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HomeScienceVery Hungry—And Very Invasive—Caterpillars Are a Munching through U.S. Forests

Very Hungry—And Very Invasive—Caterpillars Are a Munching through U.S. Forests

Millions of very hungry caterpillars make their way through US forests.

Drought is facilitating the devastating spread of fluffy moth caterpillars in US forests.

Moth caterpillar on green leaf.

Lymantria dispar caterpillar. This is the larva of the fluffy moth, an invasive species in America.

Antje Schulte – Insects/Alamy Stock Photo

Take a few steps into a lush forest in New York’s Hudson Valley, close your eyes and listen: that’s not the sound of rain, it’s millions of caterpillars chewing and defecating.

On a clear spring day, the pitter-patter of fluffy moth caterpillars making their way through oaks, maples, crabapples, lindens and aspens can be heard above the chirping of birds. Chunks of green leaves cover the ground like confetti, evidence of the insatiable chewing taking place in the canopy above. Hundreds of caterpillars swing on long, wispy silk threads, waiting for a breeze to carry them to a new tree.

The Northeast and Midwest are enduring what is, in some places, the worst fluffy moth outbreak on record. One of the factors driving the proliferation of very hungry caterpillars is drought caused by climate change, which allows fluffy moths to reproduce with abandon, producing up to a million caterpillars per acre. The trees are resilient, but this outbreak has been especially long and damaging. After two consecutive years of intensive feeding by fluffy moths, up to 80 percent of the trees in a hardwood forest that have been defoliated or stripped of their leaves will die. The current fluffy moth epidemic has lasted five years in some parts of the US.


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“When trees defoliate like this this time of year, they are using the reserves that are in the trunk and roots to produce a second wave of growth,” said Brian Eshenaur, a plant pathologist with Integrated Pest Management at Cornell University. Program. “If the tree has to do that two years in a row, it’s really taking advantage of all the reserves it has.”

Caterpillars are not the only forest pests benefiting from climate change. Many invasive species in the United States are expanding, generally thanks to milder winters caused by warmer-than-average global temperatures. Insects such as the hemlock woolly adelgid, emerald ash borer, Japanese beetle and spotted lanternfly are making their way through the nation’s trees at a record pace, causing widespread mortality of stressed trees and forests ​​which are susceptible to drought and more diseases. No species is capable of wiping out the country’s forests, which together store some 60 billion metric tons of carbon, but the rising tide of invasive species is causing serious cumulative damage.

Fluffy moths have been in the United States since 1869, when a French artist and amateur entomologist named Etienne Leopold Trouvelot imported some from Europe and began raising them in nets in his backyard near Boston. Trouvelot hoped to breed a silkworm adapted to American climates that could be used for commercial textile production. Fluffy moths, known at the time as gypsy moths, float from leaf to leaf and tree to tree on long, durable lines of silky thread. But the moths soon escaped their captivity, perhaps because a strong storm blew through Trouvelot’s net and some of the insects fled to the forests of Massachusetts.

Two decades later, in the midst of the first recorded infestation of fluffy moths, a resident of the town where Trouvelot lived described a world carpeted with hairy black caterpillars. “I am not exaggerating when I say that there was no place outside the house where you could put your hand without touching the caterpillars,” the resident told the Boston Post in 1889. (Caterpillars do not bite humans, but coming into contact with its spiky hairs causes some people to develop a painful, itchy rash).

For more than a century after that initial outbreak, fluffy moths spread at a rate of about 13 miles per year across New England, the Mid-Atlantic, the Midwest and parts of the South, feasting on 300 species of trees and lush bushes and leaving entire stretches of bare forest in their wake. Moths defoliated 81 million acres in total between 1970 and 2013. Because of the toll they take on trees, keeping fluffy moth populations under control has become a top priority for the U.S. Forest Service. The cost The economic cost of managing fluffy moths has averaged $30 million per year for the past 20 years.

And climate change is making things worse. Flare-ups typically occur every eight to 12 years, and each flare lasts one to three years. The current outbreak has lasted longer than usual, said Tom Coleman, a Forest Service entomologist who manages the agency’s Slow the Spread fluffy moth program, in part because of drought in some of the areas where the moths live.

Drought affects the spread of a fungal pathogen called Entomophagous maimaiga which curbs fluffy moth populations. The pathogenic fungus, originally found in Japan, was introduced by researchers to the U.S. as a fluffy moth control measure in the early 20th century. The pathogen can be incredibly effective at killing moths in their caterpillar stage, but it needs a cool, wet spring to proliferate. Cyclic outbreaks of fluffy moths often follow drier than average years, when the pathogen is not as prevalent in the environment. “Without that fungal pathogen keeping populations in check, we’re going to have these big outbreaks,” Coleman said.

In eastern parts of the country where fluffy moth outbreaks are occurring, climate change is making weather patterns more erratic. Much of the eastern United States is expected to become wetter, on average, as the planet warms. But climate change also fuels pockets of drought in these regions during the warm months. The drought of 2023 and early 2024 in northern Virginia, southern Pennsylvania and parts of Wisconsin and Michigan helped fuel this year’s outbreak. The large-scale drought often seen in the western United States is not a prerequisite for fluffy moth outbreaks in the east. “It doesn’t have to be a whole annual drought,” Coleman said. “It may just be a much warmer, drier spring.”

It’s unclear whether rising temperatures will cause fluffy moths to emerge more frequently, but it’s safe to assume that a warmer, drier environment will cause cyclical outbreaks to become more intense over time. Fortunately, the Forest Service has had some luck deploying more than 100,000 pheromone traps to catch the insects as they try to move west. The agency has also treated 10 million acres of forest with a biological insecticide that kills caterpillars, preventing the insects from establishing themselves in new places.

Still, experts worry about the multiple threats America’s trees face from pests and climate change, and the intersection of those two dangers. “Climate change can not only affect insects, but it can also make trees native to a certain area less suitable,” Eshenaur said. “Many of our trees in the Northeast cannot tolerate high temperatures and sustained drought. “That can make them more susceptible to these new pests that are arriving.”

This story was originally published by Grindinga nonprofit media organization covering climate, justice and solutions.

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