Spongy Moth (Lymantria dispar)
Why are there so many spongy moths around in 2023?
It's 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, and we are in the midst of one of those population booms now.
It is troubling to see trees, especially beautiful mature oaks lose leaves to these caterpillars. Keep in mind that that it is a pest most mature trees have had to endure before in their long lives.
Mature trees do rebound with new leaves to replace the ones lost to the caterpillars. This doesn't mean that the spongy moth isn't worrisome. Plus with the ongoing drought conditions, trees will have to use a lot of energy to produce new leaves, putting them under a lot of stress. And much like people who are ill, being under additional stress is not helpful.
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.
Contact a Certified Aborist for Your Private Trees
You can also contact a certified arborist for help with trees on your property.
One way to be sure your tree can endure the damage caused by spongy moth caterpillars is to reduce it's stress by keeping it well watered and postpone any pruning.
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.
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.
Golden Pest 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. The oil prevents the eggs from hatching.
If you elect this method, please use the oil as directed.
Scrape & Submerge
Use a putty knife or similar flat edged scraping tool, remove the egg mass and submerge it within a container of soapy water. (You can use household dish soap.) Keep the eggs submerged for two days before discarding them into the trash.
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 above facts do not rule out aerial spraying for the caterpillar in Madison - but performing aerial spraying is a very complicated operation. Also, aerial spraying does not completely eradicate the spongy moth. The caterpillar would still be present in areas after a successful application (many will be killed, but not all.) The process takes months and months of planning and neighborhood contacts. It is quite expensive. The spraying can only happen during a specific time in the spring during a certain time in the life cycle of the spongy moth. And, also, the weather must be favorable.
Spongy moth feeding leads to summer defoliation of our trees.
And the caterpillars can cause an itchy rash in some people.
have questions about the spongy moth caterpillar?
Yes. It was recently changed by the Entomological Society of America as part of their Better Common Names Project.
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.