Adult Cyzenis albicans, the parasite of winter moth. (Courtesy Maine Department of Agriculture, Conservation and Forestry)

BATH — In order to fight an invasive species of moth, state scientists will be releasing a species of parasitic fly that will eat the pests from the inside out in Bath on Wednesday.

Winter moths have been defoliating plants between Kittery and Mount Desert Island in Maine since 2012. Their larvae feeds on trees such as oaks, maples, apples and blueberries, in early spring, and repeated infestations can cause trees to die.

Maine Department of Agriculture, Conservation and Forestry say tens of thousands of acres of oak trees have died in Massachusetts as a result. In Maine, oaks are dying in Cape Elizabeth because of the larvae.

Parasitic flies, however, have successfully reduced winter moth populations in several Massachusetts locations, however.

“The parasitic flies only attack winter moth and the adult flies are around for just a few weeks in May making it a good biocontrol agent,” according to a press release from the department. “They have been successfully used as a control strategy in Nova Scotia, parts of western Canada and the US, as well in southern New England.”

Entomologists at the Maine Department of Agriculture, Conservation and Forestry’s Maine Forest Service will release the flies in the parking lot of the Grace Episcopal Church on Washington Street at 11 a.m. Wednesday.

The winter moth. (Courtesy Maine Department of Agriculture, Conservation and Forestry)

The flies are in cocoons in a cage to be buried in the ground until spring. The flies will be released in early May.

Wednesday’s scheduled release is part of a larger release program, undertaken in conjunction with the University of Massachusetts, with funding from the U.S. Department of Agriculture to control the winter moth across New England. Flies have been released in six other locations in south coastal Maine starting in 2013 and are starting to become established in Kittery, Cape Elizabeth and Vinalhaven.

According to the Massachusetts Audubon Society, the fly will lay eggs on leaves eaten by winter moth caterpillars. The fly’s eggs will hatch inside the caterpillars, at which point the fly’s larvae consume the caterpillar from the inside out.

“The fly pupates inside the carcass of the caterpillar and, the following spring, emerges an adult fly to mate and begin the cycle again,” according to Mass. Audubon.

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