RIVER VALLEY — Growing up in the city, I’d hear reports from our country cousins: “You can hear the caterpillars munching....”

I didn’t really believe it.

But then I went bicycling in Harriman State Park with my then-boyfriend, now-husband. It was the early ‘80s. We were desperate to see some fresh green leaves. Instead, we got tons of hairy black caterpillars.

I have never seen so many spongy moth caterpillars—formerly known as gypsy moth caterpillars—in one place before or since. The trees and bushes were bare, as if it were January. The few blades of grass left were covered with caterpillars fat with feasting but starting to crash now that everything green was gone.

If we stood still for even a few minutes, a platoon of desperate caterpillars would head toward us and creep over our shoes and bike tires. To enter the public restroom I had to push aside a curtain of caterpillars hanging from the lintel like a beaded curtain. The wash basin was filled to the brim with caterpillars writhing and dying from lack of food. If you touched one, even lightly, their innards oozed out.

That experience wasn’t enough to keep me from moving to the country, though. 

I saw a few bad outbreaks—but none as bad as that—at my York Lake home before our homeowners’ association started spraying. Spongy moths love oaks, and our place is oak heaven. We have white oaks and pin oaks, bear oaks and chestnut oaks, scarlet oaks and Northern reds, along with hickories and other types of trees they also find tasty.

Mapping the caterpillars

This year is shaping up to be a bad spongy moth year in the river valley. Every April I look for caterpillars at the first leaf break. If I find them under the newborn oak leaves, let’s say one or more per leaf, I know we’re in for it. And I found them this year. The exterior walls of my house look as if flecked with black pepper. Upon closer inspection: caterpillars!

Andy Boyar picked 200 caterpillars off the tree in Heroes Park in the Town of Highland, said Peter Carmeci, the commander of the Tusten Highland Lumberland VFW,

Outbreaks are cyclical, and the current one is cycling closer. Lori Severino of the NYS Department of Environmental Conservation (DEC) said the current outbreak in New York started in the Finger Lakes region in 2020 and spread to many other areas of the state in 2021.

“In 2022 the spongy moth population crashed, and defoliation was confined mostly to the Hudson and Champlain Valleys,” she told the River Reporter in an email. “In 2023, the heaviest defoliation was observed throughout the Hudson Valley from Westchester County to Albany. This year, DEC expects pockets of defoliation to be scattered across the lower Hudson Valley.”

As to precisely where the defoliation will happen, Severino said DEC will do aerial surveys later this spring to map it. 

Pennsylvania already has a map. Pike County has one of the highest spongy moth egg mass counts in the state, according to 6,322 surveys the PA Department of Conservation and Natural Resources (DCNR) conducted statewide on March 1. A bunch of those egg masses were found along southern Wayne County, but the rest of eastern Pennsylvania was relatively clear of masses.

Trevor Tochydlowski, forest specialist with the Wayne County Conservation District, said the border of Wayne and Pike counties is where two different types of forests meet. One type is the Northern hardwood forest, which features maples, beeches and ash, and characterizes the woods found in Wayne County. On the other side of the border spreads an oak and hickory forest. That means outbreaks in Wayne County aren’t as bad, since the caterpillars’ preferred food doesn’t blanket the county as thoroughly as it does in Pike, he said.

“Most of the egg masses we saw in Wayne were old egg masses,” he said. But, he said, Lake Wallenpaupack and areas close by will be hit hard.

Northeastern Pennsylvania has many maple producers, and Tochydlowski said they sometimes they need to spray to save their producing trees. But he doesn’t anticipate a problem with sugar maples being defoliated this year.

The PA Department of Conservation and Natural Resources does aerial spraying of state lands, and has already completed its treatment of hundreds of acres in Pike County. Tochydlowski said treatments with Btk (Bacillus thuringiensis Kurstaki), a naturally occurring soil bacteria that interferes with the spongy ​​moth caterpillars’ digestive system, have a “very limited impact on other species.”

Good news, and bad

The good news is that the leaves will grow back in mid-summer and the trees will survive, although in a weakened state. 

“Trees defoliated by spongy moths in the spring will re-flush and grow new leaves around the first two weeks in July,” Severino said. “Healthy trees can usually withstand moderate defoliation for two to three years.

“However,” she said, “severe and repeated infestations may weaken trees and make them more susceptible to other pests, diseases and environmental stressors.”

Tochydlowski said if a tree suffers two to three consecutive defoliations, “there is a high likelihood of the tree kicking the bucket.” And he said many trees have already suffered a defoliation this year during a late frost “when we lost a bunch of leaves.”

Rosa Yoo, forest health manager in the Division of Forest Health, said aerial spraying has been completed in Pike County, with some areas treated twice because of the high numbers of caterpillars. Btk is not very toxic and is also photosensitive, she said, so some of the 8,000 acres treated received a second application.

Severino said the spongy moth is always around in low numbers, with outbreaks occurring on a cycle of three to five years. “Populations in non-outbreak years are controlled by small mammals,” she said. “The main diet of the small mammals is acorns, so if there is a year that has low acorn production, the small mammal population will be lower the following year. This is turn will result in less predation on the spongy moth, which allows their population to increase. Approximately three to five years ago, there was low acorn production, which set the process in motion.”

However, there is some more good news: The defoliation that I witnessed at Harriman State Park 40 years ago is becoming a thing of the past. Years of aerial spraying and human vigilance have amounted to something like success. “DEC expects the spongy moth population to continue to decline,” Severino said. 

Tochydlowski said “human influence has made a tremendous impact” for the better. He said it’s very rare to find a landowner unfamiliar with the spongy moth or the importance of controlling it. Controls will become even easier now that drones are being developed to spray small lots so that property owners do not have to join forces on a big project.

Weather, too, has an influence. If you were wondering if all the rain this season will help kill off the caterpillars, the answer is, it just might. Spongy moths have two biological controls: one’s a virus and the other’s a fungus. The fungus tends to grow when the weather is wet, which is bad news for the spongy moth. 

And, he said, if you see a dead spongy moth caterpillar, you can tell how it met its end: If it’s hanging off the tree, stuck by its rear two suction cups and facing the ground, it was the fungus. If it’s bent over in a “V” shape, it was the virus.

But you don’t have to wait for either of those. Next year, you can take some preventative measures found on the DEC website https://www.dec.ny.gov/animals/83118.html. DEC also hosted a Facebook Live on the spongy moth with DEC Forester Rob Cole.