Spongy moth

What is spongy moth?

Spongy moth, the insect formerly called gypsy moth, is a non-native, leaf-eating caterpillar. The caterpillars hatch in spring and eat leaves for about seven weeks. DCNR says it has caused major forest damage in Pennsylvania since the 1970s, with outbreaks in 2013 to 2019 and 2021 to 2024.

Most trees survive one year of leaf loss. Penn State says trees with 30 to 50 percent defoliation will likely recover but may be weaker. Losing more than half the leaves is a major stress, and repeated years raise the risk of death, especially with drought or other pests. Conifers usually don't survive being stripped completely.

Populations rise and crash. Two natural killers, a virus (NPV) and the fungus Entomophaga maimaiga, often wipe out outbreaks, especially after a cool, wet spring. For yard trees, removing egg masses and banding trunks help, and well-timed treatment can reduce leaf loss in heavy years.

People and pets: Caterpillar hairs can irritate skin and cause rashes or itching in some people (UMaine Extension). Penn State advises wearing gloves when handling bands or caterpillars.

Signs and damage

• Leaves chewed full of holes, then whole trees stripped bare in spring and early summer
• Hairy caterpillars resting low on trunks and branches on hot, sunny days
• Tan, fuzzy egg masses on trunks, stones, outdoor furniture and buildings
• Dead caterpillars hanging limp in an upside-down V (a sign of the NPV virus)

Spongy moth identification: what to look for

• Adult: Male: dark brown with wavy black bands, feathery antennae, about 1 1/2 inch wingspan, strong flier. Female: larger, mostly white with faint dark markings, 2 to 2 1/2 inch wingspan, and cannot fly.
• Larva: Young caterpillars are tiny, black and hairy. Grown caterpillars reach 1 1/2 to 2 1/2 inches, are hairy and dark, with yellow lines, 5 pairs of blue spots up front and 6 pairs of red spots behind.
• Eggs: Tan, fuzzy, felt-like masses covered with the female's hairs. Each holds hundreds of eggs (DCNR says about 100 to 1,500). Found on trunks, branches, stones and outdoor structures.
• Damage: Chewed and stripped leaves in spring and early summer. Repeated heavy defoliation can kill hardwoods; one complete stripping can kill conifers.
• Timing: Penn State gives egg hatch as early April to late May (DCNR says mid-April to early May). Caterpillars feed about seven weeks, pupate late June to early July, and moths emerge a few weeks later. Eggs are laid in June and July.

Life cycle

One generation a year. Eggs spend the winter in masses and hatch from early April to late May. Tiny caterpillars climb the tree or drift on silk threads to new trees. They feed for about seven weeks, molting five or six times, mostly at night or on cool days. They pupate in sheltered spots in late June or July. Moths come out 10 to 15 days later, mate, lay new egg masses in midsummer, and die.

When to look: Scout for egg masses from summer through winter; mid-October to April is the time to scrape them. Watch for small caterpillars in April and May; feeding peaks in spring and early summer. DCNR says treatments usually start in May when about half the caterpillars are in their second stage.

Spongy moth look-alikes

A few things get mixed up with it:

• Eastern tent caterpillar (Malacosoma americanum): Has a solid white stripe down the back and builds silk tents in branch forks in early spring. Spongy moth caterpillars have blue and red dots and no tent.
• Forest tent caterpillar (Malacosoma disstria): Has a row of keyhole-shaped pale spots down the back instead of blue and red dots.
• Fall webworm (Hyphantria cunea): Shows up later in summer, is lighter yellow, and wraps branch tips in grayish webs.
• Spotted lanternfly egg mass (Lycorma delicatula): Gray-brown and mud-like rather than tan and fuzzy.

How to control spongy moth

Cultural control

• Scrape egg masses from mid-October through April and soak them in soapy water for a few days before throwing out. Penn State says crushing alone doesn't work well.
• Wrap a band of burlap about 18 inches wide around the trunk at chest height. Check every few days and destroy the caterpillars hiding under it. Remove bands after July.
• Clear old wood piles, building materials and dead branches where egg masses hide.

Biological control

• Nucleopolyhedrosis virus (NPV) kills caterpillars in crowded, stressed populations; dead larvae hang limp in an upside-down V.
• The fungus Entomophaga maimaiga can kill caterpillars even at low numbers when spring weather is cool and wet.
• Parasites and predators also help keep numbers down.

Chemical control

Penn State names Bacillus thuringiensis subspecies kurstaki (Btk), a bacterial product that kills feeding caterpillars, and tebufenozide, which makes larvae molt early and stop feeding. Both work only on caterpillars feeding when applied, so timing in spring (usually May) is critical. Labeled horticultural oil can be put on egg masses to cut the next hatch. DCNR uses Btk and tebufenozide by air on state and federal forest land only when numbers pass set thresholds; it does not spray private land. Use only products labeled for this use; the label is the law. Spraying large trees is a job for an ISA Certified Arborist or licensed applicator. Muthler Landscaping does not spray or treat trees; see our pesticide applicator license guide.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Check outdoor furniture, grills, trailers, campers and equipment for egg masses before moving them, and follow the APHIS self-inspection checklist if you're leaving the quarantine area.
• Don't move firewood; see our firewood rules guide.
• Plan any treatment in fall and winter if you saw lots of egg masses.

When to call a pro

Call an ISA Certified Arborist or licensed applicator if large oaks or valuable yard trees are covered in egg masses before an expected outbreak year, or if a tree was badly stripped two years in a row. For a woodlot, DCNR says private owners and towns can run their own spray programs using its guide, and Penn State suggests a consulting forester before any salvage harvest.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

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