Gypsy Moth (Spongy Moth) Outbreaks in Michigan and Wisconsin

Spongy Moth: A Recurring Threat in Michigan and Wisconsin

The spongy moth (formerly known as the gypsy moth, Lymantria dispar) remains one of the most destructive defoliating insects in the Midwest. Michigan and Wisconsin have experienced cyclical outbreaks since the moth became established in the region during the 1980s and 1990s. When populations surge, spongy moth caterpillars strip the leaves from oaks, aspens, birches, and hundreds of other tree species across vast swaths of the landscape.

The most recent population upswing — driven by several consecutive dry springs that suppressed the caterpillar-killing fungus Entomophaga maimaiga — is once again threatening forests and residential landscapes across both states, with expanding activity also reported in Minnesota, Illinois, Indiana, and parts of Iowa and Ohio.

Identifying Spongy Moth at Every Life Stage

Successful management starts with identification:

• Egg masses (August through April): Tan or buff-colored fuzzy patches about the size of a quarter to a half-dollar, found on tree trunks, branches, rocks, outdoor furniture, firewood, vehicles, and building walls. Each mass contains 500 to 1,000 eggs.
• Caterpillars (May through July): Young caterpillars are black and hairy. Older caterpillars are up to 2.5 inches long with five pairs of blue spots and six pairs of red spots along the back. They are voracious feeders, consuming entire canopies in outbreak years.
• Pupae (June through July): Dark brown, teardrop-shaped casings found in sheltered locations on bark, in crevices, and under objects.
• Adult moths (July through August): Males are brown and fly in a zigzag pattern. Females are white with dark markings and do not fly — they remain near their pupal case and lay egg masses nearby.

Trees Most at Risk

In Michigan and Wisconsin, spongy moth caterpillars preferentially feed on:

• Oaks (all species) — the preferred host across the Midwest
• Aspen and birch — heavily impacted in northern Michigan and Wisconsin
• Basswood (linden) — a common urban and forest tree in both states
• Willow, apple, and crabapple
• Tamarack — one of the few conifers they readily consume

Most healthy deciduous trees can survive one or two consecutive years of complete defoliation by re-leafing later in the summer. However, three or more consecutive years of heavy defoliation — or defoliation combined with drought — often kills trees. Conifers are far less resilient and may die after a single complete defoliation.

Control Methods for Homeowners

Egg Mass Removal (Fall through Early Spring)

The simplest and most effective homeowner action is scraping and destroying egg masses before they hatch:

• Use a putty knife or paint scraper to remove masses from surfaces
• Drop scraped material into a bucket of soapy water and let it soak for 48 hours
• Check tree trunks, firewood stacks, outdoor equipment, trailers, and play structures
• A single destroyed egg mass eliminates up to 1,000 potential caterpillars

Burlap Banding (May through July)

Caterpillars descend from the canopy during the day to seek shelter:

• Wrap a 12-to-18-inch strip of burlap around the trunk at chest height
• Fold the top half of the burlap down to create a flap
• Check under the flap daily in the afternoon, collecting and destroying caterpillars in soapy water
• This method is effective for individual yard trees but labor-intensive for large properties

Btk Spraying (May through Early June)

Bacillus thuringiensis var. kurstaki (Btk) is a naturally occurring bacterium that kills caterpillars when they ingest treated foliage:

• Btk must be applied when caterpillars are young (under 1 inch long), typically in mid- to late May in Michigan and Wisconsin
• It is specific to caterpillars and does not harm beneficial insects, birds, pets, or humans
• Available as a homeowner spray for small trees and as professional aerial or ground applications for larger areas
• State and county governments in Michigan and Wisconsin conduct aerial Btk spray programs in heavily infested areas — contact your county extension office to find out if your area qualifies

When to Call a Professional

Contact an ISA Certified Arborist when:

• Trees have been heavily defoliated for two or more consecutive years
• High-value trees show signs of crown dieback or decline following defoliation — use our diagnose tool to assess symptoms
• You have too many trees to manage with DIY burlap banding and egg scraping
• You want trunk injection treatments to protect specific high-value oaks or other trees

Frequently Asked Questions

Will spongy moth kill my trees?

Most healthy deciduous trees can survive one or two years of heavy defoliation. However, three or more consecutive years of severe defoliation, especially combined with drought stress, can kill even large, established oaks and other hardwoods. Conifers like spruce and pine are more vulnerable and may die after a single year of complete defoliation. Trees that were already stressed by drought, soil compaction, or other factors are at higher risk.

How do I get rid of spongy moth egg masses?

Scrape egg masses off surfaces using a putty knife or paint scraper and drop them into a container of soapy water for at least 48 hours. Do not simply scrape them onto the ground, as the eggs can still hatch. Check tree trunks, outdoor furniture, firewood, vehicles, trailers, and building walls from late summer through early spring. Destroying egg masses is the single most effective action homeowners can take.

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Protect your trees during spongy moth outbreaks. Find a certified arborist through the TreeIssues directory for professional assessment and treatment options.