Gypsy Moths Are Destroying Forests as the Climate Dries

Much of New England has been slammed by a gypsy moth infestation that has denuded trees of leaves on some 150,000 acres in Massachusetts alone, forestry managers say.

The gypsy moth population exploded this spring across large swaths of Massachusetts, Connecticut, and Rhode Island after drought conditions impeded the spread of a fungus that usually keeps the number of gypsy moth caterpillars under control.

It was the second year in a row that a lack of springtime rain triggered an explosion of the voracious caterpillars, which can strip the leaves from trees with alarming efficiency.

“We were blown away by how far it has spread; it’s just everywhere,” said George Boettner, an invasive pest researcher at the University of Massachusetts Amherst. “It’s the worst we’ve seen in decades.”

Caterpillar defoliation can have cascading effects on local ecosystems, Boettner said. Removing the forest canopy can make it too hot for nesting birds, which abandon their nests, and it eliminates habitat for insects that feed many bird species.

Exposed nests are also subject to infiltration by cowbirds, which, like cuckoos, lay their eggs in the nests of other species. When cowbird hatchlings emerge, they force the other chicks from the nest.

Gypsy moth caterpillars also devour oak pollen that produces acorns, an important food source for turkeys, deer, bears, and other animals.

Acorn shortages can eradicate large numbers of white-footed mice, which depend on the nuts to get through the winter. In springtime, the mice feed on gypsy moth caterpillars, helping to control their population.

“The moths are regulating their own predators,” Boettner said.

The last big gypsy moth outbreak was in 1981, when 3 million acres in Massachusetts and 15 million acres in New England were affected, he said.

Ken Gooch, forest health program director at the Massachusetts Department of Conservation and Recreation, said his agency completed aerial surveys of the state on Thursday and will have complete defoliation data in about a week.

“Last year there were 38,000 acres of gypsy moth damage, and my guess is that this year it’s close to 150,000 acres and could even be more,” Gooch said. Damage was found across Massachusetts, with Cape Cod and the north and south shores hit hardest.

Widespread defoliation has also been reported in northeastern Connecticut and much of Rhode Island, Gooch said.

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By now, most of the caterpillars have entered the pupal stage, but widespread damage is already done.

“Two years of gypsy moth infestation have caused trees to weaken because of vast defoliation,” Gooch said. “The leaves can grow back the same year if the trees are healthy. But they are stressed out from not getting enough moisture. If this happens over three or four years, we’re going to see a lot of tree mortality.”

Much of Massachusetts is suffering from moderate to severe drought. It is difficult to determine whether the rainfall shortage is related to global warming, Gooch said, “but if you look at what’s happening out West, people are saying the drought is related to climate change.”

Gypsy moths were introduced to Massachusetts in the 1860s by a French immigrant who wanted to crossbreed the moth with silkworms to create a species that would not be dependent on mulberry trees. The experiment failed, and the moths quickly spread, infesting much of the Northeast and Midwest.

In 1989, however, a fungus was introduced from Japan—possibly traveling on the shoes of Japanese researchers who came to Massachusetts—that proved highly lethal to the caterpillars.

“When it showed up, it changed the whole game,” Boettner said. “The fungus can kill 50 percent of caterpillars and 80 to 100 percent in high-density areas. We can go a very long time between outbreaks as long as there’s some rain in May and June.”

Little has been done in recent decades to eradicate gypsy moths because rainy springs have traditionally done the job.

The gypsy moth caterpillar is, however, susceptible to a certain virus.

“Labs are now producing the virus to use in aerial spraying on affected areas,” Boettner said. But the virus is expensive to make and not nearly as effective as the fungus, which releases spores that can travel over vast areas.

If drought hits again next spring, the virus may be the tree’s best defense.

“It’s a tough call to spend a lot of money to establish something like that or to just wait,” Boettner said. “All we really need is one good rain per week.”