Pest

Armillaria Root Disease: A Fungal Killer

Posted on by akabelowsky
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.

Continue reading “Armillaria Root Disease: A Fungal Killer” →

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.

Continue reading “Gall Causes Bull’s-eye Spots On Maple Leaves” →

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.

Continue reading “Dusky Birch Sawfly Larvae Defoliate Birch Leaves” →

Non-Native Caterpillars Attacking Non-Native Euonymus

Posted on by akabelowsky
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.

Continue reading “Non-Native Caterpillars Attacking Non-Native Euonymus” →

Tan Tamaracks? It Could Be Larch Casebearer

Posted on by akabelowsky
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.

Continue reading “Tan Tamaracks? It Could Be Larch Casebearer” →

Maple Petiole Borer Sawfly Causing Leaf Drop

Posted on by akabelowsky
A closeup display photo showing prematurely fallen sugar maple leaves that have a normal appearance, except for characteristic darkened petiole stubs caused from feeding damage of the maple petiole borer.

Prematurely fallen sugar maple leaves with normal appearance except for characteristic darkened petiole stubs caused from feeding damage of the maple petiole borer. / Photo Credit: Paul Cigan, Wisconsin DNR

By Paul Cigan, DNR Forest Health Specialist
Paul.Cigan@wisconsin.gov or 715-416-4920

Fallen maple leaves have been observed under Wisconsin maple trees since late May.

The fallen leaves are typically of normal shape, size and color, but have a darkened petiole stub (i.e., leaf stem) remaining. A closer examination of the petiole may reveal a hollow feeding tunnel containing tiny larvae of the insect responsible for the damage inside: the maple petiole borer.

Continue reading “Maple Petiole Borer Sawfly Causing Leaf Drop” →

Winsome Fly Biocontrol Slows Japanese Beetle

Posted on by akabelowsky
A closeup photo showing Winsome fly eggs on three Japanese beetles, including a mating pair with a female Japanese beetle below the male.

Winsome fly eggs are found on three Japanese beetles, including a mating pair (with a female Japanese beetle shown below the male). One or more eggs are laid on each beetle; eggs are nearly always laid just behind the head. The Winsome fly prefers to lay eggs on female Japanese beetles. / Photo Credit: Jacques Lasnier, Quebec, Bugwood.org

By Bill Hutchison, Department of Entomology
University of Minnesota
Hutch002@umn.edu

The Winsome fly (Istocheta aldrichi (Mesnil)), part of the Tachinidae family, is a parasitoid of the invasive pest Japanese beetle (Popillia japonica).

A parasitoid is an insect whose larvae develop in or within a host insect of a different species, eventually killing it.

The Winsome fly was introduced in 1923 in New Jersey as a biological control agent of the Japanese beetle, six years after the Japanese beetle was accidentally introduced in the United States. Since then, the Winsome fly has become established in the Northeastern states and Quebec and Ontario in Canada.

Continue reading “Winsome Fly Biocontrol Slows Japanese Beetle” →

Woolly Elm Aphid Causes Elm Leaves To Curl

Posted on by akabelowsky
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.

Continue reading “Woolly Elm Aphid Causes Elm Leaves To Curl” →

Spongy Moth Population In Decline

Posted on by akabelowsky
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.

Continue reading “Spongy Moth Population In Decline” →

Watch For Oak Wilt Symptoms

Posted on by akabelowsky
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.

Continue reading “Watch For Oak Wilt Symptoms” →