Statewide Forest Health

Increased Tree Mortality Seen As Spongy Moth Outbreak Ends

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Mature spongy moth larva on a tree at Potawatomi State Park in Door County, July 2025.

Mature spongy moth larva on a tree at Potawatomi State Park in Door County, July 2025. / Photo Credit: Wisconsin DNR

By Bill McNee, DNR Forest Health Specialist, Oshkosh
Bill.McNee@wisconsin.gov  

Wisconsin’s recent period of high spongy moth populations (an “outbreak”) came to an end in 2025, as the remaining known hotspots were aerially sprayed or naturally collapsed. Only a handful of large larvae were observed or reported by property owners.

Spongy moth populations are likely to be low across the state for at least the next few years. However, it is possible that some sites may experience a population increase before then, if local weather conditions are favorable for the insect.

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Stumping Heterobasidion Root Disease With Cellu-Treat®

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Photo of a freshly cut tree stump after treatment with fungicide to prevent infection with Heterobasidion root disease. The fungicide is dyed blue to help ensure complete coverage.

A freshly cut tree stump after treatment with fungicide to prevent infection with Heterobasidion root disease. The fungicide is dyed blue to help ensure complete coverage. / Photo Credit: Wisconsin DNR

By Tim Shively, DNR Forest Health Specialist, Eau Claire
Timothy.Shively@wisconsin.gov or 608-772-6974

Treating freshly cut conifer stumps is critical to preventing infection by Heterobasidion species, causal agents of Heterobasidion root disease (HRD).

Within this genus, H. irregulare is believed to be the only species in Wisconsin, and it causes the mortality of overstory pine and spruce trees. The DNR has developed comprehensive guidelines to assist landowners, property managers, foresters, and loggers with determining whether stump treatments are necessary to prevent the introduction or spread of HRD at a given site.

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Which Witches’ Broom Is It?

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Witches’ broom on serviceberry. The leaves of this infected shrub are dying because of prolific spore production on their undersides. / Photo Credit: Wisconsin DNR

By Tim Shively, DNR Forest Health Specialist, Eau Claire
Tim.Shively@wisconsin.gov or 608-772-6974

Witches’ broom is a term that describes a disease of many different plants. It’s a distinctive, abnormal growth typically caused by a pathogen — though in some cases it can result from insect feeding.

The namesake symptom appears as abnormally dense clusters of stunted shoots, branches, and foliage that strongly resemble a witch’s broom from storybooks and folklore. In Wisconsin, this is not often a critical forest health concern that merits management action, but Wisconsin Department of Natural Resources Forest Health staff recently observed widespread incidence of two particular witches’ broom diseases.

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Jack Pine Showing Dead, Brown Branch Tips

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Photo showing jack pine branch tips killed by the jack pine tip beetle.

Jack pine branch tips killed by the jack pine tip beetle. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health specialist, Woodruff
Linda.Williams@wisconsin.gov or 920-360-0665

Jack pine tip beetle (Conopthorus banksianae) is a tiny bark beetle that bores into the twig tips of pines. Damage from jack pine tip beetle was observed this summer on jack pine in Vilas County. In some areas, each tree showed 2-10 dead branch tips in its crown.

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Barklice Don’t Damage Trees

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Photo showing a group of immature bark lice show their characteristic striped abdomens.

A group of immature barklice show their characteristic striped abdomens. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health specialist, Woodruff
Linda.Williams@wisconsin.gov or 920-360-0665

Barklice, sometimes called tree cattle, tend to congregate in large groups on trees, which can lead some people to worry about tree health. However, they are actually quite harmless.

Immature barklice don’t have wings, so you can see their heavily striped abdomens. Adults have dark-colored wings that form a tent over the abdomen. Both immature and adult barklice congregate in groups.

These insects feed on the algae, lichen and other debris that collects on the surface of tree bark. They do not feed on the tree itself.

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DNR, UW Offer Fall Invasive Plants Workshops

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Buckthorn beneath dead ash trees at Big Foot Beach State Park. / Photo Credit: Wisconsin DNR

By Wisconsin DNR

Is buckthorn taking over your urban property? Would you like to be able to determine which plants are doing harm to the native landscape? Are you ready to arm yourself in the fight against invasives?

If so, this day-long workshop is for you!

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Sawflies On Jack Pine Are Neodiprion Maurus

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Photo showing five Neodiprion maurus larvae feeding on jack pine needles.

Neodiprion maurus larvae, feeding on jack pine, are gregarious. Can you spot all five larvae? / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health specialist, Woodruff
Linda.Williams@wisconsin.gov or 920-360-0665

In mid-July, a few sawfly larvae were found feeding on jack pine in northern Vilas County. After delving into associated literature and reaching out to a sawfly researcher at the University of Kentucky, the sawflies were identified as Neodiprion maurus. There is no official common name for this insect; it is generally referred to as “a sawfly on jack pine.”

These sawflies are gregarious and feed in groups on older needles, doing relatively minimal damage. They are often referred to as “rare” and have been found in Michigan, Minnesota and Wisconsin, as well as Alberta, Manitoba and Saskatchewan in Canada.

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Pink Striped Oakworm Seen Feeding On Red Oaks

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Closeup photo showing young pink striped oakworm caterpillars feed gregariously on northern red oak leaves.

Young pink striped oakworm caterpillars feed gregariously on northern red oak leaves. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health specialist, Woodruff
Linda.Williams@wisconsin.gov or 920-360-0665

Young larvae of the pink striped oakworm (Anisota virginiensis) have been observed feeding gregariously on northern red oak leaves in Vilas County. Northern red oak is their preferred host, but they can feed on other oaks.

Young caterpillars start life with a shiny black head capsule, but older larvae develop an orange head. Their pinkish body coloration develops as they get older, as well; young caterpillars are dark green in color.

Pink striped oakworm has several fly parasitoids, along with a couple of wasp parasitoids. Populations of pink striped oakworm in Wisconsin don’t usually grow too large or cause problems.

 

Northern Pitch Twig Moth Creates Pitch Blisters On Jack Pine

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Closeup photo showing that hollow pitch blisters get darker with age and may resemble cones until closer examination.

Hollow pitch blisters get darker with age and may resemble cones until closer examination. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health specialist, Woodruff
Linda.Williams@wisconsin.gov or 920-360-0665

Northern pitch twig moth (Petrova alhicapitana) is also called the pitch nodule maker or pitch blister moth.

It feeds on branches and twigs of young jack pine and scotch pine, causing a hollow blister of gooey pitch to form around the larvae as it feeds at the base of a lateral branch. It prefers trees that are 20 feet tall or smaller, although it can attack older trees.

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Monitor Your Trees For Spongy Moth-Related Decline And Mortality

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A photo taken June 20, 2025, shows dead and dying oaks near Whitewater Lake in Walworth County, following a period of defoliation and summer drought.

A photo taken June 20, 2025, shows dead and dying oaks near Whitewater Lake in Walworth County, following a period of defoliation and summer drought. / Photo Credit: Wisconsin DNR

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

Property owners are encouraged to monitor their trees for signs of decline and mortality, as the last few years have been marked by drought and spongy moth defoliation.

Landowners who have oak, birch, crabapple, aspen, willow, tamarack and basswood (linden) trees should be particularly watchful, because the caterpillars of this invasive insect prefer these species. Many other tree species are not preferred by the caterpillars and are less likely to be heavily defoliated, but are more likely to die if heavy defoliation should happen.

This article focuses on oak impacts.

The 2021-24 spongy moth outbreak has now collapsed statewide; no defoliation has been observed as of late July 2025. Only a few reports of single caterpillars were received by Wisconsin DNR Forest Health staff. (Note: Parts of Wisconsin may be experiencing a continuing outbreak of different caterpillars known as “oak leafroller” and “larch casebearer.”)

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