Everything you need to know about gypsy moths

The foliage-chomping insects are back in Massachusetts this year.

Brewster- 6/28/2016 Gypsy moth infestation is taking a toll on oak trees on Cape Cod. Ken Gooch, Forest Health Program Director for the Dept. Of Conservation and Recreation examined an infested oak in the pupate stage  at Nickerson State Park. Boston Globe staff photo by John Tlumacki(metro)
Ken Gooch, Forest Health Program Director for the Dept. Of Conservation and Recreation examined an infested oak in the pupate stage at Nickerson State Park in 2016. –John Tlumacki / The Boston Globe

Gypsy moth caterpillars picked the foliage off thousands of acres of trees in Massachusetts last year, raining droppings on residents and creating a hum while crunching their green meals.

It was the biggest outbreak of the caterpillars since 1981. In their wake, the hungry insects left leafless brown forests in Massachusetts, Rhode Island, and Connecticut that were even visible from space. The Bay State alone suffered more than 350,000 acres of defoliation from the caterpillars in 2016, compared to about 38,175 the previous year.  

State officials predicted earlier this year that Massachusetts would see another record year of infestation by the chomping caterpillars. But after a cold and wet spring, experts say exactly how much damage the insects will inflict when they arrive later this month remains to be seen.

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Here’s everything you need to know about the hairy moths.

What are gypsy moths?

A closeup of a gypsy moth caterpillar in 2016. —John Tlumacki / The Boston Globe

Gypsy moths are an invasive insect that, as caterpillars, have hairy bodies dotted with blue and red spots. When they hatch, they’re only about one-sixteenth of an inch, but can grow to be more than three inches long by the time they form cocoons about six weeks later. 

They were introduced to Massachusetts in the late 1860s by E. Leopold Trouvelot, who brought them to Medford from France to study the caterpillars for silk production.

It wasn’t until decades later, when caterpillars began defoliating entire neighborhoods in the area, that authorities realized they had a serious issue on their hands, said Tawny Simisky, an entomology specialist at the University of Massachusetts Extension.

Gypsy moth eggs live through the winter in clusters of hundreds of eggs called egg masses, which usually hatch in the spring around the first week of May. The egg masses can be found on tree trunks and branches, and can also be laid on other surfaces, such as the sides of homes and even lawn furniture.

The little caterpillars then start a process called “ballooning,” in which the tiny insects crawl to the highest point of the surface the egg mass was laid on and spin a silken thread to catch the wind. Then the moths float to a new location. According to Simisky, this is the main way the insects spread.

When will they start infesting New England?

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Once they’ve found a new host, the caterpillars start feeding on newly-opened spring foliage during the month of May. As the moths grow, they eat more. The effects of their feasting become visible with the defoliation of trees by mid-to-late June, according to Simisky.

They typically form a cocoon and begin to turn into moths by the end of June. The adult moths begin to emerge from the cocoons in early July, she said.

Until 1989, New England would typically get a “population explosion” of gypsy moths, followed by a collapse. But at the end of the 1980s, a Japanese fungus introduced to North America in the early 1900s called entomophaga maimaiga started killing the caterpillars. Since then, the fungus has played a large role in keeping the gypsy moth population low, along with other diseases and natural predators.

“In Massachusetts, when we’re not in drought conditions, gypsy moths go through this natural fluctuation in the population where it does get reduced because of these pathogens,” Simisky said.

 The extended drought in recent years contributed to the infestation of the gypsy moth caterpillars that occurred in 2016 because the dry conditions “knocked back” the fungus. Without it, gypsy moths, which lay between 100 and 300 eggs in an egg mass, were able to build up their population and expand quickly.

Because of this year’s cool, wet weather, the state is a couple weeks behind where it was last year, with the gypsy moths slow to start feeding due to the weather conditions, said Ken Gooch, forest health program director at the Massachusetts Department of Conservation and Recreation.

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“Where we’ve seen them, because of the cold and wet weather, they’re just sitting there,” he said. “They’re not feeding.”

People will likely notice defoliation from the caterpillars during the first three weeks of June, according to Simisky.

Gooch said the caterpillars have been spotted in generally the same areas as last year — in the western part of the state, on the Rhode Island and Connecticut borders, and stretching south of Boston through the south shore to Plymouth and parts of Cape Cod.

Experts are hopeful that the cool and wet weather this spring will help keep the caterpillars in check.

“That being said, there’s so many of them out there, we could still see significant defoliation,” said Simisky. “But I think what everyone is hoping is that we won’t see more than what we saw last year. But again, that’s hopeful that the fungus would aid in that.”

If enough of the caterpillars are killed by the fungus this year, she added, it might lead to a drop in the population next year.

Why are they a problem?

While many a tree can survive a year of its leaves being picked clean by gypsy moths, the concern is that repeated defoliation over several years might start killing trees.

The state is already seeing some pockets of tree death in areas where plants were hit hard by the last three years of drought on top of the defoliation, according to Gooch. 

“It creates hazard trees on roadways and around people’s properties,” Gooch said. “If the defoliation kept up for year after year, it eventually causes trees to get weakened. Then secondary pests come in. There are all these root decay fungi and beetles and wood borers that go after stressed and weakened trees.”

In southern New England, the gypsy moths can cause major problems because forests in the area are mostly comprised of oak trees, which the insects especially like to prey on.

—John Tlumacki / The Boston Globe

The gypsy moth can also be a nuisance in other ways.  

For some people, the hairs on its body can cause an itchy rash, which is treatable with an over-the-counter anti-itch cream.

The caterpillars’ droppings can also become a problem. Last year, according to The Boston Globe, a woman called state officials to complain her house was so covered in the droppings that she would slip as soon as she stepped outside her door.

 What can you do to cope?

Gypsy moth caterpillars on an infested oak tree at Nickerson State Park in 2016. —John Tlumacki / The Boston Globe

Gypsy moth caterpillars clinging to the sides of homes or hanging on your backyard patio can be sprayed away with a strong jet of water. But if you have ornamental plants or shade trees that are being eaten by the pests, you can also contact a licensed arborist to help manage them with a chemical spray.

You could put burlap strips on tree trunks to catch the caterpillars, for example, but that’s only “a drop in the bucket,” Gooch said.

“There isn’t a whole lot you can do as a property owner on your own that would be effective,” he said.

What you can do is try and keep your plants healthy through maintenance such as proper mulching and watering, especially if the state enters another drought, and avoid chemicals underneath the canopies of the tree.

“Anything you can do to protect the roots is a good thing,” he said.