MADISON, Wis. – The Wisconsin Department of Natural Resources (DNR) today announced that the department will be spraying areas of Devil’s Lake State Park and the Kettle Moraine State Forest Southern Unit by plane to combat spongy moth* (formerly known as gypsy moth) populations between mid-May and early June.

Spongy moth caterpillars strip trees of their leaves. Their numbers can grow to the point that they eat all the leaves off of trees, which can kill trees in some cases. Spongy moth outbreaks occur every five to 10 years, and this year’s high populations threaten tree defoliation in high-use areas of some state properties. The DNR is taking action at state parks to prevent the stress of and potential loss of valuable trees.

Since numbers of the pest are large enough to cause extensive damage this summer if no action is taken, pilots will spray areas of Devil’s Lake State Park and the Kettle Moraine State Forest Southern Unit. Planes will fly very low, about 50 to 100 feet above the treetops. Spraying may begin at dawn and continue into the early afternoon if weather conditions allow.

Staff will use an insecticide called “Foray.” It is approved by the Organic Materials Review Institute for use in organic food farming and contains Bacillus thuringiensis var. kurstaki (Btk), a naturally occurring soil bacteria that kills spongy moth caterpillars when they ingest it. Btk is not toxic to people, bees, pets or other animals. However, some people with severe allergies may want to stay indoors during nearby applications.

This treatment will minimally impact non-target species. Btk only affects caterpillars that feed on sprayed leaves within seven to 10 days after application, and the timing of this spray is specific to spongy moths. Ultimately, the ecological advantages of saving these trees from defoliation are higher than the cost of not treating them.

Treatment dates and times are weather-dependent. Find out when the DNR plans for treatments to take place:

  • Call the toll-free Spongy Moth Information Line at 1-800-642-6684 (press Option 1) in May and June to hear where spraying is scheduled to occur the following day.
  • Sign up for daily email notifications of spray activities. Subscribe or unsubscribe online at

In addition to spraying, ground treatments have been vital in keeping spongy moth populations to a tolerable level until aerial sprays can step in to reduce numbers significantly. These tools have been critical in saving historic oaks in open, high-use areas of parks. Ground treatments and biocontrol will continue to be helpful management options moving forward.

For more information on spongy moth programs and treatment options, visit

*A new common name for Lymantria dispar, spongy moth, replaced the prior name of this insect, gypsy moth, in 2022. This change was necessary because the word “gypsy” is an ethnic slur and the former common name equated people with insects. For more information, visit the Entomological Society of America website. The DNR is making name changes in all spongy moth material, including the spongy moth portal

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