Northern WI Forest Health

White Pine Blister Rust Can Devastate Young Stands

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By Tim Shively, DNR Forest Health Specialist
Timothy.Shively@wisconsin.gov or 608-772-6974

A close-up photo of a white pine blister rust canker as seen on a young white pine tree.

A white pine blister rust canker as seen on a young white pine tree. / Photo Credit: Wisconsin DNR

Eastern white pine (Pinus strobus) has historically held great ecological, cultural and economic value throughout Wisconsin, a tradition which continues today. For more than 100 years, however, it has been threatened by a non-native, invasive fungal pathogen called Cronartium ribicola, or white pine blister rust (WPBR).

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Impacts Of Winter Weather On Trees

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

A stand of pines shows heavy damage from an ice storm in April 2025.

A stand of pines shows heavy damage from an ice storm in April 2025. / Photo Credit: Wisconsin DNR

Winter damage can significantly impact the health of trees. Keeping trees healthy through the fall is important for reducing winter stress. Provide water as needed to trees until the leaves start falling (for deciduous trees) or until the ground starts freezing (for conifers).

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DNR, DATCP Teams Exemplify Interagency Teamwork

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

Wisconsin Department of Natural Resources Forest Health team leader Becky Gray and Department of Agriculture, Trade and Consumer Protection Pest Survey and Control Section manager Renee Pinski appear before the Natural Resources Board to discuss tree and forest health issues on Nov. 13, 2025.

Wisconsin Department of Natural Resources Forest Health team leader Becky Gray (left) and Department of Agriculture, Trade and Consumer Protection Pest Survey and Control Section manager Renee Pinski appear before the Natural Resources Board to discuss tree and forest health issues on Nov. 13, 2025. / Photo Credit: Wisconsin DNR

The spirit of interagency communication was on display during a joint meeting of the Wisconsin Natural Resources Board (NRB) and the Wisconsin Board of Agriculture, Trade and Consumer Protection (Ag Board) on Nov. 13.

The seven-member NRB is the governing body of the Wisconsin Department of Natural Resources (DNR); the nine-member Ag Board governs the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP). Each board sets policies and rules to be followed and enforced by its respective agencies. Members of both boards are appointed by the governor and confirmed by the Wisconsin State Senate.

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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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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.

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Take Advantage Of Mild Winter Days To Prune Trees

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An arborist uses climbing safety gear to prune an oak tree along a road.

An arborist uses climbing safety gear to prune an oak tree along a road. / Photo Credit: Wisconsin DNR

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

Winter is an ideal time to perform trimming, cutting and brush removal work on and near trees, especially oak trees.

Winter is the best time to prune because the insects and diseases that attack oak trees, such as oak wilt, are not active. Pruning and other work can be done around oak trees from after the first hard freeze in the fall until the high-risk period for oak wilt begins in April.

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Did You Know? Facts About Pine Webworm

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

Webbing and a collection of frass (caterpillar excrement) around these cones is the web nest of pine webworm. / Photo Credit: Wisconsin DNR

Pine webworm (Pococera robustella) is a native moth whose caterpillars feed on pine needles. In Wisconsin, the caterpillars prefer young jack pine and red pine. A single generation per year occurs in Wisconsin.

Young larvae mine needles, feeding within the needle. Older larvae form a compact web that they fill with small, oval pieces of frass. In early fall, the larvae drop to the ground to pupate and spend the winter.

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Cottony Ash Psyllid Found In Additional Areas

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Closeup photo showing how cottony ash psyllid nymphs feed on the lower sides of leaves and grow white wool over themselves for protection.

Cottony ash psyllid nymphs feed on the lower sides of leaves and grow white wool over themselves for protection. / Photo Credit: Wisconsin DNR

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

Cottony ash psyllid (Psyllopsis discrepans) was observed in 2023 in several areas in northern Wisconsin. Additional sites were then identified in 2024 and this year.

Some provinces in Canada have observed that psyllid numbers can explode during dry periods, so that may be why we started to see some issues in Wisconsin in 2023.

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HRD Conks Found In Different Shapes And Sizes

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Photo from above showing Heterobasidion root disease growing from an infected tree stump.

Heterobasidion root disease grows from an infected tree stump. / Photo Credit: Wisconsin DNR

By Kyoko Scanlon, DNR Forest Pathologist, Fitchburg
Kyoko.Scanlon@wisconsin.gov

Fall is a great time to look for mushrooms, including conks from trees infested with Heterobasidion root disease (HRD).

Considered one of the most destructive diseases of conifers in the northern hemisphere, the fungus causing HRD is very difficult to eradicate once established. Infestation of a conifer stand may significantly impact stand management, making early detection of the disease extremely important.

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Fall Webworm Spins Its Way To Attention

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Photo of fall webworms form a tent on a tree at Kohler-Andrae State Park in September 2025.

Fall webworms form a tent on a tree at Kohler-Andrae State Park in September 2025. / Photo Credit: Wisconsin DNR

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

The native insect known as fall webworm (Hyphantria cunea) has been generating calls and emails from curious property owners over the past few weeks.

The caterpillars don’t draw much attention on their own; instead, what catches the eye at this time of year are the large webs (or “tents”) that they create on the tips of tree branches.

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