Wisconsin Could be Seeing One of the Worst Spongy Moth Caterpillar Outbreaks in Over a Decade

Sunday, July 23rd, 2023 -- 9:00 AM

(By Danielle Kaeding, Wisconsin Public Radio) Entomologists and the state Department of Natural Resources say Wisconsin could be seeing one of the worst spongy moth caterpillar outbreaks in more than a decade.

According to Danielle Kaeding with Wisconsin Public Radio, Spongy moths, formerly known as gypsy moths, are invasive insects from Europe that were introduced in the late 1800s. They are destructive as caterpillars and are voracious eaters in that stage that can completely strip leaves from entire forests.

The caterpillars feed on up to 300 species of trees and shrubs during spring and early summer. They prefer trees that are common in mixed hardwood forests, including oak and aspen. And a single spongy moth egg mass can contain up to 1,000 eggs.

Patrick Engelken, an entomologist with the U.S. Forest Service, said this is likely the worst outbreak for some areas of Wisconsin in a decade. Large areas of the Chequamegon-Nicolet National Forest have been affected by spongy moth caterpillars, especially in the Washburn and Lakewood-Laona ranger districts. In the worst-case scenario, unhealthy trees can die from stress that’s more likely to occur in drought-prone areas. 

The DNR’s Bill McNee, a forest health specialist, said in an email that the state has seen at least 200,000 acres of spongy moth defoliation this year. He said that’s affected mostly aspen in northern Wisconsin, but the southern part of the state has also seen about 50,000 acres of oak that have been stripped of their leaves.

Spongy moth populations may spike temporarily about every 10 years. Outbreaks have been trending upward in the last couple of years, according to PJ Liesch, an entomologist with the Division of Extension at UW-Madison.

The DNR recorded 294 acres of spongy moth defoliation in 2021, but around 85,000 acres of trees experienced a loss of leaves last year. Liesch said weather is a key factor in the severity of outbreaks because spongy moths thrive with hot, dry conditions.

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