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    Caterpillars of winter moths threaten region’s trees, again

    A winter moth caterpillar munched on an oak leaf at the Arnold Arboretum, though all types of trees have been under seige this year.
    Jessica Rinaldi/Globe Staff
    A winter moth caterpillar munched on an oak leaf at the Arnold Arboretum, though all types of trees have been under seige this year.

    It’s the annual attack of the tiny, very hungry winter moth caterpillars.

    You may have seen them in your yard. On your car. Dropping into your lap.

    Over the past few days, the green inchworm caterpillars have begun ballooning down out of trees by the thousands, dangling on a wisp of thread and leaving behind a tattered, frayed canopy of leaves.


    With the annual winter moth caterpillar outbreak in full force, homeowners and tree care companies are facing a rite of spring that has become a major threat to the health of the region’s trees.

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    “There’s a tattering of the leaves — it’s a lot of holes on the leaves. A lot of this comes from the caterpillars feeding on the buds before they open,” said Joseph Elkinton, a professor of entomology at the University of Massachusetts Amherst who has for a decade been releasing a parasitic fly that has begun to have success in controlling the population of hungry caterpillars. “In one person’s yard, there might be one million winter moths.”

    The winter moth caterpillar is an invasive species from Europe that showed up in Massachusetts sometime in the late 1990s and was only definitively identified in 2003. But the pests have quickly established themselves and spread, with no natural enemy to keep the population in check.

    Experts said the severity of this year’s outbreak depends on where you live, and even varies by neighborhood and town. There are pockets where the caterpillars have severely defoliated the tree canopy, while areas down the road might be lightly affected.

    Where the caterpillars have established themselves, however, the damage has been done, aided in part by the weather.


    “If you have a long, cool spring and things are very slow to leaf out, the winter moths can be feeding within the closed buds, and decimating the foliage before it emerges,” said Andrew Gapinski, the Arnold Arboretum’s manager of horticulture, who said the trees on Peters Hill have been particularly hard hit this year.

    The winter moth is a difficult pest because its hunger seemingly knows no bounds. It eats maples, oaks, fruit trees, ash.

    “I have yet to find a tree they don’t like, frankly,” Elkinton said. “Apples, in particular, are damaged by winter moths, and we’re trying to figure that out.”

    The stress on the trees is also compounded by the lack of rainfall.

    “We’re working on three consecutive years of drought,” said Rolf Briggs, a consulting arborist at Tree Specialists in Holliston. “The stripping of foliage right when it comes out is an enormous hit on the tree’s bank account.”


    Briggs and other specialists have been busy fielding calls and spraying trees with insecticides, but several said the caterpillars are pretty much done munching and for the next week will be dropping into the soil where they will pupate and spend the summer, before developing into moths around Thanksgiving.

    ‘I have yet to find a tree they don’t like, frankly.’

    Joseph Elkinton, University of Massachusetts Amherst professor of entomology 

    A longer-term solution, however, has been quietly beginning to show success.

    The damage has been especially severe on trees on Peter’s Hill in the arboretum.
    Jessica Rinaldi/Globe Staff
    The damage has been especially severe on trees on Peter’s Hill in the arboretum.

    In Nova Scotia, a winter moth invasion in the 1950s was eventually controlled by the spread of a fly called Cyzenis albicans . The fly lays its eggs on the leaves the caterpillar eats, and once the winter moth caterpillar begins to pupate in the soil, the fly will eat the insect from the inside out.

    A decade ago, Elkinton and colleagues began releasing the flies by the thousands at sites around the state. Now, they are beginning to see success.

    In Wellesley, they have been collecting caterpillars by putting a tarp down and shaking the branches of trees. The researchers have found that about 30 to 40 percent of the caterpillars have the parasite inside them.

    This kind of control won’t help in the short-term, but as the population of flies grows, it should be able to keep the winter moth caterpillars in check. Because the flies are specialists, they also do not pose a threat to other caterpillars, Elkinton said.

    In the shorter term, many homeowners resort to spraying and insecticides. But the timing has to be right — before they’ve turned leaves into a lattice and eaten their fill.

    “There are options; you can spray,” said Ken Gooch, forest health program director for the state Department of Conservation and Recreation. “But the timing has to be correct. . . . By the time people notice, it’s too late to spray for it.”

    But many anguished homeowners face a learning curve in how to battle a relatively recent pest.

    Just over a decade ago, Briggs recalls looking up into the tree canopy on a Brookline street and seeing something unfamiliar and alarming.

    “Every sugar maple on the street had been eaten heavily,” Briggs recalled. He called the state entomologist at the time, Robert Childs, and told him about the green inchworm that was decimating the maple trees.

    “He said, ‘We don’t have that in Massachusetts,’ ” Briggs said. “I said, ‘I’m standing right here, and these trees are wrecked.’ ”

    Carolyn Y. Johnson can be reached at Follow her on Twitter @carolynjohnson.