Asian insect threatens hemlock trees in Maine

Maine has a new pest to worry about.

The Maine Forest Service confirmed recently that the hemlock woolly adelgid, a tree-killing invasive insect, has established itself in the southern part of the state.

Foresters have been concerned about the bug since it started killing trees in southern New England decades ago.

The tiny, flightless insects feed on the sap of hemlock trees. A fully infested tree can die within five years, according to the Maine Forest Service.

Since 2000, Maine has been under quarantine, banning the importation of nursery hemlocks for fear they could spread the bug. A few infested ornamental trees have been destroyed, mostly along the coast.

But the York County infestation, which is scattered over about 600 acres, has more of a natural pattern, indicating to state entomologist Dave Struble that the hemlock woolly adelgid probably reached Maine by hitching a ride with a windstorm or a bird.

“I think they’ve probably been trickling in for some time,” Struble said recently. “The good news is that the population is extremely low right now.”

The hemlock woolly adelgid most likely was brought to the United States from its native Asia on a tree. Once in North America, the insect had no natural predators and was able to develop alarming populations. In southern New England, where the infestation has grown out of control, the forest cover has completely disappeared in some places.

“Whole parks are dead,” Struble said.

Here in Maine, such an infestation would be disastrous. Maine relies on hemlock for 10 percent of its pulpwood. The tree is also used for lumber, and its bark is sold as mulch.

“To potentially lose 10 percent of your softwood is a big concern, no matter what,” Struble said. “Losing this is not acceptable.”

The ecological damage of losing hemlocks could be even greater. In some parts of southern and coastal Maine, hemlock is the primary tree providing winter shelter for deer and other forest wildlife.

State scientists are surveying York County forests this winter in an attempt to determine how pervasive the bug has become. But this time of year is not ideal. The woolly adelgid is named for the white, cottonlike egg sacs it lays on the underside of hemlock branches in the spring and fall, making it easy to spot.

During the winter, however, even verifying the presence of the microscopic bug is difficult. Scientists must inspect each individual tree, top to bottom.

“We know it when we find it, but it’s very hard to prove that it’s not there,” Struble said.

The state is also pursuing substantial federal grant money to attack the infestation once the snow melts in the spring.

But don’t look for the giant rolling clear-cuts used to combat Maine’s last major forest pest, the spruce budworm, Struble said.

“Simply destroying the trees is not the most appropriate answer,” he said.

Yet the state will consider cutting trees to destroy any small, isolated populations that are discovered.

January’s arctic weather could have some benefit, as hemlock woolly adelgids can’t survive bitter cold.

“You can get complete kill at 25 below,” Struble said. “But just how much do you want to bank on good luck and weather?”

Most likely, the state next spring will release populations of predatory Asian beetles that have shown great promise in the lab. Maine started experimenting with the beetle in 2002 in hopes of getting a jump on the hemlock woolly adelgid.

In some states, pesticide applications have shown great promise, though painting trees with insecticides is far easier on a Connecticut lawn than in Maine’s industrial forests.

Regardless of which weapons the state ultimately chooses, the battle against the hemlock woolly adelgid will be a long and frustrating one, Struble said.

“It’s like wrestling Jell-O,” he said.