Northern WI Forest Health

Native Sawyer Beetles Resemble Asian Longhorned Beetle

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An adult Asian longhorned beetle, left, and an adult whitespotted sawyer beetle, right. The arrow shows the white spot at the top of the whitespotted sawyer beetle’s wing covers, indicating it’s the native beetle and not Asian longhorned beetle. / Photo Credits: Left, Bugwood.org; right, Wisconsin DNR

Two closeup photos show the similarities and differences between an adult Asian longhorned beetle, left, with spots all over its back and striped antennae; and an adult whitespotted sawyer beetle, right, with one white spot beneath its head and between its wings.

By Michael Hillstrom, DNR Invasive Forest Pest Coordinator
Michael.Hillstrom@wisconsin.gov

More than 300 species of longhorn beetles inhabit the Midwest. The Wisconsin Department of Natural Resources (DNR) posted about one species, the invasive Asian longhorned beetle (ALB), on social media in early July.

Forest Health staff received many photos of potential ALB as a result. Fortunately, none of the photos were of ALB – which has yet to be detected in Wisconsin.

By far, the most common insect seen in the submitted photographs was a native longhorn species called the whitespotted sawyer beetle (Monochamus scutellatus), also known as a pine sawyer beetle.

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Herbicide Damage To Trees? Always Read The Label

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Closeup photo showing active ingredients listed on the front label of pesticide containers. The complete label, including instructions, may be on the front or back of the bottle and will require opening to read.

Active ingredients are listed on the front of pesticide containers (yellow arrow above). The complete label, including instructions, may be on the front or back of the bottle and will require opening to read (blue arrow above). / Photo Credit: Wisconsin DNR

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

This is your reminder to always read pesticide labels, even if you think the product you are using is the same product you’ve always used.

Do you always check the active ingredients listed on the front of pesticide containers – and read instructions on the use of the product – every time you buy a new container of pesticide? It has become more important than ever to do so.

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Japanese Beetle Defoliates Some Forest Trees In The North

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Closeup photo of Japanese beetles defoliating a basswood leaf

Japanese beetles defoliating a basswood leaf. / Photo Credit: Wisconsin DNR

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

The Japanese beetle (Popillia japonica) is an invasive insect that feeds on many plant species. The adult beetles “skeletonize” leaves by eating the material between the veins.

Adult Japanese beetles feed on the flowers and leaves of more than 300 plant species, including trees, shrubs and herbaceous plants. Forest trees can be defoliated, with open-grown or edge trees suffering the highest defoliation rates.

Japanese beetles favor birch, mountain ash and basswood (linden) trees, but they have also been observed feeding on the leaves of young oaks.

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Viburnum Leaf Beetle Appearing In Forests

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

Closeup photo showing small brown adult viburnum leaf beetles causing typical defoliation on a leaf.

Small brown adult viburnum leaf beetles cause typical defoliation on a leaf. / Photo Credit: Wisconsin DNR

Viburnum leaf beetle (Pyrrhalta viburni) is a relatively new invasive insect in Wisconsin. It originated in Eurasia and was first identified in Wisconsin in 2009.

Adults and larvae feed on the leaves of viburnums and can cause mortality after a few years of repeated defoliation. Feeding causes characteristic holes in the leaves. Beetle numbers can be very high on a single plant.

Viburnum leaf beetle has been observed defoliating native viburnum species including American Highbush Cranberry (Viburnum opulus var. americana) and Mapleleaf Viburnum (Viburnum acerifolium) in forest stands in Iron County and more recently in Oneida County.

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Locust Sawfly Defoliation Found In Oneida County

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

Closeup photo showing four locust sawfly larvae feeding on bristly locust leaves.

Four locust sawfly larvae feed on bristly locust leaves. / Photo credit: Wisconsin DNR

In early July, black locust and bristly locust in several areas of Oneida County were severely defoliated by locust sawfly.

Affected trees and shrubs have now produced additional leaves, making the defoliation less obvious. Damage was not noted in other areas of the state.

The sawflies are finished feeding, so no control is necessary.

Forest Health Specialists Plan Public Talks

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By Art Kabelowsky, DNR Outreach and Communications
Arthur.Kabelowsky@wisconsin.gov or 608-335-0167

Photo showing Wisconsin Department of Natural Resources Forest Health specialists (from left) Bill McNee, Paul Cigan and Linda Williams discuss a tree during the Cambridge Tree Tour in July 2025.

Wisconsin Department of Natural Resources Forest Health specialists (from left) Bill McNee, Paul Cigan and Linda Williams discuss a tree during the Cambridge Tree Tour in July 2025. / Photo Credit: Wisconsin DNR

Two Wisconsin Department of Natural Resources (DNR) Forest Health specialists will give presentations that are open to the public in the coming weeks:

  • Thursday, Aug. 7: Alex Hornung, Wisconsin DNR Forest Health specialist for central Wisconsin, will discuss the DNR’s Forestry program at Farm Technology Days. The event will run from noon to 5 p.m. at Clinton Farms, E8351 State Hwy 22, Bear Creek (Waupaca County). Admission is $15 per adult.
  • Friday, Sept. 5: Paul Cigan, DNR Forest Health specialist for northwest Wisconsin, will give a presentation on “Fungal Pathogens of the Wisconsin Forest” as part of the Wisconsin Mycological Society’s “Northwoods Foray” in Cable (Bayfield County). Cigan’s talk will run from 7:30 to 8:30 p.m. The Northwoods Foray will run from Sept. 4-7. Those interested in attending can visit the event’s webpage to obtain more information and sign up for a fee.

Armillaria Root Disease: A Fungal Killer

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A photo showing that after bark has been removed from a declining white spruce, the white mycelial mat of Armillaria can be seen under the bark.

Bark has been removed from a declining white spruce to show the white mycelial mat of Armillaria under the bark. / Photo Credit: Linda Williams, Wisconsin DNR

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

Armillaria is a root-rot pathogen that usually lives quietly under the soil. But when trees experience stress, the fungus can attack and colonize the roots of the stressed tree.

Armillaria can infect many different species of trees, and trees of any age or size, but the result for the tree is usually a slow decline and eventual death. Stressors can include drought and flooding, defoliation from insects or diseases or physical damage such as when roots are severed.

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New Forest Health Specialists Take To The Woods

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By Art Kabelowsky, DNR Outreach and Communications
Arthur.Kabelowsky@wisconsin.gov or 608-335-0167

It’s time for River Mathieu and Tim Shively to put all their education and research to work.

Mathieu and Shively are the newest additions to the Wisconsin Department of Natural Resources (DNR) Forest Health team. Both started on June 30, filling Forest Health Specialist vacancies.

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Gall Causes Bull’s-eye Spots On Maple Leaves

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A photo showing that maple eyespot galls have created colorful, concentric patterns on red maple leaves.

Maple eyespot galls create colorful, concentric patterns on red maple leaves. / Photo Credit: Linda Williams, Wisconsin DNR

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

Colorful “bull’s-eye” spots on maple leaves are maple eyespot galls, caused by the tiny ocellate gall midge (Acericecis ocellaris).

The adult midge lays eggs on the underside of the leaves. Once the eggs have hatched, the larvae feed on the leaves. That feeding causes the leaf to develop telltale colorful, concentric circles in that area of the leaf.

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Dusky Birch Sawfly Larvae Defoliate Birch Leaves

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A closeup photo showing that while feeding on a white birch leaf, dusky birch sawfly larvae raise into an S shape when a potential predator is noticed.

While feeding on a white birch leaf, dusky birch sawfly larvae raise into an S shape when a potential predator is noticed. / Photo Credit: Linda Williams, Wisconsin DNR

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

Dusky birch sawfly (Croesus latitarsus) is a native sawfly that feeds on white birch and occasionally river birch and yellow birch. Larvae feed in a group at the edges of birch leaves and abandon part of the midrib before moving on to another leaf.

Defoliation is typically light, with a few dozen leaves per tree being eaten. This defoliation is only problematic on young seedlings with few leaves. There may be two generations of larvae in a year.

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