Spongy Moth Update

The winter of 2023-2024 was Wisconsin’s warmest on record, worrying many whose properties had endured recent Spongy Moth outbreaks, as a warmer winter can improve pest survivability, leading to another year of heavy defoliation. However, the wet spring that followed increased the presence of two natural controls for Spongy Moth: Nucleopolyhedrosis Virus and the Entomophaga Maimaiga (EM) fungus. As a result, while homeowners contended with Spongy Moth outbreaks early in 2024, Spongy Moth populations experienced their own “outbreak” of these two infections.

NPV is naturally found in Spongy Moth populations and spreads more easily when caterpillar numbers are high. When caterpillars are infected with NPV, they usually die and are left hanging in an upside-down “V” shape. NPV infection also results in smaller egg masses: while they typically range in size from a nickel to a quarter, smaller masses closer to a dime in size indicate NPV. If you note smaller egg masses like this, it may indicate a decreasing Spongy Moth population to come.

EM fungus was introduced to Wisconsin in the late 1990s specifically to control Spongy Moths. The spores of this fungus live in the leaf litter where caterpillars feed. Spores attach to the caterpillars, eventually invading their bodies and killing them within about a week. Caterpillars that die from the fungus typically hang straight down in a vertical position.

While Spongy Moth numbers dropped in 2024, we’re not completely out of the woods. Some areas in Southeast Wisconsin may still have pockets of high pest populations. Additionally, the weather this coming Spring will play a major role in determining what happens next. A mild winter or dry spring could lead to another surge in caterpillar populations. To protect your trees, we recommend continuing treatments for those that were previously affected until Spongy Moth numbers remain low for several consecutive years.

Trees that have endured multiple years of Spongy Moth damage are likely stressed and should be monitored for signs of secondary pests or diseases. Exiting an outbreak, trees will still need continued care to recover and rebuild energy reserves. Regular watering—even for mature trees—and with proper mulching are key strategies to support your trees’ health. Keep a close watch; if something doesn’t seem right, contact your ISA Certified arborist at Wachtel Tree Science for advice.

Let’s hope for a typical Wisconsin winter and a wet spring to keep these pests in check!

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