Nearly a third of the forest canopy across Massachusetts has been consumed this year by a plague of gypsy moth caterpillars, finger-long creatures whose insatiable appetite for leaves can ultimately kill trees.
Several years of drought conditions allowed the insects to thrive to such an extent that they have chewed through about 1 million of the state’s 3 million acres of forest foliage, state surveys show.
While only a small number of trees are likely to die immediately as a result, and next year should see a sharp drop in the caterpillar population, the damage could threaten trees’ long-term health.
“Many of these trees were already stressed,” said Ken Gooch, forest health program director for the Massachusetts Department of Conservation and Recreation. “A deciduous tree needs leaves to feed itself . . . and if a tree isn’t strong enough to defend itself, it can die.”
This year’s defoliation has been about three times more extensive than last year’s, making it the second-worst on record, officials said. It trails only 1981, when the caterpillars devastated about 2 million acres across the state.
The caterpillars have damaged the state’s forests every few years since the 1860s, when the mottled moths were first brought to the United States from France. Last year’s infestation was centered on the southeastern part of the state, especially on Cape Cod, where in some areas the caterpillars’ droppings fell from trees with such regularity it sounded like a hail storm.
This year, the main pockets of defoliation are in Worcester County, between Palmer and Hopkinton, extending south into Connecticut and Rhode Island. In the hardest-hit sections, the barren landscape looks as if winter came early.
But researchers say the larger concern is what happens to the trees after the caterpillars leave them denuded. Many of them devote a significant amount of energy to producing a new crop of leaves. When they are forced to do that repeatedly, they are left weakened and more vulnerable to a range of other pests and viruses.
“For the first time in decades, we’re looking at gypsy moths being a significant issue for forest health,” said Curtis Woodcock, a professor of geography at Boston University who has studied the defoliation. “My concern is that, in the long run, they will undermine the health of the forests throughout New England.”
But the stripping of millions of trees comes with something of a silver lining. This spring’s cool, wet weather provided a breeding ground for a fungal pathogen that curbs the populations of gypsy moths.
Imported from Japan in the early 1900s, the natural pathogen requires wet conditions to thrive, and in recent years of low rainfall it had all but disappeared. But with its reemergence, state officials now expect defoliation to affect just 25,000 acres of forests in Massachusetts next year.
“The forest is resilient, and it’s rebounding now,” said Gooch, who estimated that about 95 percent of the state’s gypsy moths failed to reproduce this year. “Nature always has a way of bringing the forest back.”
Even then, the cycle will continue to play out in the coming years, as the healthy adult females that remain will lay about 300 eggs a year. Joseph Elkinton, an entomologist at the University of Massachusetts Amherst, said he has seen little mortality in the western part of the state. If the death rates remain low there, the local caterpillar population will double in size, he said.
At Wells State Park in Sturbridge, the life and death cycle has been on full display. Earlier this year, Joe Fife would walk outside his office to what sounded like a light rain, as countless caterpillars munched on leaves across the park’s 1,400 acres.
“We made sure to shower after walking outside,” said Fife, a forest and parks supervisor, noting the caterpillar pellets that fell from above. “They would sometimes get in your hair.”
But the fungus did its work, and by July few caterpillars were left.
“The ones we found looked mostly liquefied,” a sign of the fungal pathogen, he said.
Within a few weeks, nearly all of the trees appeared to have produced a new set of leaves.
“That they sprang back so quickly is a good sign,” he said.David Abel can be reached at email@example.com. Follow him on Twitter @davabel.