CAPE ELIZABETH — A new finding of winter moths here means the invasive species has spread from two Maine towns to three.
Warmer winters are blamed for the spread of the environmental pest, which feeds on hardwood trees.
Although people have spotted winter moths in other coastal communities in Maine, from Rockland to Kittery, only Harpswell and Vinalhaven have seen significant populations before this year’s explosion of moths in Cape Elizabeth, state entomologist Charlene Donahue said.
“Cape Elizabeth is probably about a year behind Harpswell,” Donahue said, where the moths showed up in 2011.
The species, Operophtera brumata, has been a major problem in Massachusetts and Rhode Island for several years, but wasn’t identified in Maine until 2005 and is a “coastal phenomenon,” she said.
“Back in 2005 and 2006, we did a survey with the male moths and picked up moths in York County and had a couple of isolated cases on some eastern peninsulas,” Donahue said. “We didn’t see any again until fall of 2011 in Harpswell and Vinalhaven. They exploded along the coast this fall.”
Although the moths are visible this time of year, any action by the town to reduce their population won’t happen until next spring, she said.
“There’s nothing they can do at this point,” she said. “It’s like closing the barn door after the horse is out; the females have already laid their eggs.”
The most visible of the moths are the males, which look like a typical brown or gray moth. The females look more caterpillar-like and are about one-quarter-inch long, and have wings, although they can’t fly.
The females lay between 50-100 eggs, which then hatch, unleashing the larval-stage moths on the leaves of an array of hardwood trees. Over a few seasons, this process begins to stress and weaken the trees, eventually killing them, Donahue said.
“The concern is reducing the growth of trees,” she said. “We’re not going to lose all our trees, it’s just one more thing.”
After winter, the next step for intervention is to spray trees with a horticultural pesticide so the eggs won’t hatch, Donahue said, although some sprays can also kill one of the predators introduced to keep the moth population in check.
In Harpswell, the plan is to release a parasitic fly into the forests, said Robert McIntyre, of the nonprofit Harpswell Heritage Apples. The flies lay eggs on the leaves in the spring, the winter moth caterpillars eat them, and then the eggs hatch inside the not-yet-moths, killing them in their cocoons.
The flies then use the cocoons to stay alive during the winter and repeat the process the following season.
McIntyre said they also have wrapped some trees, which prevent the females from climbing the trees and laying eggs, but that proves difficult when there are thousands of trees.
The Harpswell group has also been catching moths and counting them this year and has noticed dramatic swings in their population when the weather changes.
“In the beginning of November they started to appear and the numbers would be like 20 and 6. Little numbers like that,” he said. “Then, 300 to 500; the numbers went to be as high as 3,000 to 4,000. Then the number collapsed when it got cold a couple of days ago, then shot back up when it got warmer again.”
On Dec. 15, they counted almost 2,000 moths. By Dec. 17, when most of Maine saw snow, there were zero, McIntyre said.
The spread of the moths is blamed on a combination of factors, Donahue said, from natural movement to the transportation of soil to a warming climate.
“It can be as simple as transferring plants from a summer home to a winter home,” she said. “The other thing is that we have had two very mild winters, especially in 2011, and that has allowed the moths to really get a foothold.”
Scientists don’t know how low temperatures must go before the moths can’t survive, only that they have thrived in warmer winters, she said.
Another moth species, the browntail, does similar damage to trees, Donahue said, and their combination with the winter moths can do double the damage.
None of the browntail have been reported in Cape Elizabeth. But Donahue said they were a serious problem in the early 2000s and more recently in Brunswick, until their population crashed in 2011.
The invasive winter moth caterpillar was discovered in Harpswell in late May, and has now been identified in Cape Elizabeth. Scientists say the species, which was likely brought to Maine on a landscaping plant, is a threat to hardwoods, apples trees and blueberry bushes.