By Art Kabelowsky, DNR Outreach and Communications, Fitchburg;
Arthur.Kabelowsky@wisconsin.gov or 608-335-0167

An airplane sprays insecticide on trees at Devil’s Lake State Park in Sauk County. / Photo Credit: Wisconsin DNR

The Wisconsin Department of Natural Resources (DNR) has finalized aerial spraying plans for its 2024 Spongy Moth Suppression Program, with maps for the six selected treatment areas available online.

The treatment sites contain high-value trees at six state parks or forests in Columbia, Marinette, Sauk and Walworth counties. A total of 928 acres will be treated with an aerial spray of “Foray” bacterial insecticide containing Bacillus thuringiensis var. kurstaki.

Areas to be sprayed:

• Columbia County: 101 acres at MacKenzie Center.
• Marinette County: 255 acres at Governor Earl Peshtigo River State Forest.
• Sauk County: 212 acres at Devil’s Lake State Park; 187 acres at Mirror Lake State Park.
• Walworth County: 124 acres at Big Foot Beach State Park and 49 acres at Kettle Moraine State Forest – Southern Unit.

The aerial spraying will occur between early May and early June, with exact dates and times depending on caterpillar development and weather conditions. The DNR intends to avoid spraying high-use properties during the extended Memorial Day weekend if spraying has not occurred.

For up-to-date information, visit the Wisconsin Spongy Moth Resource Center and subscribe to email updates that will provide notice when spraying dates are determined. Also, the toll-free Spongy Moth Hotline, 1-800-642-MOTH (1-800-642-6684), has a recording of up-to-date spraying plans. Law enforcement, local governments, schools and nearby medical facilities will be notified before spraying.

The DNR aerial spray program complements the Wisconsin Department of Agriculture, Trade and Consumer Protection’s (DATCP) participation in the national “Slow the Spread” program. DATCP will use an airplane to treat sites in nine western counties. The program was created to slow the westward spread of spongy moth by treating newly established, low-level populations in areas not infested by this pest.

DATCP’s planned 2024 spray blocks also have been selected, and the DATCP spray block book maps are also available online.

“Slow the Spread” treatments will use either the same insecticide as the DNR treatments or a pheromone mating disruptor that prevents moth reproduction by interfering with the ability of male moths to find female moths. Learn more in DATCP’s news release.

The State of Wisconsin does not offer a state-sponsored or cost-share program for treating private property to prevent defoliation by spongy moth. Those interested in organizing a spray program at their own expense can view a DNR guide titled “Organizing An Aerial Spray For Forest Pests.”  Also, tree care businesses may be hired to apply insecticide from the ground. Management options can be viewed online through the Wisconsin Spongy Moth Resource Center.

Spongy moth caterpillars feed on many tree and shrub species, stripping trees of their leaves and potentially killing them. Spongy moth outbreaks occur every five to 10 years. In 2023, Wisconsin saw a record amount of spongy moth defoliation, with about 375,000 acres affected (mostly in northern Wisconsin).

Generally, healthy hardwoods can endure one or two years of heavy defoliation before tree mortality occurs. Healthy trees typically put out a replacement set of leaves a few weeks after defoliation. However, this uses energy reserves and makes the tree more susceptible to disease, drought stress and other insect attacks, which, in combination, can kill the trees.