Right now, the spongy moth may seem like yesterday’s problem in much of Wisconsin. After several years of severe defoliation, populations have crashed, bringing the state’s latest major outbreak to an end.
But quiet years are part of the spongy moth story.
Spongy moth (Lymantria dispar) populations rise and fall in cycles. They can remain low enough to escape notice for years before surging into outbreaks that strip oaks and other hardwoods of their leaves across thousands of acres.
Wisconsin’s latest outbreak shows just how dramatic that swing can be. It peaked in 2023, when spongy moth caterpillars defoliated roughly 373,000 acres statewide. Although the caterpillar population has since collapsed, some trees weakened by repeated defoliation are still declining.
For Wisconsin homeowners, that makes the quieter years between outbreaks important, too. Understanding the cycle can help you recognize trouble, support stressed trees, and be better prepared when spongy moth populations eventually begin to build again.
Wisconsin’s Boom-and-Bust Spongy Moth Cycle
Spongy moth outbreaks can feel sudden, but they’re part of a much longer population cycle. In Wisconsin, outbreaks may recur about every 10 to 12 years, with the periods of heavy activity themselves typically lasting two to three years.
During the quieter years, spongy moths don’t necessarily disappear. Populations can remain low enough that homeowners barely notice them before conditions allow their numbers to climb again.
Native to parts of Europe, Asia, and North Africa, the spongy moth was introduced to North America in Massachusetts in 1869 and gradually spread westward.
Wisconsin first detected spongy moth life stages in the late 1960s, and early isolated populations were successfully eradicated.
By the late 1980s, however, reproducing populations had become established in parts of eastern Wisconsin. The state recorded its first noticeable tree defoliation in Oconto County in 1997.
Since then, periodic outbreaks have become part of managing this invasive pest. Today, spongy moth is well established across much of eastern and central Wisconsin, while state agencies continue working to slow its spread farther west.
What Makes Spongy Moth Populations Boom and Bust?
Spongy moth populations don’t follow a calendar. Weather, food availability, disease, and natural enemies all help determine whether their numbers stay low, climb rapidly, or collapse.
Wisconsin’s recent outbreak shows how those forces can work together.
Dry spring weather from 2020 through 2022 limited the effectiveness of Entomophaga maimaiga, a fungus that infects and kills spongy moth caterpillars. With less disease pressure, more caterpillars survived, helping populations surge.
But the conditions eventually shifted.
Wisconsin’s wetter spring in 2024 favored the fungus, while crowded caterpillar populations allowed a naturally occurring virus called nucleopolyhedrosis virus (NPV) to spread more effectively. Together, those diseases helped drive the population crash that ended the outbreak.
That doesn’t mean every outbreak follows exactly the same script. But it does explain why spongy moth can seem overwhelming one year and surprisingly scarce not long afterward. The pest is still present; the conditions controlling its population have just changed.
What Repeated Defoliation Does to Wisconsin Trees
A bare canopy can look catastrophic, but one round of spongy moth feeding doesn’t necessarily mean a tree will die. Healthy hardwoods can often survive even severe defoliation and produce a second set of leaves within weeks.
That recovery comes at a cost. Growing replacement leaves requires a tree to draw on stored energy reserves. If heavy defoliation happens again the following year, or the tree is already dealing with drought, disease, or stress, those reserves can become increasingly depleted.
Oaks are especially important to watch. Spongy moth caterpillars strongly prefer them, and weakened oaks can become more vulnerable to secondary pests such as the twolined chestnut borer (Agrilus bilineatus), a native beetle that tends to attack stressed trees.
The effects can continue after the caterpillars are gone. Wisconsin DNR reported moderate-to-severe oak mortality in 2025 in many areas hit hard during the recent outbreak. For homeowners, a full canopy after defoliation is encouraging, but it isn’t always proof that a tree has completely recovered.
| A Green Canopy Doesn’t Always Mean Full Recovery
An oak may produce new leaves after spongy moth defoliation, but that second flush draws on stored energy. Repeated defoliation, drought, disease, or secondary pests can compound the stress, and decline may become apparent later. |
Know What to Look for Throughout the Year
Spongy moth has one generation per year in Wisconsin, but the signs and symptoms you’ll see change dramatically with the season. Knowing which life stage is active can help you spot a growing population and understand when tree damage is most likely to occur.
| Time of Year | Life Stage | What Wisconsin Homeowners May Notice |
| August–April | Egg masses | Beige, fuzzy-looking masses containing hundreds of eggs on tree trunks, outdoor furniture, firewood, vehicles, and other sheltered surfaces |
| April–May | Young caterpillars | Tiny caterpillars hatch as leaves emerge and begin feeding; some disperse to new trees on silken threads |
| May–July | Growing caterpillars | Caterpillars become larger and easier to recognize by the pairs of blue and red bumps along their backs; most defoliation occurs during the later larval stages |
| June–August | Pupae | Dark brown pupae may be tucked into bark crevices and other protected locations |
| July–October | Adult moths | Adults emerge to mate; females lay new egg masses that remain in place through winter |
Exact timing varies with location and weather. In southern Wisconsin, egg hatch can begin as early as mid-to-late April, while activity may start later farther north.
Protecting Trees Before & During an Outbreak
The best time to think about spongy moth management is before hungry caterpillars have stripped a tree’s canopy.
Watching egg masses and caterpillar activity can give homeowners an early indication that populations are building and help determine whether valuable trees may need protection.
Management depends on the size and number of trees, the severity of the infestation, and the insect’s life stage. Egg masses that are within reach can be removed during the dormant season, while some treatments are most effective after caterpillars hatch but while they are still small.
Tree health matters, too. Oaks and other susceptible trees are better equipped to recover from defoliation when they aren’t already struggling with drought or other stress. Supporting overall tree health can be especially important as affected trees replace lost foliage.
Large or high-value trees require a different approach. A professional arborist can assess current spongy moth pressure, evaluate the tree’s overall condition, and determine whether professional pest and disease control is warranted and properly timed.
That becomes especially important when repeated defoliation could put an already stressed tree at greater risk.
| Don’t Wait Until the Canopy Is Bare
Spongy moth treatments are generally most effective while caterpillars are still small and before most feeding damage occurs. In southern Wisconsin, that window often falls in mid-to-late May. Farther north, it may extend into late May or early June. |
What to Watch During Wisconsin’s Quiet Years
Low spongy moth numbers are good news, but they don’t mean monitoring isn’t necessary. Even during a statewide population bust, individual properties and small pockets can experience higher activity.
Pay particular attention to oaks and other preferred host trees that were heavily defoliated during the recent outbreak. Declining branches, thinning crowns, or other changes in tree health may signal lingering stress or secondary problems that deserve a closer look.
It’s also worth watching for the adult spongy moth itself. Egg masses on trunks and outdoor objects can offer an early indication of local populations, while increasing numbers of caterpillars or noticeable leaf loss in spring and early summer may signal that activity is picking up.
You don’t need to predict Wisconsin’s next spongy moth outbreak to protect your trees. Knowing what is normal on your property and noticing when that begins to change can give you time to respond before another heavy round of defoliation takes its toll.
Frequently Asked Questions About Spongy Moth in Wisconsin
Spongy moth outbreaks raise plenty of questions, especially when populations can look dramatically different from one year to the next. Here are answers to some of the questions Wisconsin homeowners commonly have about this cyclical tree pest.
Are spongy moths still a problem in Wisconsin?
Yes. Although the most recent major outbreak ended in 2024, spongy moth remains established across much of Wisconsin. Populations are currently low in many areas, but localized pockets of activity can still occur, and populations can build again over time.
What trees do spongy moth caterpillars prefer?
Oaks are among their favorite hosts, but spongy moth caterpillars feed on hundreds of tree and shrub species. Other preferred hosts in Wisconsin include birches (Betula spp.), aspens (Populus spp.), crabapples (Malus spp.), willows (Salix spp.), and lindens (Tilia spp.).
When populations are high, caterpillars may feed on less-preferred species as well.
Will a tree survive spongy moth defoliation?
Often, yes. A healthy hardwood can generally survive one or even two years of heavy defoliation and may produce a second set of leaves.
Repeated defoliation is more concerning because it drains energy reserves and leaves trees more vulnerable to drought, disease, and secondary pests.
How can I tell if spongy moth populations are increasing on my property?
Look for egg masses from late summer through spring and watch for increasing numbers of caterpillars after they hatch. Egg masses are typically beige or tan and have a fuzzy or velvety appearance.
Finding numerous fresh egg masses can indicate greater caterpillar activity the following spring.
When should spongy moth treatment begin?
Timing depends on the treatment, but insecticides used against caterpillars generally work best while the caterpillars are still small and before extensive feeding damage occurs. In Wisconsin, that typically means spring.
Planning ahead is especially important if you have large or high-value trees that may require professional treatment.
Can I remove spongy moth egg masses myself?
Egg masses that are safely within reach can be scraped into soapy water and left to soak before disposal. Simply knocking egg masses onto the ground does not reliably kill the eggs. Egg masses high in trees or widespread infestations may require a different management approach.
When should I call an arborist about spongy moth damage?
Consider a professional assessment if a valuable tree has experienced heavy or repeated defoliation, shows thinning or dieback afterward, or has a large number of egg masses or caterpillars.
An arborist can evaluate both spongy moth pressure and the tree’s overall health to determine whether treatment or additional care is appropriate.
Stay Ahead of the Next Spongy Moth Cycle
Wisconsin’s current break from widespread spongy moth damage is good news for homeowners and trees alike. It’s also a good time to pay attention to trees that are still recovering and watch for signs that local populations may be changing.
If you’re concerned about a tree that was heavily defoliated or you’re noticing egg masses, thinning foliage, or other signs of stress, an ISA-certified arborist can help you understand what’s happening and what your tree needs next.
HH Accurate Arborists serves homeowners throughout the Greater Madison area with science-based tree care. Contact HH Accurate Arborists to schedule an assessment and help keep your trees healthy through Wisconsin’s changing pest cycles.
Sources & References
- Wisconsin Department of Natural Resources – “Spongy Moth”
Wisconsin-specific resource covering spongy moth history, outbreak cycles, defoliation, tree impacts, population trends, and management.
https://dnr.wisconsin.gov/topic/foresthealth/spongymoth - Wisconsin Department of Natural Resources – “Forest Health Annual Reports”
State forest health reporting documenting Wisconsin’s recent spongy moth outbreak, population decline, defoliation trends, and lingering oak mortality.
https://dnr.wisconsin.gov/topic/foresthealth/annualreports - University of Wisconsin–Madison Extension – “Spongy Moth”
Wisconsin resource covering spongy moth biology, identification, life cycle, natural population controls, tree impacts, and homeowner management.
https://hort.extension.wisc.edu/articles/spongy-moth/ - Wisconsin Department of Agriculture, Trade and Consumer Protection – “Spongy Moth”
State resource covering spongy moth distribution, host trees, quarantine areas, monitoring, and efforts to slow its spread in Wisconsin.
https://datcp.wi.gov/Pages/SpongyMoth.aspx - USDA Forest Service – “Spongy Moth”
National forest health resource covering defoliation, tree recovery, repeated feeding, and factors that can increase tree mortality risk.
https://research.fs.usda.gov/srs/understory/spongy-moth