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Rain-Boosted Fungus Keeping Caterpillars Down in Maine

July 05, 2011 - Augusta - May showers bring June flowers, mosquitoes, and – this year at least – the collapse of the noxious browntail moth caterpillar population in several Maine communities.

Rainy weather last month fueled the growth of a naturally occurring fungus, Entomophaga aulicae, that virtually wiped out the caterpillars in Brunswick, Bath, Topsham and Bowdoinham, according to Charlene Donahue, Maine Forest Service Forest entomologist.

It is cause to celebrate, she said. "It's really beneficial for people and for trees."

Last year, the browntail stripped leaves from more than 7,000 acres of trees in Maine, Donahue said.

The invasive orange-patched pests are known for their toxic microscopic hairs, which can cause a blistery, oozy rash or respiratory distress for people. Hairs can drift in the air and remain toxic for a year or more, affecting people into subsequent seasons.

Marty Folsom of Lucas Tree Experts in Portland said he remembers getting the "island itch" from visits to Peaks Island as a kid.

"It's more of a people problem than a tree problem for people who get their yard sprayed (to kill the caterpillars)," he said. "It's a health issue."

In April, the forest service warned that caterpillar populations could be high this year, based on the large number of webs observed. So the rainy weather and subsequent fungus averted what could have been a nasty situation in some areas.

While monitoring the caterpillars this month, Donahue said she noticed they were dying.

"Their carcasses were hanging on the webs with a halo of fungal spores around them," she said.

"If one gets sick, they all get sick."

The pest boomed in the early 2000s in Cumberland County, with diminished populations in 2006 and 2007, Folsom said.

Some towns paid for aerial spraying of the caterpillars in the 1990s, but public spraying has been largely discontinued due to costs and environmental issues.

State law now restricts spraying next to coastal waters, because of concerns about the effect of pesticides on lobsters and other marine life.

Donahue said the browntail population will probably be down in Brunswick, Bath, Topsham and Bowdoinham for a number of years, as well as in Vaughn's Island Preserve in Kennebunkport, where populations have also crashed.

But the caterpillars can still be found in Freeport, Falmouth and other communities and islands around Casco Bay where the fungus didn't reach, Donahue said. The population has varied since the moths were discovered in the United States in 1910.

"It's been a problem since the early '90s in one place or another," she said. "I don't think it will stay away for good in any of them."

Colleen Stewart, Portland Press Herald, July 2011

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