The complete guide to spongy moth caterpillars for bird lovers

Dear Avant Gardener, Last week we woke up to a massive infestation of invasive spongy moths. They’ve just started laying eggs and the trees are now covered in hundreds of thousands of the eggs. Apparently these eggs are supposed to hatch in two weeks, and I’m terrified the resulting boom will irreversibly decimate our tree foliage. What can I do to protect my trees from this invasion?! — Marina, Millbrook, NY

Ew, gross — or maybe not. The Hudson Valley is apparently at the tail end of a three-year spongy moth population explosion, which happen roughly every 10 to 15 years. Your photo and video illustrate how pervasive these moths (formerly called gypsy moths) can be during these times. I imagine knowing how destructive they are makes their omnipresence creepy, unlike witnessing thousands of beloved and endangered monarch butterflies at their Mexican overwintering grounds.

Fortunately, spongy moth damage is not irreversible; most trees recover within the season. And there are physical and — yes — psychological ways to make spongy moth invasions less disturbing. Moreover, this year’s eggs won’t hatch until next May, so you have time to respond thoughtfully. 

Skip the “organic” pesticides

First, do no harm. That means don’t use pesticides — including “organic” pesticides. Many of us, including me, grew up thinking organic means good for the environment. However, just because crops treated with a “natural” substance are safe for human consumption does not mean that substance is safe for wildlife.

Pesticides derived from natural sources (such as biological pesticides) may be used in producing organically grown food. — EPA

Btk (Bacillus thuringiensis var. kustaki), recommended broadly on the internet for killing spongy moth caterpillars, is a biological pesticide that’s listed as organic by the OMRI (Organic Materials Review Institute). However, spraying forests or residential yards with “organic” Btk wipes out all types of caterpillars, not just spongy moths. And killing all caterpillars in an area decimates the food available there for nestlings. This is serious because moth, butterfly, and bird populations are declining at alarming rates.

It’s worth noting that the USDA developed a pesticide, branded GYPCHEK, that kills spongy moths without harming other caterpillars. Unfortunately, it’s not available commercially. Private sector pesticide manufacturers determined that the market was too unstable to make producing GYPCHEK profitable. The Federal government manufactures small quantities of GYPCHEK and New York State sprayed it by plane over six forests last May.

"The areas chosen for spongy moth treatment reflect some of New York's most valuable ecosystems," says DEC Forester Rob Cole. "Among our considerations in choosing treatment areas were the protection of endangered moth species in Allegany State Park, as well as several rare plants, butterflies, and birds in the Rome Sand Plains." — New York Department of Environmental Conservation

Accept the things you cannot change

My first recommendation is to reframe how you experience the spongy moths descending on your yard: Accept them as part of the natural world around you. Spongy moths have been slowly expanding their range in North American since they were brought here more than 150 years ago. They are here to stay. While it’s possible to minimize damage to specific trees on your property (I’ll tell you how in a minute), you cannot keep spongy moths out of your yard.

Can you accept the spongy moths as just a nuisance? I imagine it’s the invasive nature of spongy moths and continued but unsuccessful efforts to stem their tide that makes us respond differently to spongy moths than to the various periodic cicadas native to eastern U.S. Even though cicadas emerge in huge broods that can kill trees, they are considered a “wonder” and “at most a mild nuisance because of the noise they produce.” 

If you do nothing, most hardwoods like your oak will survive even complete defoliation, although successive years of defoliation can make it more susceptible to insects and diseases that will eventually kill it. 

A healthy leaf-bearing tree will likely leaf out again as the caterpillars disappear in July/August, though leaves will probably be smaller than usual. If a tree loses ALL its leaves and does not grow any new ones in late summer, check it in the spring. If it still does not leaf out next spring, it has died.  — New York Department of Environmental Conservation

If a conifer loses more than half its needles, it will probably die; fortunately, conifers are not preferred hosts.

Change the things you can

As you’ve probably already discovered by now, you can reduce the number of caterpillars that hatch on and consume the leaves of your trees and shrubs by removing and killing their egg masses. Each egg mass contains as many as 1,000 eggs. Removing them is time-consuming, so focus your efforts on trees that would create a hazard or significantly impact your enjoyment of your yard if they died. Favored hosts like the oak in your video are at greatest risk.

Favored hosts include oak, apple, alder, basswood, birch, poplar, sweet gum, willow, and hawthorn. Less favored host species include hickory, maple, cherry, cottonwood, elm, black gum, larch, sassafras, and hornbeam. Some mortality even occurs in white pine. — Invasive.org

Here’s how:

1. Identify the areas you can reasonably treat within your time constraints. It will be a combination of favorite trees and easy-to-access places. Include outdoor furniture and other places where moths lay eggs, in addition to trees and shrubs.

2. Protect yourself with gloves, eye wear, and a mask. (Some people are allergic to histamines in spongy moth hairs.)

3. Put on a playlist of upbeat music. Dance beats work well.

4. Use a plastic putty knife — or something equally dull that won’t damage bark — to scrape egg masses into a large container full of soapy water. A large yogurt container and Dawn dish soap work well.

5. Leave the egg masses in soapy water for 48 hours to kill them, then dispose of the contents down the drain.

When the eggs hatch, which will be next spring for those laid now, you can keep the caterpillars from eating the leaves of specific trees by making a sticky barrier with duct tape or a burlap band trap. However, both these methods will stop all caterpillars indiscriminately, so focus on important trees of preferred hosts if you choose to apply one.

If all else fails, make a snag

If one of your trees dies, then please create a snag. Dead trees are arguably better for wildlife than live ones. The National Wildlife Federation recommends we maintain three snags per acre, far more than are currently kept in residential yards. The biggest downside is the cost, if any, of reducing the likelihood of the tree or parts falling on people or property. Learn more in Wait! B4 You Cut Down That Tree.

— The Avant Gardener

Why, How, Wow!

Why?

Spongy moths are yet another example of a non-native species wreaking havoc among organisms that did not evolve with them — and therefore did not evolve defenses against them. 

Like many invasive species, spongy moths were brought here intentionally. In the late 1860s, Etienne Trouvelot brought spongy moths to Massachusetts from Europe, hoping to develop a hardy silkworm through cross-breeding. The experiment failed and moths soon established themselves in the lot next to his Medford home. Twenty years later, the first defoliation was recorded — fruit and shade trees in a 360 square mile area around Medford.

Spongy moths are now established in 20 states and eastern Canada. Among their competitive advantages are their diurnal nature, prolific egg-laying, and their caterpillars’ unpalatable hairy covering and broad appetites. These advantage are growing as light pollution and loss of native vegetation endanger our native moths, which are mostly nocturnal and have highly specialized diets as caterpillars.

This moth is a significant pest because the caterpillars have voracious appetites for more than 300 species of trees and shrubs, posing a danger to North America's forests. The caterpillars defoliate trees, leaving trees vulnerable to diseases and other pests and can eventually kill the tree. — USDA

Despite these advantages, natural predators keep spongy moths in check most years. But an imbalance — like a bad acorn year that results in a lower squirrel population the following year — periodically sets off a population explosion like the current one in New York State. Eventually, the moths’ voraciousness so reduces food supplies that populations fall back to normal levels.

How

Like many people, I notice and like birds more than caterpillars. However, I’ve learned that the survival of most bird species depends on caterpillars. I now actively manage my yard to maximize the number of caterpillars it supports — and I hope you will, too.

Most of North America’s terrestrial bird species, some 96 percent in fact, rear their young on insects rather than seeds and berries (Peterson 1980), and we are learning that in most of those species, the majority of those insects are caterpillars or adult moths. Caterpillars are so important to breeding birds that many species may not be able to breed at all in habitats that do not contain enough caterpillars (Narango et al. 2018). — Doug Tallamy in Nature’s Best Hope,

A single nest of baby birds eats more than 6,000 caterpillars in a two-week period. And the number of caterpillars and birds that survive one spring affects the numbers born the following spring, and so on. Entomologist Doug Tallamy’s research shows a clear relationship between the decline of bird populations and the decline of caterpillars.

[T]errestrial birds for which insects are an essential source of food have declined by 2.9 billion individuals over the last 50 years, while terrestrial birds that do not depend on insects during their life history have gained by 26.2 million individuals, a 111-fold difference. — Doug Tallamy in Ornithological Applications

Wow!

The snag we created last year from a huge, dying black cherry (Prunus serotina) in our suburban yard is now a bird magnet. One evening this week, Pete saw a ruby-throated hummingbird dive-bombing a redbelly woodpecker and ran in for his camera. By the time he came out, the hummingbird had buzzed away, but within 10 minutes he captured the redbelly woodpecker, a northern flicker (another woodpecker), numerous cedar waxwings, house finches, a blue jay, and a robin. I am so happy to provide habitat for these birds, especially the woodpeckers.

Northern Flickers are widespread and common, but numbers have decreased by an estimated 1.2% per year between 1966 and 2019 for a cumulative decline of 47%. — AllAboutBirds.org

Related Resources

• Want to host more moths and butterflies? Find host plants with the National Wildlife Foundation’s Native Plants Finder.

• Interested in hosting big, beautiful butterflies? See my host plant recommendations in 10 Butterflies to Host.

• Curious which invasive species are most prevalent in your state? Try the interactive EDDMapS Species Status Report by State.