizootic [animal epidemic] outbreak of a fungus that killed them,” the forest entomologist said.

Donahue predicted that for the Brunswick area, “the browntail moth caterpillar population will be down, and probably down for a number of years.” She said that the caterpillar population at the Vaughn’s Island Preserve in Kennebunkport also appears to have crashed.

The browntail moth is an invasive species that arrived in the United States in 1910 on nursery stock coming from Europe, then moved through Massachusetts, Maine, New Hampshire and Nova Scotia before the population collapsed. The only place where it is now found in North America is the coast of Maine and Cape Cod, Donahue said.

The caterpillar, distinctive because of the two patches of bright orange on its end, contains toxic microscopic hairs on it to keep birds from eating it. Unfortunately, those hairs can cause a blistery, oozy rash or respiratory distress for people who come into contact with them.

The hairs break off the caterpillars and circulate in the air. The caterpillar also molts, and the dried skin containing the hairs can drift, also causing problems for people. The hairs remain toxic for a year or more, so people still can be affected in subsequent seasons.

Donahue said that while monitoring the caterpillars most recently, she saw the carcasses hanging on the webs “with a halo of fungal spores around them.” The caterpillar larvae emerged from the webs and began feeding on their primary food source, oak and apple trees, before being killed by the fungus.

So far, there have been far fewer reports of toxic reactions, plus the trees have leaves, Donahue said. The fungus occurs “only when you have cold, wet weather in the spring,” she noted.

On the down side, the caterpillar is “doing very well” in Freeport, Falmouth and the Casco Bay islands, the MFS entomologist said. It also has been found in Augusta.

These areas were not affected by the fungus, and people still can suffer from adverse reactions in those parts of the state, Donahue warned.

“It’s a ‘whack-a-mole’ kind of thing,” she explained. “It’s gone down in Brunswick, and now it’s popped up in other places.”

People should remember that even though the caterpillars are gone or done feeding for this year, the hairs still are there, Donahue said. Caution should be used when mowing, raking and going into brushy areas where the browntail moth has been. The toxin in the hairs remains active for a year or more.

The Maine Forest Service has a list of licensed pesticide companies who work on the browntail moth caterpillar, and that list is available to the public, Donahue said.

For information about the browntail moth caterpillar, go to: http://www.maine.gov/doc/mfs/btm08.htm

For information about precautions to take regarding the browntail moth caterpillar, go to:http://www.maine.gov/doc/mfs/btmprecautions08.htm

For more information about pesticides, go to: http://www.maine.gov/agriculture/pesticides/

For the list of licensed pesticide companies dealing with the browntail moth caterpillar, call Charlene Donahue, MFS forest entomologist, at: (207) 287-3244 or email: Charlene.donahue@maine.gov

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