A spongy moth infestation (formerly called gypsy moth) can strip trees bare in a matter of weeks. Caterpillars feed on more than 300 species of trees and shrubs, with oaks hit hardest, and a single egg mass can contain over 600 eggs. If you’re seeing masses of caterpillars crawling up tree trunks or leaves disappearing from the canopy, here’s what you’re dealing with and what you can do about it.
A quick note on the name: in 2022, USDA APHIS officially replaced “gypsy moth” with “spongy moth,” named after the insect’s distinctive spongy-textured egg masses. You’ll still see both names used, but they refer to the same species.
How To Identify Spongy Moths at Each Stage
The easiest stage to spot is the egg mass. These tan, oval blobs are roughly 1 inch by 1.5 inches and feel spongy to the touch. You’ll find them in sheltered spots: under patio furniture, inside woodpiles, on the undersides of branches, or tucked into bark crevices. At higher populations, they become visible right on tree trunks and branches. Egg masses are present from August through May, giving you a long window to find and destroy them before they hatch.
Caterpillars emerge between early May and early June, depending on spring temperatures. They grow to about 2 inches long over the next six to eight weeks. The key identifier is a row of red and blue dots running down their backs, which distinguishes them from native caterpillar species feeding at the same time.
How Infestations Destroy Trees
Spongy moth caterpillars pass through five or six growth stages (called instars), molting between each one. Young caterpillars chew small holes in leaves and feed during the day. As they grow larger, they shift to nighttime feeding, hiding in bark crevices or leaf litter during daylight hours. By the final stages, they consume entire leaves, leaving only the thickest veins behind. This is when the real damage happens: 80 to 90% of all defoliation is caused by the largest caterpillars in their last two growth stages.
Oaks are the primary target, especially white and red oak species. Aspen, birch, basswood, willow, apple, and hawthorn are also preferred hosts. During severe outbreaks, caterpillars expand their diet to include maple, elm, cherry, pine, spruce, and other conifers. A healthy deciduous tree can usually survive one year of complete defoliation by regrowing leaves later in summer, though at significant energy cost. Older or already stressed trees may die after a single severe defoliation event. Two or more consecutive years of heavy feeding dramatically increases mortality.
Skin Irritation From Caterpillar Hairs
Spongy moth caterpillars are covered in tiny hairs that can cause skin reactions ranging from mild itching and small red bumps to more persistent, eczema-like rashes that last for weeks. You don’t always need direct contact. During heavy infestations, these hairs break off and drift on the wind, causing reactions in people who never touched a caterpillar. The hairs can also cling to clothing, outdoor furniture, and laundry hung outside to dry.
If you develop a rash, ice and over-the-counter antihistamines can help with itching. Wash any clothing that may have picked up caterpillar hairs thoroughly. Topical steroid creams may reduce eczema-like reactions, though results vary.
What To Do: A Seasonal Action Plan
October Through April: Destroy Egg Masses
This is the single most effective step for homeowners. Every egg mass you destroy eliminates 600 or more potential caterpillars. Scrape egg masses off trees, furniture, and structures into a container of soapy water. Be careful not to gouge the bark, and know that any fragment of the egg mass left behind can still hold viable eggs. Alternatively, soak the egg mass in place using a horticultural oil specifically labeled for spongy moth. Do not use motor oil, mineral oil, or other substitutes, as these can harm trees and aren’t effective.
Late April: Install Barrier Bands
Right after eggs hatch, wrap tree trunks with barrier bands to stop tiny caterpillars from climbing back up after falling or ballooning down on silk threads. Use duct tape or another nonporous material coated with a commercially available sticky product like Tanglefoot or petroleum jelly. Never apply sticky material directly to bark. For thin-barked trees like birch or young maples, wrap butcher paper around the trunk first to protect it. You’ll need to reapply the sticky coating periodically as rain washes it away and caterpillar bodies accumulate on it.
Early June: Switch to Collection Bands
Once caterpillars are larger, replace barrier bands with burlap collection bands. Older caterpillars feed at night and crawl down the tree at dawn to hide. A strip of burlap wrapped around the trunk at chest height gives them a hiding spot you control. Check under the burlap each afternoon, scrape caterpillars into soapy water, and repeat daily during peak feeding.
Btk Spray for Larger Properties
For properties with many affected trees, a biological spray called Btk can reduce caterpillar numbers significantly. This naturally occurring soil bacterium produces protein crystals that are toxic only to caterpillars, thanks to the unique alkaline chemistry of their digestive systems. It’s harmless to humans, pets, and most other wildlife. When caterpillars eat sprayed leaves, they stop feeding within hours and die within a few days from a combination of starvation, digestive damage, and bacterial growth inside their bodies.
Timing matters. Young caterpillars are far more sensitive to Btk than older, larger ones. The spray should go on soon after eggs hatch in spring, and leaves need thorough coating since caterpillars must eat the bacteria for it to work. Btk won’t kill every caterpillar on your property, and older larvae can often survive exposure. During an active outbreak, foliage needs to be sprayed each year to maintain any suppressive effect.
What Naturally Ends an Outbreak
Spongy moth populations are cyclical. Outbreaks typically build over several years, then crash. Two natural enemies drive these collapses. A virus called nucleopolyhedrosis (NPV) infects only spongy moths. Caterpillars ingest the virus, which destroys their internal organs. You’ll notice dead caterpillars hanging limply in an inverted “V” shape from tree trunks and branches during a viral epidemic. Their bodies release more virus particles into the environment, accelerating the spread through dense populations.
A fungus originally introduced as a biological control agent also plays a major role. It thrives in warm, wet spring conditions and can devastate caterpillar populations when humidity is high. Together, these two pathogens are the main reason outbreaks don’t last indefinitely, though they typically need high caterpillar density to spread effectively, meaning trees may endure one or two bad years before the population crashes on its own.
Quarantine Rules for Moving Outdoor Items
If you live in or near an area under spongy moth quarantine, regulations restrict what you can move out of the designated zone. Quarantine areas exist across many states in the eastern U.S. and are expanding westward. Regulated items include firewood, logs, and branches over four inches in diameter, nursery stock, Christmas trees, and outdoor household articles like patio furniture, boats, trailers, recreational vehicles, dog houses, flower pots, tents, and awnings.
Before moving any of these items out of a quarantine area, you’re required to inspect them for egg masses, caterpillars, pupae, or adult moths at any life stage. This applies even to items that seem unlikely hosts. Egg masses easily hide on the undersides of lawn furniture, inside wheel wells, or on the hulls of boats. Firewood is one of the most common ways spongy moths spread to new areas, which is why “buy it where you burn it” has become a standard guideline. Shippers operating commercially within quarantine zones must document every outbound load, specify its destination, and treat or remove any life stages found during inspection.