Northern WI Forest Health

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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Veiled Polypore Fungi Growing On Red Pine Bark

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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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Heavy Seed Crop And Thin Tops On Maples

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A photo of a maple tree branch showing the production of large seed crops, creating off-color leaves that are smaller than normal into early summer.

Maple trees producing large seed crops may have off-color leaves that are smaller than normal into early summer. / Photo Credit: Linda Williams, Wisconsin DNR

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

Scattered maple trees across parts of the Northwoods have produced heavy seed crops this year.

When trees produce so many seeds, it reduces the amount of energy available for producing leaves. As a result, those leaves can appear smaller and off-color into the summer. With fewer leaves produced, these trees will continue to look very thin throughout this year’s growing season.

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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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Tan Tamaracks? It Could Be Larch Casebearer

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A photo showing severe defoliation of a tamarack tree by larch casebearer, with most needles mined out by the larvae.

Severe defoliation of a tamarack tree by larch casebearer shows most needles mined out by the larvae. / Photo Credit: Linda Williams, Wisconsin DNR

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

If you’ve noticed tamarack trees with tan or light-brown foliage, you’re probably seeing the work of the larch casebearer.

As foliage began to expand this spring, the caterpillars of this invasive moth became active and began to feed. The tiny caterpillars mine out the needles of tamarack and use them as “cases” to protect themselves, hence their name.

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Yellow Needles On White Pine This Spring

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A closeup photo showing current-year white pine needles emerging among yellow needles that show spots and banding.

Current-year white pine needles emerge among yellow needles that show spots and banding. / Photo Credit: Linda Williams, Wisconsin DNR

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

In North Central Wisconsin, some Eastern white pine had needles that turned a bright yellow color in early June, then quickly dropped from the trees by the end of June.

These trees will now look rather thin throughout the summer, until new foliage can fully expand and start to fill in the crown. Most damage has been noted in Oneida and Vilas counties, with scattered instances in other counties. Continue reading “Yellow Needles On White Pine This Spring” →

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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