Winter Moth Caterpillars Prepare to Defoliate Region

The winter moth caterpillar can wreak havoc on trees. Introduced into the United States from Europe via Canada, is most commonly observed in late fall, early winter as a whitish adult moth and in spring as a tiny green caterpillar. (Audubon Society)

The winter moth caterpillar can wreak havoc on trees. Introduced into the United States from Europe via Canada, is most commonly observed in late fall, early winter as a whitish adult moth and in spring as a tiny green caterpillar. (Audubon Society)

By TODD McLEISH/ecoRI News contributor

Those annoying white moths that seemed to be everywhere between Thanksgiving and Christmas, and which sometimes formed thick fluttering clouds, are preparing to unleash even greater devastation to the trees in southern New England than they did last year.

Winter moth caterpillars defoliated about 27,000 acres of trees in Rhode Island last spring, according to data from the Rhode Island Department of Environmental Management (DEM), and one local expert says the insects will do even greater damage this year.

Heather Faubert, who runs the Plant Protection Clinic at the University of Rhode Island and has been monitoring the approaching onslaught, was hoping that the late cold spell in early April was going to kill off many of the emerging caterpillars. But that doesn’t appear to have happened.

“I think we’re in trouble this spring,” she said. “Every year since they arrived, it’s been getting worse, and I can’t imagine why it won’t get worse this year, too.”

Winter moths are a European insect that arrived in North America in the 1950s, beginning in Nova Scotia. They spread to Cape Cod by the 1990s and were first discovered in Rhode Island in 2004.

“They expand slowly, so there are still some places in Rhode Island where they aren’t found in large numbers yet,” Faubert said. “But they’re coming. The first year they arrive in a new area, you might see the moths in winter but not see many caterpillars the next spring. But once they get going, they grow exponentially.”

A male winter moth, left, and a female winter moth. The latter can’t fly. (NOFA/RI)

A male winter moth, left, and a female winter moth. The latter can’t fly. (NOFA/RI)

The URI entomologist said the adult moths appear in late November and fly around through December looking for a mate. Killing them at that time does little good, since only the males fly. Unseen flightless females lay their eggs on the trunks and branches of trees, and the caterpillars emerge in late March.

“Hatching typically takes about eight days to complete, but because of the cold weather in April it took about a month this year,” Faubert said.

The tiny caterpillars then crawl into the buds of the earliest budding trees — mostly maples, but also apple and other fruit trees — and immediately start eating. If the buds of fruit trees are consumed, the fruit crop may be lost for the year. As leaves emerge on trees of all varieties, the caterpillars feed on them until the end of May, when they drop to the ground, burrow into the soil, and pupate before emerging as adult moths in November.

While one year of defoliation doesn’t typically harm a healthy tree, Faubert said that defoliation three years in a row can kill almost any tree. And this year, Rhode Island and the rest of southern New England may have a large population of gypsy moths to contend with as well.

“Trees that are defoliated by winter moth and then again by gypsy moth later in the season probably won’t recover,” Faubert said.

Last year, gypsy moth caterpillars hit oak and other hardwood trees in rural areas hard.

Last year, gypsy moth caterpillars hit oak and other hardwood trees in rural areas hard.

Gypsy moth caterpillars defoliated about 43,000 acres of Rhode Island forest last year, thanks to a dry May that aided their survival. In years when it’s rainy in May, the moisture abets several diseases that get passed back and forth between gypsy moth caterpillars, causing the population to crash. So Faubert has her fingers crossed that Rhode Island will experience a wet May.

Homeowners seeking to protect their trees from winter moth defoliation should take action immediately. Faubert said it’s best to spray insecticide on trees while the winter moth caterpillars are still small.

“Waiting until the trees are halfway defoliated won’t really do much good,” she said.

According to DEM, the recommended treatment against the caterpillars is a pesticide containing the relatively safe bacterium Bacillus thuringiensis or Bt. Caterpillars die when they consume leaves sprayed with the pesticide, which is most effective before the caterpillars are full grown.

“That’s the safest thing to do,” Faubert said. “The problem is that the spray only lasts from three to five days before it breaks down. So if you have a prized tree you’re trying to save and there are untreated trees nearby, caterpillars may get blown onto your tree after the pesticide is no longer effective. So one shot of insecticide may not do the job.”

Physical barriers such as sticky tape or grease applied to the base of trees isn’t considered effective at stopping winter moths.

One strategy that Faubert has experimented with is biological control. Researchers have identified a parasitic fly that is known to control the spread of winter moths in their native Europe. The fly lays its tiny eggs on tree leaves, and when the caterpillar consumes the eggs while eating the leaves, the eggs hatch inside the caterpillar and eat the caterpillar from the inside out.

Faubert released groups of the parasitic flies in seven locations in Rhode Island between 2011 and 2014. But, she said, “if it’s going to work, it takes years.”

In the meantime, she advises residents concerned about their trees to contact a local landscaper or arborist who can assess and treat their trees. The Rhode Island Nursery and Landscape Association has a list of those who can do the job.

Rhode Island resident and author Todd McLeish runs a wildlife blog.