How to Get Rid of Those Caterpillars Devouring Your Trees

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Published Jun 28, 2022

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If you live in the Northeast, Midwest, or Middle Atlantic states, you may have noticed an unwelcome visitor in your garden this spring: Spongy moths (formerly called gypsy moths). These voracious pests hatch out as teeny caterpillars starting in mid-to late spring, depending on where you live. “They can defoliate thousands of acres of forest,” says entomologist Michael Skvarla, PhD, assistant research professor of arthropod identification at Penn State University. “They have only one generation per year, so they’re usually done hatching by early June.”

From the time they emerge, they’re eating your garden. They chow down on more than 500 different plants, though their favorites are trees such as oak, aspen, birch, cedar, willow and all kinds of fruit trees. “They avoid foods they don’t especially like, such as catalpa and dogwood, but once preferred foods are gone, they’ll switch to plants they don’t like. Once those are gone, they’ll eat plants they avoid in other situations, such as conifers,” says Skvarla.

While some years you barely notice these insects, a significant outbreak occurred in the summer of 2021 and has continued this year. So, is your garden doomed?

Here’s everything you need to know about these pests:

What do spongy moths look like?

The tiny caterpillars, or larvae, emerge about ¼ to ½-inch long and mature into fat, black, hairy caterpillars about 1 to 2 inches (or more!) long with two rows of bumps down their backs. The first third of the dots are blue, the second third are red; no other caterpillar looks like this, says Skvarla.

You’ll find them wandering all over the place on trees and the side of your house; you also may have to deal with lots of slimy caterpillar poo on your deck, patio, or walkways (just hose it off). You may see them dangling from trees, too, then ballooning on a tiny silk strand to another location. Most end up feeding on trees close to where they hatch, though studies have shown they can be carried by the wind up to a half-mile away.

Eventually, the adults look for a place to hide, spin a cocoon, and pupate, says Skvarla. They love to take cover in areas such as between ridges of tree bark, under loose bark, under vehicles and eaves, on outdoor play sets. The cocoons have sparse yellow hairs protruding (if you can identify it, destroy it). Check out how to ID the various stages of their life cycles here.

Tomasz Klejdysz/Getty Images

Why Are There So Many This Year?

Originally, these caterpillars were imported to Massachusetts in 1869 by an entrepreneur who was hoping to find an alternative insect for silk production. Unfortunately, a few made a break for it, reproduced and started munching their way across the Northeast. State and federal programs have slowed the spread the last 20 years, but these insects still are a threat, defoliating 80 million acres since 1970.

Every 10 or 15 years or so, there’s a population explosion. Outbreaks usually last two to four years until naturally occurring diseases, a virus known as NPV and a fungus, cause the numbers to die off. “Unfortunately, it’s impossible to predict when this outbreak will end,” says Skvarla. “But the virus that usually keeps spongy moths under control works best at high population levels, so the longer the population is high, the better the chance there is every year that the virus catches up and causes the population to crash.”

Will spongy moths kill my trees?

Maybe. “Most healthy trees can take one to three years of defoliation, but if an outbreak goes beyond that, trees could die,” says Skvarla. Newly planted or young trees or trees stressed by drought or other pests are more at risk. Also, conifers which have lost significant amount of needles may die. If you have a special or expensive tree, contact a certified arborist who may be able to help with management.

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How Do I Get Rid Of Gypsy Moths?

First, it’s going to take perseverance! It also depends on what stage of the life cycle they’re in. For the newly hatched ones, spray with a naturally occurring biological pesticide called Bacillus thuringiensis kurstaki, or BTk, which is specific to spongy moths. However, this only works on the tiny ones; once they’re all grown up, this is not effective. Another technique for catching baby caterpillars is to use sticky bands wrapped around tree trunks, though this also may catch beneficial insects.

Using other chemical products isn’t recommended because they will kill many other beneficial insects, too, such as bees. Unfortunately, the best method for managing adult caterpillars is to start squishing (and wear gloves because the hairs can cause skin irritation in some people). Yep, it’s gross—and it’s definitely effective. If you’re squeamish, use a putty knife to knock them into a bucket of soapy water to drown them. To aid in collection of adult caterpillars, wrap tree with burlap barrier bands; they’ll congregate beneath them so it will be easier to complete your search-and- destroy mission.

how to get rid of caterpillars gypsy moth females

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In late summer, the moths emerge and start laying egg masses. The females (who don’t fly) emit pheromones to attract the males. She can up to 1,000 eggs in a 2-inch-long fuzzy egg mass, which hatches the next spring. The egg masses can be found on trees but also on any hard surface such as campers, boats, grills, firewood and play sets (so make sure to inspect these types of items if you’re moving to an area of the country where these things haven’t spread).

In late fall throughout winter, watch for these egg masses. Scrape them into a bucket of soapy water, or place them in a plastic bag in the hot sun. Don’t smash the egg masses because, believe it or not, some eggs may survive to hatch. While you’re not going to eradicate these destructive pests, your efforts may help reduce their numbers in your garden next year. At the very least, you’ll feel like you’re doing something to fight back.

Arricca Elin SanSone

Freelance Gardening Editor

Arricca Elin SanSone is a gardener with more than 15 years of experience. In addition to PureWow, she writes for Prevention, Country Living, Veranda, The Spruce and many other national publications. She also trials new plant cultivars and field tests garden products to evaluate practicality and durability.

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