Insect

Does Cold Weather Bug The Bugs?

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A map of Wisconsin showing cold temperatures on Jan. 25, 2026.

Cold temperatures across the state of Wisconsin on Jan. 25, 2026. / Graphic Credit: National Weather Service

By River Mathieu, DNR Forest Health Specialist, Fitchburg
River.Mathieu@wisconsin.gov or 608-772-2758

Wisconsin experienced a polar vortex in late January 2026, and temperatures got chilly throughout the state, even reaching close to -40⁰ F in some places! Although it has warmed up, the cold weather is still fresh in everyone’s minds, leaving insect and forest lovers alike wondering how insects survive temperatures that cold.

The Wisconsin Department of Natural Resources Forest Health staff has received several questions over the past month about how insects survive cold weather in winter and whether the January temperatures were cold enough to kill insect pests.

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Protect Valuable Ash Trees Against Emerald Ash Borer

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An ash tree in Oshkosh shows damage from woodpeckers feeding on emerald ash borer larvae beneath the bark.

An ash tree next to a walking trail in Oshkosh has been damaged by woodpeckers feeding on emerald ash borer larvae beneath the bark. / Photo Credit: Wisconsin DNR

By Bill McNee, DNR Forest Health Specialist, Oshkosh
Bill.McNee@wisconsin.gov or 920-360-0942

Property owners with healthy, valuable ash trees are encouraged to treat them with insecticide this spring to protect against emerald ash borer (EAB).

The invasive pest is currently the most damaging threat to trees in Wisconsin, killing more than 99% of the ash trees it infests.

Woodpecker damage during the winter is often the first sign that an ash tree is infested. The birds remove pieces of bark while feeding on EAB larvae inside the tree. Insecticide treatment of infested ash trees is more likely to be successful if the trees have low or moderate levels of woodpecker damage.

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Wisconsin Sees First Recovery Of EAB Parasitoid Oobius Agrili

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A plastic pan trap for wasps is hung from a tree branch.

A yellow pan trap used for trapping Oobius agrili wasps. / Photo Credit: United States Department of Agriculture Animal and Plant Health Inspection Service

By River Mathieu, DNR Forest Health Specialist
River.Mathieu@wisconsin.gov or 608-772-2758

One small adult wasp collected by United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) staff at Riedel Redmond Nature Preserve in Mosinee (Marathon County) was recently identified as Oobius agrili.

USDA APHIS staff collected several wasp specimens in yellow pan traps and sent them to a U.S. Forest Service expert for identification. This marks the first recovery of O. agrili in Wisconsin.

Oobius agrili is an emerald ash borer (EAB) parasitoid that was first released in Wisconsin in 2011 and has been released annually since then. O. agrili parasitizes EAB eggs on the bark of ash trees, consuming the EAB larvae inside the egg. Continue reading “Wisconsin Sees First Recovery Of EAB Parasitoid Oobius Agrili” →

Now Is Time To Seek Out Spongy Moth Egg Masses

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Large spongy moth egg masses on a tree main stem.

Large egg masses are seen on a tree at the Kettle Moraine State Forest-Southern Unit in 2021. / Photo Credit: Wisconsin DNR

By Bill McNee, DNR Forest Health Specialist, Oshkosh
Bill.McNee@wisconsin.gov

It certainly has been chilly in recent weeks, but spring is coming. When the weather warms up, the annual return of spongy moth caterpillars will begin.

The overall spongy moth population is currently low in Wisconsin. However, egg mass numbers may remain high enough to cause nuisance problems and heavy tree defoliation on individual trees or in small areas.

There’s a way for property owners to help keep the population down: Get out now to locate and properly dispose of spongy moth egg masses, which were produced by adult moths last summer. Each spongy moth egg mass contains hundreds of eggs that will hatch into hungry, leaf-eating caterpillars in the spring.

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Heterobasidion Root Disease Resources Updated

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Heterobasidion root disease fruiting bodies grow on base of tree

A tree shows fruiting bodies of Heterobasidion root disease at the base of its trunk. / Photo Credit: Wisconsin DNR

By Kyoko Scanlon, DNR Forest Pathologist
Kyoko.Scanlon@wisconsin.gov or 608-235-7532

In 2025, Heterobasidion root disease (HRD), a destructive fungal disease, continued to be found in Wisconsin counties where the disease had previously been detected. There were no new county finds in 2025.

Based on the findings in 2025, the maps in the HRD stump treatment guidelines and online viewer have been updated. The 6-mile and 25-mile buffer zone maps in the guidelines are updated annually and good through Dec. 31, 2026.

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Hemlock Borer Attacks Stressed Hemlock

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Closeup photo of a hemlock borer larva feeding inside channels in a hemlock tree.

A hemlock borer larva inside channels in a hemlock tree. / Photo Credit: Steven Katovich, Bugwood.org

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

Woodpeckers are often the first to know that a hemlock tree is being attacked by hemlock borer.

Woodpeckers will flake off the outer layers of bark to get at the tasty hemlock borer larvae under the bark, leaving the tree with a reddish hue that, once you’ve seen it, is unmistakable. The woodpecker damage quickly makes it obvious that the tree was fully infested from top to bottom by hemlock borer.

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Be Mindful Of Invasives During Holiday Clean-Up

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A stack of discarded Christmas trees awaits processing at a drop-off facility.

A stack of discarded Christmas trees awaits processing at a drop-off facility. / Photo Credit: Wisconsin DNR

By Wisconsin DNR

Invasive plants and insects don’t mind playing the long game. After all, they’re just trying to survive and thrive, like the rest of us.

They’ll hang out as part of your holiday decorations throughout the season, just on the chance humans might dispose of them improperly, giving them an opportunity to find a new home in your yard or neighborhood.

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Watch For Woodpecker Damage On Ash Trees

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Woodpecker flecking on the bark of a dying ash tree is visible at Winnebago County Community Park in Oshkosh.

Woodpecker flecking on the bark of a dying ash tree is visible at Winnebago County Community Park in Oshkosh. Flecking occurs when the birds peck away at the trees to feed on emerald ash borer larvae beneath the bark. / Photo Credit: Wisconsin DNR

By Bill McNee, DNR Forest Health Specialist
Bill.McNee@wisconsin.gov

The Wisconsin Department of Natural Resources (DNR) encourages property owners to watch for woodpecker damage to their ash trees this winter.

Woodpecker damage, often called “flecking,” happens when birds peck away some of a tree’s bark to access the larvae underneath. Flecking is a common early sign that an ash tree might be infested with emerald ash borer (EAB), an invasive insect. EAB is the most damaging threat to Wisconsin trees, killing more than 99% of the untreated ash trees it infests.

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Increased Tree Mortality Seen As Spongy Moth Outbreak Ends

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Mature spongy moth larva on a tree at Potawatomi State Park in Door County, July 2025.

Mature spongy moth larva on a tree at Potawatomi State Park in Door County, July 2025. / Photo Credit: Wisconsin DNR

By Bill McNee, DNR Forest Health Specialist, Oshkosh
Bill.McNee@wisconsin.gov  

Wisconsin’s recent period of high spongy moth populations (an “outbreak”) came to an end in 2025, as the remaining known hotspots were aerially sprayed or naturally collapsed. Only a handful of large larvae were observed or reported by property owners.

Spongy moth populations are likely to be low across the state for at least the next few years. However, it is possible that some sites may experience a population increase before then, if local weather conditions are favorable for the insect.

Continue reading “Increased Tree Mortality Seen As Spongy Moth Outbreak Ends” →

Report Invasive Hemlock Insects This Winter

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Small, white hemlock woolly adelgid eggs found on the needles of a tree in Bastian, Virginia, in 2017.

Small, white hemlock woolly adelgid eggs are found on the needles of a tree in Bastian, Virginia, in 2017. / Photo Credit: Bill McNee, Wisconsin DNR

By Mike Hillstrom, Forest Invasive Pest Coordinator, Fitchburg
Michael.Hilstrom@wisconsin.gov or 608-513-7690

Hemlock woolly adelgid (HWA) looms as a major threat to Wisconsin’s hemlock trees. As the infestation of these invasive, aphid-like insects spreads in Michigan and other states in the eastern United States, Wisconsin is preparing to respond when it arrives.

HWA is most evident in winter and spring as fluffy, white, woolly balls that are present at the base of hemlock needles. These fluffy, waxy balls cover adult female adelgids and eggs.

Continue reading “Report Invasive Hemlock Insects This Winter” →