The Gypsy Moth Has a New Name, Could Do More Damage This Year
If you've noticed an abundance of hungry black caterpillars devouring your trees or crawling around on your patio, you may be dealing with an invasion of spongy moths.
These insects, formerly called gypsy moths, are poised for another year of damage in certain parts of the U.S..
Spongy moths (Lymantria dispar) were renamed recently by the Entomological Society of America (ESA) in order to update common insect names that are offensive to people of various cultures and ethnicities.
The previous name was removed due to its use of a derogatory term for the Romani people, with the new name selected by a group of entomologists, scientists and Romani scholars.
"Spongy moths" also better describes the insect's distinctive sponge-like egg masses.
State and federal programs have slowed the spread in the last 20 years—but they’re still a big threat in many regions of the U.S. because they can defoliate entire forests. Parts of the U.S. and eastern Canada are seeing some of the largest outbreaks in decades.
Here's what else you need to know about this invasive species:
How Bad is the Spongy Moth Outbreak?
In 2023, we’re seeing the fourth year of a significant outbreak. "Populations began to increase in 2020 in Michigan with about 950,000 acres defoliated," says Tom Coleman, PhD, entomologist/ Slow the Spread Program Manager with USDA Forest Service.
"Aerial surveys mapped spongy moth defoliation in the Northeast and Midwest with 2.5 million acres defoliated in 2021, and 1.7 million acres defoliated in 2022, mostly in Michigan and Pennsylvania."
So far, populations still are contained to the Midwest, Northeast, and southern U.S., though they may be spreading.
Spongy moths were identified in states as far west as California and as far south as Florida last year. "Additional monitoring will determine if these populations have established," says Coleman.
When Will the Spongy Moth Outbreak Be Over?
Generally, population explosions of these moths occur cyclically and eventually collapse due to natural predators such as mice or specialized wasps, or two naturally occurring diseases, a virus known as NPV and a fungus.
"The virus builds up in high population densities and will likely contribute to the outbreak collapse this year and next year," says Coleman.
"It takes several years for the virus to build. The fungus can also build up in high spongy moth populations, but its spread is facilitated by wet spring conditions."
The good news? "Defoliation declined from 2021 to 2022, so the virus and fungus are likely contributing to this decline," says Coleman. Overall, numbers are down in the Northeast and Midwest, though building in Virginia.
How Can I Tell If I Have Spongy Moths?
They're voracious insects. Spongy moths can eat about 11 square feet of leaves and grow 1,000-fold in weight as they develop. You may see the caterpillars munching on many trees and shrubs because they feed on more than 300 species.
Some of their favorites include oak, birch, cedar, and fruit trees. "But they’ll even eat conifers," says entomologist Michael Skvarla, PhD, assistant research professor of arthropod identification at Penn State University.
Look for the tiny ¼-inch to ½-inch-long caterpillars after they hatch in early to mid-May. By June they’re 1 to 2 or more inches long, hairy, with two rows of little bumps down their backs.
The first third of the dots are blue, the second 2/3 are red dots. "No other caterpillar looks like this," says Skvarla.
If they strip a tree, the caterpillars move to the next. This is when you may spot them on your patio or side of your house, looking for a new food source or a place to spin a cocoon and pupate, says Skvarla. They also leave behind a lot of messy caterpillar poo on decks and patios (just hose it off!).
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Will Spongy Moths Kill My Trees?
That depends. “Most healthy deciduous trees can take 1 to 3 years of total defoliation, but if an outbreak continues beyond that, we could see tree death,” says Skvarla. Saplings, new transplants or drought-stressed trees may be more vulnerable. Also, conifers won’t regrow needles so they may be at higher risk of dying.
How Do You Get Rid of Spongy Moths?
You're not going to eradicate them, but you may be able to reduce the population with these steps:
First, positively ID what you're seeing. Check out how the various life cycle stages here.
If you see tiny caterpillars, spray the affected tree with a naturally occurring biological pesticide called Bacillus thuringiensis kurstaki, or BTk. This is the specific strain that targets this moth. But this only works on the little ones, not on the full-sized caterpillars. Other chemicals aren’t recommended because they kill beneficial insects, too, says Skvarla.
For adult caterpillars, create barrier bands on your trees by wrapping your affected tree trunks with folded-over burlap; the older caterpillars look for a place to hide during the day and will get hung up here, so you can scrape them into a bucket of soapy water and drown them.
For large numbers of caterpillars on your deck or side of your house, knock them down with the hose, sweep them up and drown them in soapy water. Or squish them (effective but messy).
For a huge problem or an expensive tree you’re worried about, a certified arborist may be able to help with management.
What Should You Do If You Find Spongy Moth Eggs?
Positively ID them. From late summer onward, look closely to find the 2-inch long egg masses covered in fine hairs between ridges of bark, on firewood, or in crevices under vehicles, outdoor furniture, play sets, campers, tents, and RVs (this why you should inspect these items if you're moving so you don't accidentally spread them).
Scrape egg masses off with a putty knife into a bucket of hot, soapy water; don’t smash them because some may survive and hatch next spring. Or place them in a plastic bag in the hot sun. Sure, you won't find all the egg masses, but you'll feel a teeny bit better about reducing the numbers in your own garden next year.
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