State Forestry Officials Bring In Parasitic Flies to Kill Winter Moths in South Portland

Nov 29, 2017

Forestry officials are expanding an effort to combat winter moths, an invasive species that kills oak trees and other leafy trees and shrubs, into South Portland.

Maine Forest Service entomologist Charlene Donahue says Wednesday morning she and her colleagues buried a cage of cocoons, that will emerge in the spring as Cyzenis albicans, a kind of parasitic fly that preys exclusively on winter moths.

The flies have been used successfully in the Pacific Northwest, Massachusetts, Nova Scotia, Cape Elizabeth, Peaks Island, Harpswell, Vinalhaven Island, and Kittery.

When the flies emerge from their cocoons in the spring, they'll lay their eggs on the plants that winter moths eat. Winter moth caterpillars will consume the eggs, and the fly larvae will develop inside them.

The caterillars won't die from this, says Donahue. "It's not until the caterpillar drops down to the ground and makes a cocoon that the fly maggot then eats the winter moth. Then they're inside this wonderful little cocoon that the winter moth made for them, and they have a perfect place to spend all summer, fall, winter and spring until they come out the following May."

The flies aren't seen as a threat to the overall ecosystem because they've evolved to specifically prey on winter moths.

Donahue says that's a big part of the reason they've been widely used for this purpose. "This is certainly something the Maine Forest Service is concerned about, is not only the health of the trees in controlling the winter moth, but making sure if we bring something in it's not going to have unintended consequences." 

As the population of winter moths declines, the population of the parasitic flies will shrink along with it.

The flies will start to emerge in early May. They're not attracted to humans or human homes. The Forest Service also asks people who see winter moths to report it to them.