Each egg mass laid by a spongy moth carries 300 or more eggs. Caterpillars will emerge in spring. Egg masses can be removed with care during fall and winter months to prevent the emergence of caterpillars. Photo Credit: / Wisconsin DNR

By Bill McNee, DNR Forest Health Specialist, Oshkosh;
Bill.McNee@wisconsin.gov or 920-360-0942

The Wisconsin Department of Natural Resources (DNR) is encouraging property owners to seek out and dispose of spongy moth egg masses, which were produced by adult moths during the summer.

Each spongy moth egg mass contains hundreds of eggs that will hatch into hungry, leaf-eating caterpillars in the coming spring. Large numbers of these invasive caterpillars can be a tremendous nuisance that may cause tree mortality.

Treating or removing egg masses in the fall will help to reduce the pest population and can help to reduce the intensity of tree defoliation next summer. Property owners have two options:

• Gently scrape egg masses into a container of soapy water to soak for a day or two before throwing them in the trash. DNR has produced a short instructional video available online.
• Spray the masses that are safely within reach with horticultural, mineral or neem oil that can be applied to trees. These oils are available online and at many local garden centers, hardware stores and big-box stores. Do not use motor oil or other lubricants, as these can harm the tree and be a pollutant.

Carefully scraping spongy moth egg masses off trees and into a cup of soapy water will help to hold down caterpillar damage in 2025. / Photo Credit: Wisconsin DNR

Checking for egg masses also helps property owners predict the level of nuisance caterpillars and tree defoliation that is likely in 2025. Those estimates can, in turn, help landowners and managers decide if action should be taken in the spring.

An overall spongy moth population decline was seen in Wisconsin this summer due to two caterpillar diseases, with some sites reporting a collapse in the pest’s population. However, egg mass numbers remain high enough to cause nuisance problems and heavy tree defoliation in some locations. Old masses with no viable eggs will appear faded and will feel spongy when touched.

Populations usually reach the highest levels on:

• Preferred tree species (oak, crabapple, birch, etc.) growing in mowed lawns.
• Large oaks with rough bark, especially in or adjacent to mowed lawns.
• Dry sites with sandy soil and abundant oak.

Visit the Wisconsin Spongy Moth Resource Center for information about egg mass oiling and removal, physical controls, population survey methods and insecticide applications to high-value host trees. An arborist or forester may also be a good resource for anyone dealing with spongy moths.