June 21, 2024

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

On a clear spring day, the pitter-patter of spongy moth caterpillars feeding through oak, maple, crabapple, basswood and aspen trees can be heard over the sound of birds singing. Bits of green leaves litter the ground like confetti – evidence of the insatiable chewing that takes place in the canopy above. Hundreds of caterpillars bounce on long, spindly silk threads, waiting for a breeze to carry them to a new tree.

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

“When trees are defoliated like this at this time of year, they’re using reserves that are in the trunk and in the roots to put out a second flush of growth,” says Brian Eshenaur, a plant pathologist at Cornell University’s Integrated Pest Management Program. “If the tree has to do this two years in a row, it really exploits all the reserves it has.”

A dark brown and blue speckled caterpillar sits on a green leaf that has been chewed several times
A spongy moth in its caterpillar form on an oak leaf, its favorite food.
Sebastian Willnow/Picture Alliance via Getty Images

The caterpillars are not the only forest pests that benefit from climate change. Many invasive species in the US are expanding, generally thanks to milder winters caused by warmer-than-average global temperatures. Insects like the hemlock woolly adelgidthe emerald ash borer, the Japanese beetle and the spotted lanternfly are chewing through the country’s trees at a record pace – leading to widespread tree mortality and stressed forests which is susceptible to drought and more diseases. No single species is capable of taking down the nation’s forests, which collectively hold mountains about 60 billion metric tons of carbonbut the rising tide of invasive species is doing serious cumulative damage.

Spongy 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 suitable for American regions that could be used for commercial textile production. Sponge moths, then known as gypsy moths, float from leaf to leaf and tree to tree on long, durable lines of silky thread. But the moths soon escaped from captivity, perhaps due to a violent storm torn by Trouvelot’s netand some of the bugs decamped to the Massachusetts forest.

Two decades later, in the midst of the first sponge moth infestation on record, one resident of the town where Trouvelot lived described a world covered in black, hairy caterpillars. “I’m not exaggerating when I say that there wasn’t a place on the outside of the house where you could put your hand without touching caterpillars.” the resident told the Boston Post in 1889. (The caterpillars do not bite humans, but coming into contact with their spiny hairs causes some people to develop an itchy and painful rash.)

For more than a century after that initial outbreak, sponge 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 foliage leaving trees and shrubs and whole tracts of forest bare in their wake. The moths defoliated 81 million hectares cumulatively between 1970 and 2013. Because of the toll they take on trees, keeping sponge moth populations in check has become one of the U.S. Forest Service’s highest priorities. The economic cost of managing sponge moths has averaged $30 million per year over the past 20 years.

The sun shines against a blue sky as a man looks up at a tree bare of leaves at its tips.
A member of the Massachusetts State Forest Health Program checks on trees defoliated by spongy moths.
Suzanne Kreiter / The Boston Globe via Getty Images

And climate change is making things worse. Outbreaks usually occur every eight to 12 years, and each surge lasts one to three years. The current outbreak has lasted longer than usual, said Tom Coleman, a Forest Service entomologist who oversees the agency’s Slow down the spread sponge moth program, partly due to drought in some of the areas the moths inhabit.

Drought affects the spread of a fungal pathogen called Entomophaga maimaiga which curbs spongy moth populations. Originally found in Japan, the fungal pathogen was introduced to the US by researchers in the early 1900s as a spongy moth control measure. The pathogen can be incredibly effective at killing the moths in their caterpillar stage, but it needs a cool, wet spring to multiply. Cyclical outbreaks of sponge moths often follow drier-than-average years, when the pathogen is not as prevalent in the environment. “Without the fungal pathogen keeping the populations in check, we get these big outbreaks,” Coleman said.

In the eastern parts of the country where sponge moth outbreaks occur, climate change make weather patterns more erratic. Much of the eastern US is projected to get wetter, on average, as the planet warms. But climate change is also fueling pockets of drought in these regions during warm months. Drought in 2023 and early 2024 in northern Virginia, southern Pennsylvania, and parts of Wisconsin and Michigan helped this year’s outbreak. Drought on the scale often seen in the American West is not a prerequisite for outbreaks of sponge moth in the east. “It doesn’t have to be a year-round drought,” Coleman said. “It could just be a rather warmer, dry spring.”

It is unclear whether rising temperatures will cause sponge moths to emerge more frequently, but it is safe to assume that a warmer, drier environment will cause cyclical outbreaks to become more intense over time. Fortunately, the Forest Service had some luck deploying more than 100,000 pheromone-tipped traps to catch the bugs as they try to push west. The agency also treated 10 million hectares of forest with a biological insecticide that kills the caterpillarswhich prevents the bugs from settling 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, 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. This can make them more susceptible to these new pests that are coming in.”

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