Large egg masses are seen on a tree at the Kettle Moraine State Forest-Southern Unit in 2021. / Photo Credit: Bill McNee, Wisconsin DNR

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

Part Two of a report on spongy moth in Wisconsin in 2024 and 2025. Part One was published in December.

The Wisconsin Department of Natural Resources (DNR) is encouraging property owners to examine their property for spongy moth egg masses and plan for action this spring if needed. Each spongy moth egg mass contains hundreds of eggs that will hatch into hungry, leaf-eating caterpillars this spring. Large numbers of these invasive caterpillars can be a tremendous nuisance that may cause tree mortality.

Spongy moth egg masses are tan-colored lumps about the size of a nickel or quarter and are found on trees, buildings and other outdoor objects. They may also be found in protected places such as firewood piles and birdhouses. Egg masses produced in 2024 will feel firm and appear darker in color than older egg masses, which appear faded, feel spongy and do not contain viable eggs.

A pair of egg masses on a tree at the Muir trailhead of the Kettle Moraine State Forest-Southern Unit are much smaller than normal, indicating that the moths were affected by viral or fungal diseases that were common in the summer of 2024. / Photo Credit: Bill McNee, Wisconsin DNR

Predict Impacts With Egg Mass Surveys

An egg mass survey can help property owners and communities predict the level of defoliation in 2025 and determine if active management is needed. A “Protect Your Trees From Spongy Moth” video is available online to assist property owners in conducting predictive egg mass surveys.

Wisconsin experienced a wetter-than-average spring in 2024, and this was favorable for diseases that killed large numbers of spongy moth caterpillars. Sites where heavy caterpillar mortality was seen last summer are unlikely to have high numbers of egg masses unless the population was very high. If many of the egg masses are dime-sized or smaller at a site, it is likely that the outbreak will end in 2025.

Treat Or Remove Egg Masses

When current-year egg masses are found, treat or remove those safely within reach. Consider contacting an arborist or forester about management options if many masses are present and cannot be removed.

Treating or removing egg masses will help to reduce the intensity of tree defoliation and nuisance caterpillars 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. The DNR has produced a short instructional video.
• Spray the masses that are safely within reach with horticultural, mineral or neem oil that is labeled for application to trees. Some products are specifically formulated for use against spongy moth egg masses. These oils are available online and at many local garden centers, hardware stores and big-box stores. They are usually applied when temperatures are well above freezing. Do not use motor oil or other lubricants, as they can harm the tree and become a pollutant.

A spongy moth egg mass is scraped into a cup of soapy water at Kettle Moraine State Forest-Southern Unit in 2022. Scraping egg masses into soapy water and discarding them in two days will help to protect trees and forests. / Photo Credit: Darrek Sams, Wisconsin DNR

Other Treatment Options

Planning for an aerial spray of larger forested areas in the spring of 2025 should begin as soon as practical if not already started. An aerial spray guide is available to advise organizers. In recent years, for-hire applicators have used airplanes, helicopters and drones to apply insecticide from the air.

Please note: The state of Wisconsin does not offer a state-sponsored or cost-share program for treating private property to prevent defoliation by spongy moths.

Visit spongymoth.wi.gov for additional information about suitable options this spring, including physical controls and insecticide applications that can be applied by a property owner or tree-care business.

Soil and trunk injections of insecticide are an option for protecting small numbers of larger trees and are done in the spring. Property owners interested in these injections should contact a tree care business well in advance of the treatment. Directories of for-hire arborists can be found on the Wisconsin Arborist Association and International Society of Arboriculture websites.

Workers inject insecticide into oak trees at Big Foot Beach State Park in Lake Geneva to control spongy moth caterpillars. / Photo Credit: Wisconsin DNR