Winter moths may be losing battle with parasitoid flies in Cape Elizabeth - Press Herald

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A man walks a dog through Fort Williams Park in Cape Elizabeth last week. The town and local land trust haven’t banded trees against winter moths this fall because recent parasitoid fly releases appear to be working. Gregory Rec/Staff Photographer

CAPE ELIZABETH — The town and local land trust decided not to band trees against the invasive winter moth this fall because recent releases of parasitoid flies appear to be working.

In recent years, Cape Elizabeth’s trees have been among the hardest hit by the non-native moth, which can turn tender leaves into see-through lace and killed 300 acres of the town’s venerable oaks. The damage is done in the spring, during the insect’s caterpillar stage, and the moth has taken a noticeable toll on trees in coastal Maine communities from Kittery to Vinalhaven.

To prevent widespread defoliation and tree mortality, public and private property owners each autumn have banded or wrapped tree trunks with various barriers meant to keep the female moths from climbing to lay eggs on branches.

And since 2013, the Maine Forest Service has repeatedly introduced thousands of parasitoid flies as a biological control to combat widespread winter moth populations in Cape Elizabeth, South Portland, Portland, Peaks Island, Harpswell, Kittery, Vinalhaven and Boothbay. A parasitoid is an insect whose larvae kill their hosts.

This year, Todd Robbins, Cape Elizabeth’s tree warden, found “a few holes” in the leaves of maple, oak and other trees typically targeted by the moth.

“I think the parasitic releases are having an effect,” said Robbins, a licensed arborist. “I felt it was a good time, from a town perspective, to take an observational year and see how our trees do without integrated pest management.”

Todd Robbins, Cape Elizabeth’s tree warden, stands beneath trees in Fort Williams Park that haven’t been banded this fall to protect them from winter moths because recent parasitoid fly releases appear to be working. Gregory Rec/Staff Photographer

Other methods of integrated pest management include dousing trees with horticultural oil that smothers the moth’s eggs and larvae.

The Cape Elizabeth Land Trust is following Robbins’ lead, posting a notice on its website that it’s “taking a break from treating individual trees for winter moth on our properties,” including banding.

Declining winter moth populations in recent years indicate that townwide controls, such as the introduction of the parasitoid fly Cyzenis albicans, might have made banding and other individual tree treatments unnecessary, the trust reported.

“Because large-scale defoliation hasn’t happened the past couple of years, the risk of not banding this year is limited, even if the winter moth population has an unexpected rebound,” the trust explained.

However, Robbins and the trust agreed, banding is still an option for homeowners who want to protect trees that also might be suffering from other challenges, such as other pests, disease, recent drought conditions or advanced age.

A winter moth rests outside a kitchen window Saturday evening in South Portland on the Cape Elizabeth line. Kelley Bouchard/Staff Writer

Named for their habit of emerging November through January, the small tan moths came to North America from Europe in the early 1900s. Infestations were seen first in Nova Scotia in the 1930s and then in the Pacific Northwest in the 1970s, according to the forest service. The moths arrived in Massachusetts in the early 2000s, spreading south into Rhode Island and north into Maine.

While the winter moth problem was first recorded in Cape Elizabeth in 2011, Robbins believes it started several years earlier based on his examination of outer rings on freshly cut stumps of dead trees. The moths prefer oak trees but also will infest maple, ash, elm, aspen and various fruit trees and bushes.

Entomologists believe the moths were brought here when people transplanted perennial flowers and shrubs from gardens to the south where the soil was infested with winter moth cocoons. The moth’s range grows in much the same way, especially among gardeners who like to swap plants with friends or sell them at yard sales or fundraisers.

Winter moths emerge from the ground on warmer days in late fall or early winter to mate and lay eggs. When they are plentiful, the male moths form eerie gray clouds around street lamps and porch lights after sunset.

Female moths have vestigial wings, so they must climb trees to lay eggs along branches. The moths die soon afterward. The eggs can survive the harshest winter weather.

Defoliation follows the next spring, when the green, inch-long caterpillars hatch and feast on soft leaves and buds. Then, they drop to the ground and burrow into the soil to form cocoons and begin the pupal stage.

A tree at Fort Williams Park in Cape Elizabeth that hasn’t been banded against winter moths this fall because recent parasitoid fly releases appear to be working. Gregory Rec/Staff Photographer

Tree banding and parasitoid fly releases are intended to interrupt the moth’s life cycle.

For the last several years, town officials and the Cape Elizabeth Land Trust have rallied residents and volunteers to band hundreds of trees. Barriers such as sticky tape and polyester batting around tree trunks keep the female moths from climbing up to lay eggs.

The parasitoid fly, which only targets the winter moth, lays its eggs on leaves that are ingested by the moth larvae. The fly eggs then hatch into maggots inside the moth caterpillars and lie dormant until late summer, when the fly maggots eat and kill the moth larvae as they pupate underground.

Tom Schmeelk, a forest entomologist with the Maine Forest Service, validated Cape Elizabeth’s decision to take a year off from banding trees. He reported diminished winter moth populations in Cape Elizabeth and neighboring South Portland.

“They’re seeing a remarkable turnaround with winter moth,” Schmeelk said. “In the years following the fly releases, the defoliation has been less and less.”

Schmeelk said winter moths will never be wiped out. Damaging infestations can pop up again and still persist in Kittery and Boothbay, he said. But integrated pest management, including banding and parasitoid fly releases, can be implemented when needed to bring the moth population back into equilibrium and prevent widespread defoliation and mortality, he said.

On the down side, Schmeelk said, reducing the moth population means entomologists have been harvesting fewer parasitoid flies for releases in other infested areas. It’s a problem that requires careful planning by forest scientists.

That’s a challenge that Todd Robbins, Cape’s tree warden, is glad to have as he focuses on protecting the town’s trees from browntail moth, hemlock wooly adelgid, emerald ash borer and other hazards.

“We have enough other threats to deal with,” Robbins said.

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