What most people call a gypsy moth “nest” is actually an egg mass, a tan, fuzzy patch roughly 1 to 3 inches long that the female moth deposits on tree bark, siding, firewood, or almost any outdoor surface. (The insect has been officially renamed the spongy moth, a nod to the spongy texture of these egg masses.) Unlike tent caterpillars, spongy moths never build silk tents or webs. If you’re seeing a webbed structure in a tree fork, you’re likely dealing with a different species. The flat, hair-covered egg mass is the only “nest” spongy moths produce, and each one can hold 500 to 1,000 eggs waiting to hatch in spring.
What Egg Masses Look Like
A fresh spongy moth egg mass is roughly oval, rusty to pale brown, and covered in fine hairs that give it a felt-like texture. When you press on a new egg mass, it feels hard and full. They’re usually darker in color and have a neater appearance than older ones. By the following year, old egg masses fade, become powdery and fragile, and feel soft and spongy to the touch. Those spent masses are empty and not a threat.
Females lay egg masses in sheltered spots: the undersides of branches, bark crevices, the base of tree trunks, and up into the canopy. But they’re not picky. You’ll find them on house siding, outdoor furniture, tires, rocks, firewood stacks, and fallen branches on the ground. If you store or transport firewood from an infested area, you can easily carry egg masses to new locations without realizing it.
How to Tell Them Apart From Tent Caterpillars
The confusion between spongy moths and tent caterpillars is common because both species defoliate trees in spring. The differences are straightforward once you know what to look for.
- Spongy moth egg masses are tan, covered in fine hairs, and sit flat against bark or other surfaces. The caterpillars never spin silk tents of any kind.
- Eastern tent caterpillar egg masses are dark, spindle-shaped, and wrapped tightly around small twigs with a rough, varnished texture. Their caterpillars build prominent silk tents in the forks of branches.
- Forest tent caterpillars lay eggs similar to eastern tent caterpillars but do not build silk tents. They leave small silk mats on leaves where they rest.
If you see a conspicuous white web in a tree, that’s almost certainly a tent caterpillar or fall webworm, not a spongy moth. Spongy moth damage shows up as chewed leaves with no webbing at all.
When Eggs Hatch
Females lay their single egg mass between mid-June and late July, depending on the region. The eggs then overwinter and hatch in spring. In the lower Midwest and mid-Atlantic states, hatching typically happens between early and mid-May. Farther north, it can be delayed until late May or mid-June. Warm spring temperatures accelerate the timeline, while a cold, late spring pushes it back. The window between September and late April is when egg masses are sitting dormant and most vulnerable to removal.
How to Remove Egg Masses
Physical removal is the simplest and most effective approach for a homeowner. Use a putty knife, stiff brush, or paint scraper to scrape egg masses into a plastic bag or container. Then cover them with soapy water (regular dish detergent is fine) and let them soak for a full day or two before throwing the bag away. This step matters: egg masses scraped onto the ground without soaking can still hatch in spring.
Look for egg masses on a dry day between September and late April, when they’re easiest to spot on bare trunks. Check the full length of the tree, from the base all the way up as high as you can see, along with the undersides of branches. Don’t forget to inspect nearby rocks, fallen branches, and any outdoor equipment or structures.
Dormant Oil Spray
If you have too many egg masses to scrape individually, horticultural oil offers an alternative. Products labeled for spongy moth, such as Golden Pest Spray Oil, work by suffocating the eggs so they never hatch. Apply the oil on a day when temperatures are above 40°F. A practical tip from University of Wisconsin Extension: add a few drops of food coloring to the oil so you can track which masses you’ve already treated.
Skin Irritation From Moth Hairs
The fine hairs covering egg masses are largely harmless, but spongy moth caterpillars are a different story. Their tiny bristles carry histamines that can cause an itchy, red rash on contact. Some people react more strongly than others. Wearing gloves when handling caterpillars or scraping egg masses is a simple precaution, especially if you have sensitive skin. The rash is not dangerous but can be persistently uncomfortable.
How Many Egg Masses Signal a Problem
A handful of egg masses on a single tree is worth removing but not necessarily alarming. The real concern is density. When new egg masses are numerous across multiple trees in your yard, the following spring is likely to bring heavy defoliation. New masses (hard, dark, intact) are the ones to count. Old masses (faded, powdery, flat) have already hatched and can be ignored.
Healthy deciduous trees can survive one or two years of heavy defoliation by pushing out a second flush of leaves in midsummer. But repeated years of severe feeding, especially when combined with drought, weaken trees significantly and can kill them. Evergreens like spruce and pine are at higher risk because they don’t regrow needles as readily. If you’re counting dozens of fresh egg masses across your property, treating them before spring, either by scraping or oiling, is the most effective way to protect your trees for the coming season.