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Why the canopy-destroying spongy moth caterpillar has had a quiet year in Vermont

A caterpillar is pictured on a chewed up leaf.
Josh Halman
The spongy moth caterpillar, or lymantria dispar dispar, defoliated tens of thousands of acres of Vermont oaks and maples in 2021 and 2022.

You may remember the spongy moth caterpillar from the last couple of summers. The fuzzy insects clung to houses and trees across Vermont and chewed their way through foliage, leaving a mess of damage behind.

In 2022, the invasive caterpillars defoliated nearly 43,000 acres in Vermont, and the prior year just under 51,000 acres. This year, they seem to be almost non-existent.

To find out more about the caterpillars' absence this year, Vermont Public's Mary Williams Engisch spoke to Josh Halman, a forest health program manager with the Vermont Department of Forest, Parks and Recreation. Their conversation has been edited and condensed for clarity.

Mary Williams Engisch: We'll talk about the damage that the spongy moth caterpillar caused in Vermont's forests and in people's backyards in just a moment, but first, set the scene. What is the spongy moth caterpillar? What does it look like?

A man wearing an orange vest and baseball cap smiles for the camera. He's in some snowy woods.
Josh Halman

Josh Halman: So, it's a caterpillar that varies in size as it grows. You know, it starts off very, very small, and can grow to a couple inches in size. And they typically have pretty fuzzy hairs on them, and sometimes have a blue and red coloration as they age on the top of them.

And as you mentioned, they love foliage, particularly oak foliage. And so often folks will first see them when they're eating the leaves of those trees.

Josh, I understand these caterpillars have a three-year cycle. Can you recount the spongy moss activity and the damage that it did in Vermont for the first two years of that cycle, which at this point began in 2021?

That's right. So when defoliation was first reported in 2021, we had scattered reports come in. But then as the season progressed, we received more and more landowner reports that they were seeing considerable defoliation on the landscape.

What we think might have led up to this is the fact that some of the natural controls for spongy moths are a fungus and a virus that can spread through the population. When you have dry weather, those two things are not very active. And going into that 2021 growing season, we were in the middle of a rather dry spell in Vermont.

And so we think that those those two controls weren't really that functional at that point which allowed the population to grow and to contribute to this outbreak that we're hopefully on the heels of here.

But this year, it seems it's different. It's quiet. What changed?

Yeah, a couple things have changed. One is that in any of these outbreaks situations, we're used to seeing this growth of the population of the past — in this case of spongy moth — and then once it hits a certain peak, that population starts to die out.

And in large part that's due to those natural enemies, the fungus and the virus that are present on the landscape really taking hold and controlling those numbers, as well as other opportunistic feeders. Some birds will eat the caterpillars as well but we've also had ample rainfall, which has allowed those natural controls to fully take hold and to really knock those numbers of spongy moth caterpillars down.

For Vermonters who don't recall — or they've completely blocked it out of their minds because in some areas the damage and just the presence of the lymantria dispar was nightmare-level stuff — xan you gently jog our memories about what it looked like and what it sounded like this time of year last summer?

Yeah, so last summer when these caterpillars were active, if they were on your property, you probably recall lots of munching sounds, and unfortunately quite a bit of caterpillar poop raining down on the property. Which can make it a real nuisance around houses. And of course, the other issue is that it has an impact on tree health when these outbreaks are persistent for too long.

So it can be something that can really kind of drive you crazy if they're right around your house. And like you mentioned, there have not been many reports this year, which is probably great news for those landowners that have experienced this the last two years.

 An aerial shot of Vermont shows defoliated trees.
Josh Halman
An aerial photo shows forest defoliated by spongy moth caterpilars in Vermont.

And I do recall last year, too, the defoliation happened early enough in the season that a lot of trees were able to re-lead. How important is it for Vermont foliage to get a break from all of that activity this year?

The tree is pulling on resources, carbohydrates, that sort of thing to initiate that bud break and create that second set of leaves. So it's taxing on the trees and stressful. So to get a break from that certainly sets the trees that were affected previously up for greater success and greater health going into the future.

And Josh, we understand that there are many natural cycles, weather patterns, things like that. And they're changing as the climate warms. Does climate change have anything to do with what we're seeing with the spongy moth caterpillar this year?

Yeah, climate change can have an impact on extreme weather events or weather patterns in general. And so in that regard, that kind of the dryness — the unusual, dry conditions that we had leading into this outbreak — could be impacted by climate change. You know, if we continue to have these periodic droughts that last for more than a year or so that's going to have an impact on the functionality of those natural controls and other ecosystem processes.

And lastly, Vermonters and visitors love to get out in the forest to hike, cycle, walk and just really enjoy nature and the scenery. Going forward, what can we expect with the spongy moss caterpillar?

So the good news with the spongy moth caterpillar is that these outbreaks are periodic. Spongy moth caterpillars are always around the state. It's just whether or not they grow into sizable populations where they have this outbreak occurring.

We've had periodic outbreaks in Vermont for decades of spongy moths, but they do not happen that frequently. In fact, our records that we've been tracking for these past decades shows that we've had a spongy moth outbreak on average between every 10 to 15 years or so.

So once this current outbreak comes to a close, the public can expect that we won't have another outbreak of spongy moth for a few years at least.

Have questions, comments or tips? Send us a message.

Mary Williams Engisch is a local host on All Things Considered.
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