Southern WI Forest Health

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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Gall Causes Bull’s-eye Spots On Maple Leaves

Posted on by akabelowsky
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

Posted on by akabelowsky
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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Veiled Polypore Fungi Growing On Red Pine Bark

Posted on by akabelowsky
Closeup photo of veiled polypore fungi showing as yellow, leathery fruiting bodies on the bark of dead red pine trees.

Veiled polypore fungi show as yellow, leathery fruiting bodies on the bark of dead red pine trees. / Photo Credit: Linda Williams, Wisconsin DNR

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

You may have seen these leathery, yellow fungi growing on recently dead red pine trees and wondered if they were what killed the tree. The short answer: No, they did not kill the tree.

The veiled polypore (Cryptoporus volvatus) is a decay fungus that grows on the bark of red pine in the year after they die. They can be found anywhere on the trunk of the tree.

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Non-Native Caterpillars Attacking Non-Native Euonymus

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A wide view of a shrub covered by Euonymus caterpillars' web branches as they feed on the leaves of Euonymus species.

Euonymus caterpillars web branches as they feed on the leaves of Euonymus species. / Photo Credit: Wisconsin DNR

By Michael Hillstrom, DNR Forest Health Specialist
Michael.Hillstrom@wisconsin.gov

Euonymus caterpillars (Yponomeuta cagnagella) made a dramatic appearance in June, webbing and defoliating Euonymus shrubs in several counties.

Multiple species of Euonymus may be attacked, including spindle tree, winged (e.g., burning bush), eastern wahoo, winter creeper and others. Winged euonymus is listed as Restricted in Wisconsin’s invasive species rule NR40.

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Woolly Elm Aphid Causes Elm Leaves To Curl

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Closeup photo of a group of elm leaves that show varying levels of wrinkling and curling, due to feeding by wooly elm aphids on the underside of the leaves.

A group of elm leaves shows varying levels of wrinkling and curling, due to feeding by wooly elm aphids on the underside of the leaves. / Photo Credit: Linda Williams, Wisconsin DNR

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

Woolly elm aphids (Eriosoma americanum) are a minor pest of American elm. The aphids’ feeding causes leaves to curl at the edges and develop a characteristic wrinkled appearance.

Uncurling the leaves exposes many pale gray aphids on the underside of the leaves, which are protected by the rolled leaf.

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Spongy Moth Population In Decline

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A photo of a tree trunk showing spongy moth caterpillars killed by nucleopolyhedrosis virus (NPV) hang in an inverted “V” orientation; caterpillars killed by the fungus Entomophaga maimaiga hang vertically.

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.

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Watch For Oak Wilt Symptoms

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A wide-angle photo showing an oak tree in the center that died rapidly from oak wilt during the summer of 2024.

An oak tree (center) that died rapidly from oak wilt during the summer of 2024. / Photo Credit: Wisconsin DNR

By Michael Hillstrom, Wisconsin DNR Forest Health Specialist
Michael.Hillstrom@wisconsin.gov or 608-513-7690

Oak wilt symptoms typically begin to appear in southern Wisconsin in late June and in northern Wisconsin during July.

Watch for browning leaves, starting at the top of the canopy and progressing downward. Most leaves will fall from infected trees as they die, typically within two to four weeks.

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What’s That Under The Bark Of Red Pines?

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A close-up photo showing pitch blobs near the base of a red pine, indicating attack by red turpentine beetle.

Pitch blobs near the base of a red pine indicate attack by red turpentine beetle. / Photo Credit: Linda Williams, Wisconsin DNR

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

Red turpentine beetle and pine root collar weevil are the most common insects found attacking near the base of red pine trees in Wisconsin, whether near ground level or just below ground level.

Here are some tips on identifying and controlling these pests.

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Web-Spinning Caterpillars Not Spongy Moth

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

You may already be seeing the web nests created by eastern tent caterpillars, which hatch early in the spring and start feeding on emerging leaves as the weather warms up.

Eastern tent caterpillars can sometimes be mistaken for forest tent caterpillars or spongy moths. Check out the DNR’s comparison fact sheet, which includes pictures for identification and control options. Continue reading “Web-Spinning Caterpillars Not Spongy Moth” →