Northern WI Forest Health

After A Quiet Year, Conduct Spongy Moth Egg Mass Surveys This Fall

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The only mature spongy moth caterpillar detected at Harrington Beach State Park in Ozaukee County in July 2026 crawls up a tree’s mainstream. / Photo Credit: Wisconsin DNR

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

Wisconsin’s recent period of high spongy moth (Lymantria dispar) populations came to an end in 2025 and the population remained very low in 2026.

Only a handful of caterpillar reports were received, most often coming from southern Jefferson County and eastern Dane County. Most notably, numerous bur oaks were reported to be infested with several hundred caterpillars per tree in and around Cambridge, Fort Atkinson and Lake Mills; light feeding damage was visible.

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Parts Of Northern Wisconsin Experience “Exceptional Drought” Conditions

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

map

A Wisconsin map showing drought conditions as of October 6, 2026. Map created by David Mocko, NASA/GSFC/SSAI. / Graphic Credit: Wisconsin DNR

Property owners and land managers in the northern half of Wisconsin are encouraged to monitor forest stands and ornamental trees for signs of decline and mortality due to the intensity of the current drought. In late September, parts of several counties reached the “Exceptional Drought” condition on the U.S. Drought Monitor (see maps). This is the most severe drought category used and it has only been seen in Wisconsin one other time since the U.S. Drought Monitor was created in 1999 (source: National Weather Service).

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Drought Impacts Young Aspen In North Central Wisconsin

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A portion of an aspen stand showing many leaves dropped due to drought, taken Sept. 2, 2026. / Photo Credit: Wisconsin DNR

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

Drought impacts can show up abruptly or have a slower impact, with symptoms becoming visible over the next one to two years.

North central and northwestern Wisconsin have faced significant drought conditions over several months this summer.

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Sapstreak Of Sugar Maple An Oldie But A Goodie

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DNR Forester Paul Etten stands at the base of a sugar maple tree that has small leaves and a thin crown. The tree was dissected and found to have extensive sapwood streaking. / Photo Credit: Wisconsin DNR

By Alex Hornung, DNR Forest Health Specialist, Plover
Alexandra.Hornung@wisconsin.gov

There are some tree diseases that really get a lot of attention and others that get lost in historical documents, only popping up now and again to ignite excitement within the local plant pathologist community.

Maple sapstreak is one of the latter. This year, a site in Shawano County was found to have several trees experiencing symptoms of the disease. The symptoms led to a positive identification of maple sapstreak.

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White Pine Weevil Also A Problem On Spruce

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Damage to terminal leader of an understory white pine in Portage County. / Photo Credit: Wisconsin DNR

By Alex Hornung, DNR Forest Health specialist, Plover
Alexandra.Hornung@wisconsin.gov

This is a great time of year to be looking for or observing damage from the white pine weevil.

The insect is native and causes terminal leader mortality in susceptible species. Despite the name, there are numerous species of trees that are attacked in addition to white pine, including Norway spruce, jack pine, white spruce and many more.

The damage from white pine weevil is more evident when the needles on the main leader are still attached and are brown in color, primarily in the month of July through the winter of the year that the damage occurred. It is more difficult to see previous years’ damage, because the needles fall off and the dead terminal leader will either snap off or be bare. When the leader is killed, the branches below will grow in place of that leader and will eventually correct by self-selecting a new leader.

Chip cocoons and a larva are found inside the killed terminal leader of a white pine tree. / Photo Credit: Wisconsin DNR

Every year, reports come out of the northern and central zone of damage, sometimes reaching 20% or more in pine/spruce plantings where trees are open-grown. An increased number of reports in plantings have been coming out of the central zone throughout this field season. This may be due to an increase in populations of the weevil this year, or a simple function of thriving plantings that have robust terminal leaders more susceptible to attack.

In forested stands, pesticides are not the primary method of control due to practicality and cost. Trees will very often self-correct, and natural predators will do better when there is an adequate population of the weevil to sustain them. Impacted trees may end up with a noticeable crook in the stem as they continue to grow. These trees can be targeted for removal during regularly scheduled thinnings as the trees mature.

Terminal leader damage is evident on planted white pine and spruce trees in Waushara County. / Photo Credit: Wisconsin DNR

For more information about white pine weevil in Wisconsin, please refer to this article from 2023, written by Linda Williams, Wisconsin Department of Natural Resources Forest Health specialist for the Northeast zone.

Fluffy White Stuff On Your Alder? It’s Woolly Alder Aphid

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Woolly alder aphids congregate on alder stems to spend the winter. / Photo Credit: Ryan Brown, Vilas County Forest

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

Woolly alder aphids spend their winters on alder stems and summers feeding on maple leaves. Large congregations of these aphids can be visible from a distance, and the bright white patches of aphids may catch your eye.

These plump bluish aphids, with white fluffy growths on their abdomens, are called woolly alder aphids when they’re found on alder – and maple blight aphids during the part of their life cycle when they’re feeding on maples.

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Needle Rust On Spruce, Fir And Hemlock

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closeu[p of balsam fir needles with needle rust

Balsam fir needles infected with needle rust turn bright yellow. / Photo Credit: Wisconsin DNR

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

Although this summer has been dry in north-central Wisconsin, there was enough moisture this spring to promote some needle rusts. Needle rusts are fungi that infect needles of various conifers. Most rusts require two hosts to complete their life cycle.

A look at common types of needle rust:

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Pitch Borer Mass On White Pine

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A pitch mass on a tree's main stem.

A pitch “glob”, with oozing pitch running down the trunk of a young white pine infected with pitch mass borer. / Photo Credit: Wisconsin DNR

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

Have you seen big globs of gooey pitch on the main stems of eastern white pine trees?  Or maybe you’ve seen them on spruce?

For those with an adventurous side, grab a stick and scrape off some of that gooey glob. You might be rewarded with the sight of large larvae feeding beneath all of that goo. This is the larvae of our native pitch mass borer. Adults are clearwing moths that look a bit like wasps.

Peeling reveals a pitch borer "goo" mass on a damaged tree.

Using a stick to pry away some of the gooey pitch on a tree exposes a pitch mass borer larva underneath. / Photo Credit: Wisconsin DNR

Adults lay eggs on the bark. When eggs hatch, the tiny larvae chew into the bark and begin feeding on the inner bark. They feed enough to irritate the tree, causing more sap and pitch to be produced, which the larvae feed on.

Pitch mass borer can take two to three years to complete development, remaining in that same location throughout their life and continuously irritating the inner bark to promote continued pitch production. Pupation occurs under the glob of pitch and adults then chew their way out. Old pitch globs will harden and darken over time, remaining on the tree for a year or more before finally weathering off.

Hosts include pines (eastern white, jack, scotch and Austrian) and spruces (blue and Norway).

Management on forest trees is not necessary. Severely damaged and deformed trees can be removed during the first thinning. Management on yard trees includes maintaining healthy trees (which are less attractive to the adults), watering during droughts, avoiding pruning during mid-summer (which can attract adults to lay eggs near the fresh wounds), destroying any larvae under the globs of pitch and removing and disposing of severely infested trees.

A pitch mass borer larva on a tree.

Scraping more pitch off a tree’s main stem shows a pitch mass borer larva on the end of the stick. / Photo Credit: Wisconsin DNR

A photo showing the whitish color of some pitch mass borer larvae.

Pitch mass borer larvae can be shades of white to pink, depending on the host. / Photo Credit: Wisconsin DNR

Forest Health Staffers Plan Public Appearances

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Workers inspect a tree's health.

Linda Williams, DNR Forest Health specialist for Northeast Wisconsin, inspects a tree during a training session. / Photo Credit: Wisconsin DNR

By Art Kabelowsky, DNR Forest Health Communications, Fitchburg;
Arthur.Kabelowsky@wisconsin.gov or 608-335-0167

Linda Williams, Wisconsin Department of Natural Resources Forest Health specialist for Northeast Wisconsin, will speak at multiple “Learn About Your Land” teaching events scheduled in September.

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Aspen Leaf Beetle Defoliation Found In North

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Closeup of beetles, and a cluster of eggs, on an aspen leaf.

Young larvae of aspen leaf beetle feed on the undersides of aspen leaves. The arrow points to the cluster of eggs that produced the larvae. / Photo Credit: Wisconsin DNR

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

Aspen leaf beetles have recently been spotted defoliating young quaking aspen trees in Vilas and Oneida counties.

Both adults and larvae will feed on aspen leaves. The defoliation level detected was light to moderate.

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