'Absolute Nightmare Scenario': Caterpillar Outbreak Defoliating Northeast Trees
The emergence of the 17-year cicadas has dominated bug news of late, but in the northeastern parts of the U.S. and Canada, another cyclical menace has emerged that has the potential to do more lasting damage.Several weeks ago, Mark Boltz-Robinson started noticing that a couple of red oaks on his Monkton property were thinning at the top, and he wondered if it might be due to stress from drought conditions.
“But then one of them literally was, like, overnight, [I] woke up the next morning and it was like: ‘Woah, this tree is gone. What the heck?’ And then of course they started showing up, crawling all over the property.”
Boltz-Robinson is referring to an invasive critter known familiarly as the gypsy moth caterpillar. Rather than use an outdated name, we’re going to refer to them in this story as LDD caterpillars, shorthand for their Latin name: Lymantria dispar dispar. But people in this part of Monkton might have other … unprintable … names for them as well.
Neighbors report finding LDD caterpillars in their hair while they mow the lawn, covering the foundation of their houses and dangling from trees while they walk. Some are making a sport of trying to squish them with their cars while they drive along the dirt roads in town.
There are so many caterpillars in the trees that it constantly sounds like it's raining. Except, it's not raining. It's caterpillar poop — called frass — falling through the trees.
Boltz-Robinson is also Monkton’s tree warden and chair of the town forest committee. And it’s clear that he really cares about his trees. But this summer, there’s not much for him to admire.
“Almost everything I want to have on the property, that I enjoy having on the property, they’re decimating all of it, regardless," he said. "The spruce trees, the fir trees, the oaks, the birch, the maples, the apples, any number of berry bushes. So no hope for blueberries.”
He lifts up a branch of what would have been a beautiful raspberry bush, but it is completely defoliated. The 80-year-old apple tree that makes the centerpiece of his yard has no leaves whatsoever, and only a few tiny apples are left — covered with hairy caterpillars.
Over the course of the early summer, these caterpillars molt five or six times, growing up to two inches in length before they go into their cocoons at the end of June. They have yellow heads and five pairs of blue dots followed by six pairs of red dots down their fuzzy backs.
The caterpillars originated in Europe, but made their way to the U.S. more than 100 years ago as part of an economic experiment.
“They were brought over by a French scientist named Trouvelot in 1869,” explained Vermont State Entomologist Judy Rosovsky. “He was trying to find something to compete with silk worm moths. They got out of his lab, and 20 years later there was this big outbreak, horrifying the people of Medford, Mass. and vicinity.”
From there, the caterpillars spread, and they now cause damage to forests and suburban landscapes in about half the U.S. states. Every now and then their populations surge, causing a major outbreak in one place or another.
This year, Vermont is experiencing its first major outbreak in 30 years. Rosovsky says she’s particularly heard from people up and down the northwestern part of the state, but reports are coming in from around the northeast, including Ontario and New York.
These invasive insects can completely defoliate the landscape, starting with things like oaks and apple trees but moving pretty indiscriminately to other trees and bushes once they’ve chewed those bare. In some places, bare hillsides resemble forests in the dead of winter, not the full foliage of early summer. They’re known to eat more than 500 different species of tree.
It’s pretty bad this year for a number of reasons: One is drought conditions, which decrease the LDD’s main predator—a fungus called Entomophaga maimaiga [say: en-toe-MOFF-uh-guh my-MAY-guh]. Without the fungus, the population of these caterpillars, which already has a natural ebb and flow, is allowed to grow largely unchecked.
And there’s a pandemic angle, too. In normal years, the Vermont Department of Forests, Parks and Recreation conducts aerial surveys to check on forest health and look for issues caused by caterpillars and other pests. If it was clear that the LDD caterpillars were going to cause a problem, the state could have sprayed a caterpillar-killing bacteria over hard-hit areas this spring.
But it didn’t seem like a smart idea to send groups of people up into tiny planes together during COVID-19, so Rosovsky says the department wasn’t able to see the full potential scope of this outbreak.
“They ended up trying to use fire towers and mountain tops to do some surveillance, which was helpful, but it’s not quite the same thing," she said. "And because the population wasn’t that high last year, we were imagining we’d get something like an outbreak, but we weren’t entirely sure where it would be most concentrated."
Plus, Rosovsky admits, “We hoped we’d have a wet fall or a wet spring.”
Wet conditions would have fueled the growth of that fungus, helping to keep the caterpillar boom in check. But both the fall and spring have been significantly dry -- nearly the entire state is currently experiencing abnormally dry or moderate drought conditions.
And the drought conditions could be a compounding problem alongside the LDD caterpillars.
“Drought stress and defoliation -- those are two stresses on the trees, so they’re really in for a hard time,” Rosovsky said. “They can withstand some defoliation. They can withstand some drought. But the combination is not good for their long-term health.”
Rosovsky doesn’t currently think we’re in for a massive die-off of oak trees in Vermont. As she points out: most of the mature trees in these forests have seen caterpillar outbreaks before. The trees that look so bare right now may even be able to regrow some leaves later this summer, once the caterpillars become moths.
But for property owners like Boltz-Robinson, the fact that this outbreak could continue into next year and beyond is worrying. Surveying the scope of the damage in his yard alone, he remarks, “I have to just hope for the best: that enough of them will have enough resilience to rebound. One season of this, sure. But if next year turns out similar, it’s going to be an absolute nightmare scenario to the landscape of all of Addison County and northwestern Vermont.”
What Can You Do?
Foresters, municipalities, and state agencies may conduct spraying in heavily affected areas next spring, when the caterpillars are small. The most common spray to combat LDD caterpillars is a bacteria called Bacillus thuringiensis, usually called Bt. Individuals can also use bacterial or chemical sprays, but this can be cost-prohibitive for many homeowners.
If you have a few trees in particular that you’d like to help protect, you can wrap the tree all the way around in duct tape and apply a sticky substance to the tape ring. Many caterpillars are hesitant to cross the tape and climb the tree. When the caterpillars get bigger, they often drop to the ground during the day, to avoid predators, climbing back up the trees at night. So you can wrap a burlap or paperbag ring around your trees, folding the top over to create a kind of skirt around the trees. The caterpillars may hide in the folds of your “tree skirt” during the day. You can then collect the caterpillars and kill them.
It may also be beneficial to water trees that are overstressed, helping to mitigate the effects of drought and encourage refoliation in the latter part of the summer.
Kill the pests one-by-one
Or by the hundreds! While it may seem like a Sisyphean task, every caterpillar killed now is one that does not reproduce. If you want to try this route, get a bucket of soapy water and drop individual caterpillars, pupae in their cocoons, or whitish egg masses (these appear later in the summer and into fall/winter) into the bucket overnight to kill them.
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