A tiny moth is munching on Pennsylvania’s most commercially valuable tree, the black cherry, turning large swaths of the Allegheny National Forest brown and eating into future timber sale profits.

The cherry scallop shell moth, an insect pest native to Pennsylvania and the eastern United States, has defoliated cherry trees on more than 17,000 acres in the Allegheny National Forest and a total of 56,000 acres in the public and private forests around the national forest in the northwestern corner of the state, according to a recent aerial survey by the Pennsylvania Bureau of Forestry.

“This is the first time in more than 20 years that we’ve experienced an outbreak,” said Andrea Hille, a silviculturist for the national forest, in a U.S. Forest Service news release last week. While a moth infestation, even one that lasts for multiple years, rarely kills black cherry, she said some decline in tree growth and overall health of the black cherry trees is likely.

The moth infestation and resultant defoliation on the national forest land is visible to forest visitors in Warren, McKean and Elk counties, especially along State Route 6 between Kane and Sheffield, around the Kinzua Reservoir and in the Russell City and Ridgeway areas.

The state Bureau of Forestry in the Department of Conservation and Natural Resources is also monitoring the moth on the commonwealth’s 2.2 million acres of forest land, said Houping Liu, a forest entomologist in the bureau.

“The moth is a native species found across the state, but this year, for the first year in a while, it’s doing more damage than in most years, and people are noticing it,” said Mr. Liu, although he noted no state forests are experiencing the defoliation impacts seen in the Allegheny National Forest.

The scallop shell moth, Hydria prunivorata, gets its name from the pattern of alternating dark and light scalloped lines on its wings. According to a U.S. Forest Service fact sheet, the adult moths are found in the trees from late May to early August and lay eggs on the underside of cherry tree leaves. The eggs hatch from July through early August, and the yellow-and-brown-striped caterpillars feed voraciously on the tree leaves, defoliating the trees and stunting tree growth.

The declines in tree growth and health show up in narrower, discolored growth rings when the trees are cut, and those blemishes reduce the wood’s grade and value, said Jason Roblee, sawmill manager at Firth Maple Products in Crawford County, which specializes in black cherry and maple woods.

“We won’t see the stuff that’s happening this year … show up at the saw mill for several years, but for the last few years, we’ve been getting wood from central Pennsylvania, where you can see the ‘defoliation ring,’ sometimes two or three rings,” Mr. Roblee said, “and we struggle to sell wood with those rings in it.”

He said the price of cherry logs that normally sell for $1,200 to $1,300 per thousand board feet can plummet by $400, depending where in the wood the dark brown rings are located. The rings can eliminate a cherry log from use for high-quality veneer wood, valued from $6 to $8 a foot, and turn it into a “sawlog” with a price tag of 87 cents a foot, he said.

Major outbreaks of the moth and the resulting defoliation to black cherry and other cherry trees occur cyclically and usually in regions of the state, not the whole state. The last to hit the Allegheny National Forest occurred around 20 years ago. Moth populations also spiked in the late 1960s and early 1970s in various regions of Pennsylvania. Mr. Liu said cherry trees in forested areas of Indiana, Bedford and Westmoreland counties were hit hard by the moth in 2009.

“It’s very possible the moth populations are still building in the state and next year will be even bigger for the moth,” said Tim Tomon, a forest pest management specialist with the state Bureau of Forestry.

The cherry scallop shell moth is the latest in a growing list of forest pests that includes the emerald ash borer, the hemlock woolly adelgid and the tent caterpillar, which is heavily infesting white and red oaks in the eastern part of the state this year.

Mr. Liu said although the state is concerned about the cyclical defoliations of black cherry, it doesn’t normally spray insecticides to control them.

“The pests have been here for a long time and the trees have been here for a long time, so the trees have some sort of resistance to native pest species,” he said. “The moth is always in the woods to some degree. They just peak on certain years.”

First Published: August 10, 2015, 4:00 a.m.