According to the Wisconsin Department of Natural Resources, we are seeing a very high number of spongy moth caterpillars throughout the state. We are experiencing the same outbreak here in Madison. The population boom is due to a variety of environmental factors that have made conditions favorable for this species to thrive.

The spongy moth caterpillar is a concern because they can defoliate trees, putting them under more stress in what has been a dry and stressful summer for trees.

Also, these caterpillars can cause skin or respiratory irritation when touched. The insects have hairs, or setae, that trigger these reactions. According to UW Extension, these setae can lodge into clothing that has come into contact with the insect, so those articles should be washed separate from other clothes to prevent further irritation.

What can you do to help your trees now?
At this time of year, for trees on your private property, the DNR recommends installing burlap skirts around the trunk of the tree. The DNR has an excellent, and brief, video showing how to appropriately install one, including how to monitor, safely collect, and exterminate the caterpillars without exposing yourself to irritation.

You can also focus efforts on reducing other stressors for your trees to be sure they can endure any damage created by spongy moth caterpillars. For example, be sure your tree is sufficiently watered and delay pruning.

“Think of a tree dealing with spongy moth caterpillars like a person with a cold,” says City of Madison Forester Ian Brown. “When you are sick, you want to be sure you’re getting plenty of rest and eating well to give yourself the best opportunity to recover.”

You are also welcome to squish any spongy moth caterpillar you see under you shoe as each caterpillar removed is one less that can help continue the outbreak (but please be mindful not to touch them with your clothes or bare skin due to the potential irritation they can cause).

What can you do prevent future spongy moth outbreaks?
The best thing is to destroy egg masses of the caterpillars when found from mid-October through mid-April. They are a fuzzy, brownish patch typically found on tree trunks, but can also be found on sheds, under picnic benches, and other flat surfaces.

Each mass contains 600 to 1,000 eggs that can hatch into caterpillars come the spring. Focusing on the egg masses will have a much larger impact on the overall future population of this troublesome invasive species. And it is at this stage of the moth’s life cycle where Urban Forestry crews focus their efforts on treating egg masses when found.

UW-Extension has a thorough season-by-season breakdown of what homeowners caring for trees should look for to prevent spongy moths, and how to effectively eradicate them.

Additional Spongy Moth Resources
The Wisconsin Department of Natural Resources has spongy moth information portal that provides a wealth of information about tree management, and further links to the Department of Agriculture, Trade, and Consumer Protection’s aerial spraying programs for rural areas.

The University of Wisconsin Extension also has a thorough website dedicated to the spongy moth, with links to additional resources.

If you have additional questions about spongy moths, or other urban forestry concerns, please contact our office at 608-266-4816 between the hours of 7:00am and 3:00pm Monday to Friday. You can also email Urban Forestry at