How To Get Rid Of Gypsy Moth Caterpillars

It all started when an entrepreneur in Massachusetts imported European gypsy moths in 1869 in hopes of finding an alternative insect for silk production; his effort failed miserably in more ways than one. Unfortunately, a few caterpillars escaped, reproduced, and spread. These destructive insects are now found throughout the Northeast, west to Wisconsin, and south to Virginia. "Gypsy moths can defoliate hundreds of acres and decimate a forest," says entomologist Michael Skvarla, PhD, assistant research professor of arthropod identification at Penn State University. "They haven't been an issue for the last 10 or 15 years, but in 2021, we're seeing a significant outbreak."

Generally, population explosions 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. In the meantime, it's decidedly not so pleasant to watch them chowing down on your trees, covering the side of your house, or crashing the family picnic. Plus, a huge infestation may result in a lot of, well, caterpillar poo, on decks and patios. The good news is that gypsy moth caterpillars don't bite, though if handled, their hairs can cause skin irritation—and their waste simply can be hosed off. While you can't eliminate gypsy moths, you can take a few steps to try to reduce the population and feel a little less anxiety-ridden that your entire garden is their personal buffet.

Here's what you should know to help control gypsy moths in your garden.

How can I tell if I have a gypsy moths in my yard?

Fortunately, state and federal programs have slowed the spread in the last 20 years—but they're still a big threat because they've defoliated 75 million acres in the U.S. since 1970. You may see the caterpillars, also called larvae, munching on your trees and shrubs; they feed on more than 300 species. Some of their favorites include oak, birch, cedar, and fruit trees—but they'll even eat conifers if they get hungry enough, says Skvarla. Typically, you'll see the tiny ¼-inch to ½-inch-long caterpillars 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.

Will the gypsy moth 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. However, saplings or new transplants or trees stressed from drought may be more vulnerable because they don't have the reserved energy stores or established root systems. Also, conifers won't regrow needles so they may be at higher risk of dying.

You may see gypsy moth caterpillars in other places in your yard, too.

If they strip a tree, the caterpillars move on to the next one. This is when you may spot them wandering around on your patio or on the side of your house, looking for a new food source or a place to hide out, spin a cocoon, and pupate, says Skvarla. They look for a sheltered area such as between ridges of bark, under loose bark, or in crevices under vehicles, outdoor furniture or play sets. The cocoon has a distinctive appearance with sparse yellow hairs protruding from it (if you positively identify it, you can destroy it). Check out how to ID the various life cycle stages here.

How do I deal with gypsy moth caterpillars?

If you catch them in their baby caterpillar stage when they first hatch in May, you can try spraying with a naturally occurring biological pesticide called Bacillus thuringiensis kurstaki, or BTk. This is the specific strain that targets gypsy moths. Unfortunately, once the caterpillars all full-sized, this treatment is less effective. Other chemicals aren't recommended because they kill beneficial insects, too, says Skvarla. If you have a huge problem or an expensive tree you're worried about, an arborist may be able to help with control.

For adult caterpillars, you can create barrier bands on your trees by wrapping your affected tree trunks with folded-over burlap; the older caterpillars will look for a place to hide during the day, so you can remove and destroy them by scraping them into a bucket of soapy water. For large numbers of caterpillars, say, on your deck or side of your house, knock them down with the hose, sweep them up and drown them in a bucket of soapy water. Or just squish them (effective but messy).

Go on a search and destroy mission for the eggs.

In late summer, the moths emerge from their cocoons and mate. Females don't fly but emit pheromones to attract the males. Then a female lays hundreds of eggs in a 2-inch long flat, fuzzy egg mass, which hatches the following spring. The egg masses usually are found on tree trunks but also may be on any hard object including under campers, boats, and grills—which is why these items should be inspected if you're moving to an area gypsy moths have not yet infested (check here for states where they're currently found).

In late fall and throughout the winter, keep an eye out for these masses. Scrape them off with a putty knife and dump into a bucket of hot, soapy water; don't try to smash them because some may survive and hatch next spring. Or place them in a plastic bag in the hot sun. It's time-consuming (but also a little satisfying!) and may help reduce the numbers in your own garden next year.

How to Deal with Gypsy Moths

BTk Biological Control

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Arricca Elin Sansone Arricca SanSone has written about health and lifestyle topics for Prevention, Country Living, Woman's Day, and more.

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