Spongy Moth (Lymantria dispar)
VIDEO: Wisconsin DNR How to Remove & Destroy Egg Masses from Your Trees (Length: 20 seconds)
It was really a combination of environmental factors that created opportune conditions for the caterpillar/moth coupled with the fact that every 10 to 15 years, there is a boom in the spongy moth population.
It was troubling to see trees, especially beautiful mature oaks lose leaves to these caterpillars. And the caterpillars can cause an itchy rash in some people - so it is best not to touch them.
Keep in mind that that the spongy moth, and even population booms like what we saw in 2023, is something most mature trees have endured before in their long lives.
The spongy moth has been in America since 1869 (that's not a typo - it has really been over 100 years), and it has been in Wisconsin since at least the 1970s.
Mature trees will rebound with new leaves to replace the ones lost to the caterpillars. But that defoliation is a stressor to the trees as it requires a lot of energy to produce more leaves.
If you stack that stressor of needing to flush new leaves on top of drought conditions and other stressors like problems from climate change or other pests - it becomes more worrisome.
Imagine it like a person being sick. For a person to fully recover, it's best for them to be well rested and have plenty to drink - "push the fluids" as the doctors always say.
Same is true with trees. Best to avoid stress, and keep them watered.
If you have concerns about spongy moths on trees on your private property, residents should use these resources.
The University of Wisconsin Extension has season-by-season guide for homeowners of things you can do to protect your trees.
More information about the moth covering everything from its life cycle to the species history can also be found on the UW-Extension website.
State of Wisconsin
The State of Wisconsin Department of Natural Resources has a a thorough spongy moth portal that has links to the same UW-Extension site above for yard tree management and also other information about woodlot management and more.
Dane County Office of Energy & Climate Change
Dane County has a webpage of resources and photos for residents.
Contact a Certified Aborist for Your Private Trees
You can also contact a certified arborist for help with trees on your property.
Help your trees endure the damage caused by spongy moth caterpillars is to reduce it's stress by keeping it well watered and postpone any pruning.
Deploy Mulch the Right Way
There are right and wrong ways to apply mulch around your tree. Doing it wrong could actually be harmful.
The basic benefits of mulch is that it helps your trees retain moisture, moderates soil temperatures, and prevents soil compaction.
The basic rules are you want it about 2 to 4 inches deep, and you do not want to mound it up near the tree. In other words, do not create "mulch volcanoes." Piling up excess near the base of the tree is bad.
More detailed resources about how to mulch the right way can be found here:
To help control the moth population, residents can destroy the egg masses.
The best time to do this is from October to April.
Masses look like fuzzy brownish patches that can be found on tree trunks, under picnic tables, along sheds, RVs, and many other flat surfaces. Here are some sample photos to help you know what they look like. Here is a zoomed in photo from the WI DNR. Here is a photo from the Dane County spongy moth website showing the egg masses more true to size found under a picnic table.
Egg masses can hold up to 1,000 eggs, so destroying them on your property can help control this pest.
Here are the two methods that are most effective at destroying the egg mass.
You cannot simply scrape them and try to squish them. They are too strong to be ground by the heel of your shoe, and if left on the ground they will still hatch.
Option 1: Scrape - Soak - Trash
Use a putty knife or similar flat edged scraping tool, remove the egg mass and submerge it within a container of soapy water. Use household dish soap.
Keep the eggs submerged for two days. Then toss the soggy masses into the trash.
Option 2: Golden Pest Oil / Horticultural Oil
Apply a product called golden pest oil to the egg masses. This is a natural soybean oil that can be found at garden centers, though it may be hard to find in amounts appropriate for a homeowner and it can be quite expensive. But the oil, when correctly will prevent the egg masses from hatching.
If you elect this method, you must use the oil as directed. The oil directions will tell you to mix it with water before applying to the eggs. You need to do this step so it will soak into the egg mass.
In past years, the Wisconsin Department of Natural Resources (DNR) had offered communities the option to pay for a state-arranged aerial spray program for both public and private property for this pest as part of a suppression program. This suppression program did not eliminate the pest all together but it reduced the population and the effect it has on trees. The DNR has ended this program in places where the moth is established (like in Madison).
A different state agency, the Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP) has a separate spraying program, but it focuses solely on counties near the Iowa and Minnesota border to help slow the spread of the moth into those states.
The City of Madison is participating in a genetic study of spongy moths conducted by the James Lab at the University of Toronto.
A single spongy moth pheromone trap will be installed on an existing tree within the known infestation area on the west side of Madison. This trap will not cause damage to the tree.
This type of trap is not used for mass removal, but is used to collect species as part of scientific research and monitoring.
Engineering and Forestry are in the process of determining spongy moth impacts in this area and next steps.
have questions about the spongy moth caterpillar?
Yes. It was changed by the Entomological Society of America as part of their Better Common Names Project.