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

Three spongy moth egg masses are found on a tree branch at Kettle Moraine State Forest Southern Unit in Walworth County. / Photo Credit: Bill McNee, Wisconsin DNR

Now that spongy moth egg laying is complete for 2023, it’s a good time to look for and dispose of the egg masses produced by adult moths over the past two months.

Spongy moth egg masses are tan-colored lumps about the size of a nickel or quarter, and can be found on trees, buildings and other outdoor objects. They may also be found in protected places such as firewood piles and birdhouses.

Newly produced egg masses will feel firm and appear darker in color than older egg masses, which appear faded, feel spongy and do not contain viable eggs. The current-year egg masses will not hatch until next spring.

Scraping a spongy moth egg mass into soapy water at Kettle Moraine State Forest Southern Unit. / Photo Credit: Bill McNee, Wisconsin DNR

Aerial surveys done in July recorded about 373,000 acres of spongy moth defoliation in Wisconsin. Most of this defoliation was observed in northern Wisconsin (primarily Ashland, Bayfield and Marinette counties), with about 70,000 acres of defoliation observed in the southern counties.

At present, it looks as if the outbreak will continue in many areas as well as spreading into new areas where no defoliation occurred. DNR staff received numerous reports of many adult moths in areas that were not defoliated this summer, and this is a sign that these areas are likely to have defoliation in 2024. Marinette County generated the most landowner reports of this nature. Egg mass densities were noticeably lower in southeast Wisconsin after two outbreak years but remain high enough to likely cause problems with nuisance caterpillars and defoliation in 2024.

It is important to examine high-value host trees (including oak, birch, crabapple, linden/basswood, aspen and willow) because egg mass numbers can predict an unacceptable level of defoliation. Then you can plan ahead for control, if desired. A map of quarantined counties shows where the pest is considered to be “generally established.”

Populations typically experience the fastest growth rate and reach very high numbers on:

• Dry sites with sandy soil and abundant oak.
• Mowed lawns with preferred tree species.
• Large oaks (bur, in particular) with rough bark, especially on or adjacent to mowed lawns.

When egg masses are found, treat or remove those safely within reach. Accessible masses can be sprayed with horticultural oil (available online or at many home and garden centers) or gently scraped into a container of soapy water to soak for a few days before being discarded in the trash. Do not use motor oil or other lubricants, as these can harm the tree and be a pollutant.

Look to treat or remove “fresh” egg masses produced during the current summer. More egg masses are usually seen if surveys are done once leaves have fallen from the trees. Contact an arborist or forester for management and control options if many masses are present.

Visit spongymoth.wi.gov for information on egg mass oiling and removal, physical controls and insecticide application to individual trees. An egg mass survey can be done to help property owners and communities predict the level of defoliation next summer, and will help to determine if active management is needed.

An aerial spray guide is also available for landowners and groups interested in protecting a larger area of high-value host trees.