Spongy moth caterpillars killed by nucleopolyhedrosis virus (NPV) hang in an inverted “V” orientation; caterpillars killed by the fungus Entomophaga maimaiga hang vertically. / Photo Credit: Wisconsin DNR
By Michael Hillstrom, Wisconsin DNR Forest Health Specialist
Michael.Hillstrom@wisconsin.gov; 608-513-7690
Spongy moth caterpillars began hatching at the end of April in 2025 and are now actively feeding on leaves. In most Wisconsin locations, though, the population will not be as high as it has been in the last few years.
Wisconsin has had the largest outbreak in state history over the past few years. Fortunately, the outbreak collapsed in many areas of the state in 2024. The wet spring weather in April and May of 2024 allowed a fungus and a virus that infects spongy moth caterpillars to cause heavy mortality. Although some hot spots still exist in south central and northeast Wisconsin, we expect those populations to decline in 2025.
Most landowners who have had spongy moth defoliation in recent years will likely see greatly reduced populations and minimal to no defoliation in 2025. Spongy moth populations are cyclical, with outbreaks occurring approximately every 5 to 10 years, so another population spike is not expected in the near future.
Landowners are encouraged to monitor trees that were heavily defoliated during the outbreak over the next few summers for signs of decline or mortality. Heavily defoliated trees are susceptible to insects and diseases for a few years while they recover from the defoliation stress. Consult a forester or arborist through the Wisconsin Arborist Association or Trees Are Good online directories.
Three state properties were aerially sprayed in May for spongy moth to keep high-value trees in state-owned campgrounds and recreational use areas in good health. The State of Wisconsin does not offer a state-sponsored or cost-share program for citizens to conduct aerial spraying on private land, but private contractors are available for landowners and groups interested in spraying in the spring of 2026.
For more information about spongy moth management options, visit the Spongy Moth Resource Center.