Insect

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.

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DNR Debuts Fact Sheet On White Grubs

Posted on by akabelowsky
Closeup photo showing a white grub in topsoil, feeding on a root.

A white grub feeds on a root. / Robert L. Anderson, USDA Forest Service, Bugwood.org

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

White grubs, the larvae of Wisconsin’s native May/June beetles (Phyllophaga spp.), can cause significant damage to forest plantings.

To help those who plan to plant seedlings and saplings, the Wisconsin Department of Natural Resources’ (DNR) Forest Health team has produced a fact sheet on white grubs, aimed specifically at forest plantings.

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Winsome Fly Biocontrol Slows Japanese Beetle

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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.

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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.

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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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More Spruce Budworm Defoliation Expected

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A close-up photo showing young spruce budworm larvae beginning to feed as buds break, causing the new growth to crook where feeding damage occurs. Webbing is also common.

Young spruce budworm larvae begin feeding as buds break, causing the new growth to crook where feeding damage occurs. Webbing is also common. / Photo Credit: Linda Williams, Wisconsin DNR

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

From early indications, the spruce budworm outbreak in some areas of northern Wisconsin is preparing to continue for yet another year.

During the third week of May, expanding buds on balsam fir trees were already showing damage from the tiny larvae.

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Forest Tent Caterpillar Outlook Stays Low

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A close-up photo showing forest tent caterpillar larvae displaying gregarious behavior and feeding on a tree leaf.

Forest tent caterpillar larvae displaying gregarious behavior and feeding on a tree leaf. / Photo Credit: Paul Cigan, Wisconsin DNR

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

Forest tent caterpillar (FTC) populations do not appear to be making a significant return this growing season.

Spring egg mass surveys indicate that populations continue to remain low, continuing the longest documented interphase between outbreaks on state record, at 22 years and counting. The interphase provides some relief for the health of aspen stands recovering from several consecutive years of recent spongy moth defoliation.

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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” →

UW-Madison’s Raffa Talks Trees’ Natural Defenses

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Photo of Ken Raffa, professor emeritus with the University of Wisconsin-Madison Department of Entomology.

Ken Raffa, professor emeritus with the University of Wisconsin-Madison Department of Entomology. / Photo Credit: University of Wisconsin-Madison

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

Do you pity the poor tree, forced to stand defenseless against the ravages of insect pests, fungi and disease?

Then, you have not listened to Dr. Ken Raffa discuss the impressive natural defenses trees have developed.

Raffa, a professor emeritus with the University of Wisconsin-Madison’s Entomology Department, recently appeared on Proforest Podforest podcast to discuss the topic “Inside the Tree’s Natural Defense.”

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La Crosse County Added To Spongy Moth Quarantine

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A closeup photo of a spongy moth caterpillar consuming a leaf.

A spongy moth caterpillar consumes a leaf. / Photo Credit: Bill McNee, Wisconsin DNR

By Wisconsin Department of Natural Resources (DNR)

La Crosse County has become the latest Wisconsin location to be added to the state and federal spongy moth quarantine, joining most eastern and central Wisconsin counties already considered to be infested by the invasive insect.

The Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) announced the change on April 1, based on trapping data collected last year.

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