By Bill McNee, DNR Forest Health Specialist, Oshkosh;
Bill.McNee@wisconsin.gov or 920-360-0942

A tree shows high numbers of spongy moth caterpillars, though the majority of the caterpillars have been killed either by virus (hanging in an inverted V) or fungus (hanging in a stretched line, head down). The virus and fungus helped to quell a potential outbreak in many areas of the state in 2024. / Photo Credit: Bill McNee, Wisconsin DNR

As the 2023 spongy moth season ended, there were hopes for a return to colder and wetter conditions in 2024 so that the spongy moth outbreak would decline or even come to an end.

What wound up happening in Wisconsin was the “lost winter,” the warmest since recordkeeping began in 1895. Given this warmth, there were concerns that the widespread spongy moth outbreak would continue for a third consecutive summer, with good caterpillar survival, ongoing defoliation and high rates of tree mortality in already stressed forests.

Fortunately, Wisconsin experienced a wetter-than-average spring in 2024, and this was favorable for caterpillar-killing diseases. DNR staff saw and received many reports of spongy moth caterpillar mortality from both nucleopolyhedrosis virus (NPV) and the fungus Entomophaga maimaiga, which benefits from rainy conditions. This increase in mortality was expected, given the age of the outbreak and above-average rainfall.

Reported larval mortality levels generally ranged from moderate to heavy, with some landowners reporting nearly 100% mortality. Heavy caterpillar mortality was seen in counties where defoliation was observed, as well as in counties where populations were noticeable but not high enough to cause significant defoliation. The outbreak was reported to have collapsed in the northwest counties and most of southern Wisconsin.

The Wisconsin Dells area and Marinette/Florence counties remain the most heavily infested areas of the state as of late 2024. It is likely that there will be a greatly reduced outbreak next year, and it may come to an end in 2025. Egg masses are commonly dime-sized and smaller, indicating that a site is late in the outbreak cycle and a population collapse is likely at that site in the next growing season. Pockets of high populations are likely to linger in both southern and northern counties.

Defoliation Mapped In 2024

A Wisconsin map showing highest spongy moth defoliation areas in 2024. / Map Credit: Wisconsin DNR

Aerial surveys conducted this summer recorded about 334,000 polygon acres of spongy moth defoliation (where more than 10% of the polygon was defoliated). This total was down from last year’s record of 375,000 acres. More than 80% of this defoliation was observed in northern Wisconsin (primarily Bayfield, Florence and Marinette counties), with about 20% recorded in southern counties.

The last few summers show that spongy moth can do very well in the colder climate of far northern Wisconsin when weather conditions are favorable.

Counties with the most recorded defoliation in 2024 were Marinette (180,606 acres), Bayfield (43,145 acres), Florence (34,258 acres), Sauk (21,573 acres) and Richland (16,148 acres).

Defoliation was generally less severe than in 2023. The most affected tree species were oak in the southern counties and aspen and oak in the northern counties. Aerial survey data found that oak defoliation was generally recorded as more intense (trees greater than 75% defoliated) than aspen defoliation (trees 50 to 75% defoliated). Other defoliating insects, such as oak leafroller, were also active on oak and other tree species this summer, increasing the total area of forest defoliation.

Decline And Mortality In Previously Defoliated Trees

A stand of trees that have been defoliated by spongy moth in June 2024 at Governor Thompson State Park. / Photo Credit: Darrek Sams, Wisconsin DNR

Monitored oak sites in southern Wisconsin found variable impacts of drought and defoliation. Impacts ranged from a few observable signs to heavy tree mortality. Healthy hardwoods can typically endure one or two years of heavy defoliation before tree decline and mortality occurs.

Healthy trees typically produce a replacement set of leaves a few weeks after defoliation. This process (“refoliation”) uses energy reserves and weakens the tree, making it more vulnerable to mortality in combination with other insects, diseases and/or drought stress. Heavy defoliation, in combination with another stress factor, often leads to tree mortality in a single year.

Aerial surveys conducted in summer 2024 mapped approximately 2,400 polygon acres of heavy oak mortality in five southern counties, mostly in the vicinity of Palmyra (Jefferson County). The oak stands had endured several droughty summers, along with one or two years of heavy spongy moth defoliation. Additional oak mortality is likely present in areas not surveyed.

A photo taken from an airplane shows oak mortality near Whitewater Lake and Palmyra in July 2024. / Photo Credit: Wisconsin DNR

Increases in oak decline and mortality are expected in both southern and northern Wisconsin over the next few years, following several summers of defoliation and drought. Property owners with oak and other preferred species should annually monitor their trees for signs of decline and mortality and contact an arborist or forester if needed.

In particular, weakened oaks often linger for 1-3 years before finally dying. Watering yard trees during dry periods can help to reduce tree stress and increase the likelihood of the tree’s survival.

Weakened oaks are commonly infested by an opportunistic beetle known as “twolined chestnut borer.” A DNR fact sheet is available to provide identification and management advice to property owners impacted by spongy moth defoliation and recent droughty conditions.

Slow-The-Spread And Regulatory Quarantines

A Wisconsin map showing counties quarantined for spongy moth in blue, counties outside of quarantine where egg masses were found by Slow the Spread surveyors in yellow and counties outside of quarantine where surveyors did not find egg masses in white. / Map Credit: Wisconsin Department of Agriculture, Trade and Consumer Protection

In 2024, no counties were added to the regulatory quarantine administered by the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP). Douglas County is the most recently added county (in 2023), and 53 of Wisconsin’s 72 counties are currently included in the spongy moth quarantine area, as shown in blue on the map. More information can be found on the DATCP Spongy Moth website.

In the fall of 2024, Wisconsin DATCP’s Slow the Spread program found life stages that indicate a small, reproducing population (all stages other than male moths) in nine non-quarantined counties that were surveyed. Most of those locations will be treated to reduce populations to levels where they cannot contribute to the spread.

Coming in the January Forest Health News: Planning for 2025 spongy moth control efforts.