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: Bill McNee, Wisconsin DNR

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

The Wisconsin Department of Natural Resources (DNR) encourages property owners to examine their trees for spongy moth egg masses and determine whether control measures are needed this spring.

Property owners who have oak, birch, crabapple, aspen, willow and linden (basswood) trees should be particularly watchful because the caterpillars of this invasive insect prefer these species. However, they will feed on many other kinds of trees and shrubs.

The pest is considered generally established across most of Wisconsin, except the western third of the state. High spongy moth populations are most likely to be seen within parts of the quarantined area but may be present in non-quarantined counties.

Although the spongy moth outbreak of the last few years has declined or collapsed in most locations, pockets of high populations and tree defoliation may still exist this coming summer.

Looking at egg mass numbers on a specific property is the best way to determine if there is a potential problem. These masses are tan-colored lumps about the size of a nickel or quarter. Egg masses are found on trees, buildings and other outdoor objects and may also be found inside 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. The presence of many egg masses that are dime-sized or smaller is a sign that the spongy moth outbreak is past its peak intensity and likely to end at that site this summer.

Many egg masses at the base of a tree on the Kettle Moraine State Forest – Southern Unit in August 2021. Each egg mass can contain 300 or more eggs. Those caterpillars will emerge in early spring. / Photo Credit: Bill McNee, Wisconsin DNR

Egg mass hatch usually begins in mid-to-late April in southern Wisconsin. Before hatch begins, horticultural oil (available from many retailers and garden centers) can be applied when temperatures are above freezing to prevent the masses from hatching. You can also use a knife, chisel or similar tool to scrape unhatched egg masses into a can of soapy water; leave the egg masses in the water for a couple of days before disposing of them in the trash.

When low or moderate numbers of egg masses are present, physical controls can often reduce pest impacts and make insecticides unnecessary.

Insecticide treatments, including foliage sprays and soil and trunk injections, may be a suitable control option after the caterpillars have hatched. A professional treatment is likely needed when there are high numbers of egg masses on large trees. Insecticide treatments are most effective when the caterpillars are small (mid-to-late May in southern counties and late May or early June in northern counties).

Planning for insecticide treatments early can ensure trees will be able to receive treatment ahead of a last-minute rush when pesticide application businesses may already be very busy. Resources for finding a qualified pesticide applicator include the Wisconsin Arborist Association, the International Society of Arboriculture and online directories. Property owners are encouraged to monitor their trees, particularly oaks, during the summer. Host trees that were heavily defoliated or drought-stressed over the past few years are at a higher risk of dying, especially if they are not protected with insecticide and lose a high percentage of their leaves again this year. Low-vigor oak trees are commonly attacked by Armillaria root disease fungi and the native beetle, twolined chestnut borer, contributing to tree decline and death. Consult an arborist or forester for additional management recommendations.

Additional Recommendations

Property owners are encouraged to examine their trees and take action. Specifically:

• Visit the Spongy Moth Resource Center for management information. Options include egg mass oiling and removal, physical controls and insecticide application.
• Forest management may need to be postponed in areas that were defoliated or will be heavily defoliated this coming summer. Trees will be stressed from heavy defoliation and will often be unable to handle the additional stress from forest thinning.
• Monitor for crown dieback and twolined chestnut borer infestation in trees that were stressed by drought and defoliation over the past few years.
• Consult an arborist or forester for additional management recommendations. When seeking an arborist, check both of these directories: Wisconsin Arborist Association and International Society of Arboriculture. A forester directory is available here.