Species Spotlight: Spongy Moth, Lymantria dispar

Here’s a hot topic: Spongy Moth (formerly: “Gypsy Moth”). It seems like it’s EVERWHERE. I’ve fielded several calls this season already about the issue. “What the heck is happening to the woods?” Maybe I can shed a little light on what we’re seeing.

One of the best illustrations of this I’ve experienced has been with a recent drive up the I-87 toward Lake George. There’s a point, right as you cross over the Hudson River near Moreau Lake State Park, that the landscape changes very suddenly. It almost seems as if time has rolled back and you’re driving through the forest in early spring again; the trees aren’t fully leafed out anymore and there’s just a lot of blue sky where there ought to be lush, green canopy. That’s the power of Spongy Moth caterpillars. These things emerge in the Spring in massive quantities, crawl up to the canopy, and consume every single last morsel of leafy vegetation with a strong preference for Oak.  Why do you hear the pitter-patter of rain on a sunny day? It’s nothing to worry about, just the cute sound of millions of pooping caterpillars.

These caterpillars are hard to miss during a population boom. They grow rapidly but max out around 2.5 inches in length and are easily recognized due to their double rows of colorful spots; 5 blue and 6 red. They’re also very bristly- be careful when handling these as they can cause irritation. The adult moths, typically present in mid-summer, are around 2 inches in length.  The females, tend to be a light cream color with brown line patterns on the wings and, while they do have fully developed wings, the females are generally flightless. Males, on the other hand, are just a bit smaller, and are brownish gray, with those grand, feathery antennae (typical to male moths).

Females lay egg masses directly on the trunk of the tree (or anywhere they please, really). The masses, spongy to the touch, are very distinctive light brown, fuzzy lumps, about the size of a quarter. The spongy texture of the eggs is what gave rise to the name, Spongy Moth. Each mass may contain upwards of 700 eggs and, during population booms, these egg masses are EVERYWHERE. They’re also easily confused for Spotted Lanternfly Egg masses, which are similarly found on many surfaces and are about the same size, but are more of a drab gray color, and have the appearance of dried mud.

Native to Eurasia, Spongy Moth was introduced to the US byway of Massachusetts in the late 1800s, with the (failed) intent of developing a new breed of silkworm that would kick off an American silk industry. As the story goes with most of the world’s contemporary biological invasives, Spongy Moth was introduced into a novel ecosystem, with few natural predators to keep it in check. Sure enough, the Spongy Moth spread like wildfire.

Anecdotally, Spongy Moth plays a subtle role in a surprising amount of US history. Its arrival here can be connected with the Civil War-induced turbulence of the global textile industry, and through its destructive, unchecked nature, it was a major driver for the widespread use of the infamous pesticide, DDT, thereby laying a stone in the road to the modern environmental movement.

Spongy Moth has been on the American landscape long enough that it’s considered to be a naturalized component of the forest community. This means, I’m sorry to say, that it’s probably not going anywhere. It also means that Spongy Moth has struck a sort of uncouth and uneasy balance with the native ecosystem. For well over a hundred years now, populations of this voracious caterpillar have risen and fallen with some regularity. For two or more years we may see these startling defoliation events in our forests, followed by a decade or more of relative silence.

This boom-and-bust cycle is due largely to, you guessed it, BiologicalControl! In the context of Spongy Moth, there are numerous biocontrol agents at work but most of the credit goes to a couple of unique pathogens; a fungus, Entomophaga maimaigaI, and a viral disease, the Nucleopolyhedrosis Virus, or NPV for short. The work of these two bio-controls is evident when you see collections of dead Spongy Moth caterpillars on trees that have wasted away or folded bodily in half, backward, forming an upside-down “V” shape. These controls only really kick in with a bang when Spongy Moth populations are at their peak, leading to the dramatic crash in infestations. Bird populations and small mammals also take full advantage of the tasty, caterpillary-smorgasbord.  

If you have Spongy Moth caterpillars at home or see them at our preserves (Hollyhock Hollow, for instance), don’t panic. While the defoliation of the trees can be shocking, they’re usually hardy enough to bounce back after the caterpillars have stopped feeding, even with a few consecutive seasons of infestation.  On a positive note, just think about all the rich caterpillar fertilizer being distributed across the forest floor. Smaller trees and shrubs may have a harder time recovering, so if you have a few ornamentals or special trees in your care, check out some of the options that may be available to you. There are all kinds of clever contraptions that can be affixed to trees to prevent the swarms of caterpillars from moving up into the canopy. There are also a few options for chemical control, one of the most commonly used of these is called “BTk”, or Bacillus thuringiensis kurstaki, a naturally occurring soil bacteria that disrupts the digestive system of the caterpillars. As with any chemical options, you should do your research before making purchases, or (even better) contact a licensed applicator or Cooperative Extensions Office. You can also do your best to scrape off the egg masses from trees into buckets of soapy water, reducing next year’s infestation.

As I mentioned, the history of Spongy Moth in the US is an interesting tale. For better or worse, it has certainly left its mark on the legacy of our forests. There are many, many resources available for further information. I’d recommend first visiting our own Capital Region PRISM’s website (capitalregionprism.com)and Cornell’s Integrated Pest Management guidelines. Spongy Moth is an excellent forest health primer for budding naturalists. The rise and fall cycle of the infestation and the reverberating effects of defoliation events in our forest communities can be fascinating to watch. As I like to say, “Keep your eyes up and your mouth closed”.

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Marshall Lefebvre
Stewardship Coordinator

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