Northern WI Forest Health

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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Pine False Webworm Uncommon In Wisconsin

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Messy webbing filled with frass (droppings) and pieces of needles are signs of pine false webworm feeding on this young white pine. The arrow points to a pine false webworm larvae. / Photo Credit: Wisconsin DNR

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

Pine false webworm (Acantholyda erythrocephala) is not a very common insect in Wisconsin, though it is present. They prefer to feed on white pine in Wisconsin, though reference materials say they will also feed on red pine, jack pine and Scotch pine.

Damage was reported as “minor” in the central part of the state from 1953 to 1961. Later, in 1995, 1997, 2002, 2004 and 2006, it was once again observed as a minor pest feeding on red or white pine in the central part of the state, particularly Portage, Waupaca and Waushara counties. In 2024 and again this year, a few scattered young white pine were observed being significantly defoliated in Oneida and Vilas counties.

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Japanese Beetle Populations Still Building In The North

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Beetles feeding on a basswood leaf.

Japanese beetles defoliating a basswood leaf. / Photo Credit: Wisconsin DNR

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

The Japanese beetle (Popillia japonica) is an invasive insect that feeds on many plant species. The adult beetles “skeletonize” leaves by eating the material between the veins.

Adult Japanese beetles feed on the flowers and leaves of more than 300 plant species, including trees, shrubs and herbaceous plants. Forest trees can be defoliated, with open-grown or edge trees suffering the highest defoliation rates.

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Spruce Budworm Hits Hemlock And Tamarack

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Closeup of hemlock branches showing missing and damaged needles.

Hemlock branches show missing needles, and needles with feeding damage, as the result of spruce budworm infestation. / Photo Credit: Wisconsin DNR

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

Spruce budworms are finished feeding for the year, but the damage is still being noticed. That’s especially true in areas where defoliation was severe, such as northeastern Oneida County and southeastern Vilas County.

Spruce budworm prefers to feed on balsam fir and spruce, but in areas with very high populations and severe defoliation, they will also feed on hemlock and tamarack.

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Tamaracks Busy Refoliating After Larch Casebearer Defoliation

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Wide view of stand of trees

Tamarack trees showing the effect of larch casebearer defoliation. / Photo Credit: Wisconsin DNR

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

Earlier this spring, scattered areas of tamarack in North Central and Northeastern Wisconsin showed tan or light brown foliage. Sometimes that is due to frost, but this year it was the work of an insect called larch casebearer.

As the foliage started to expand this spring, the larch casebearer became active and began to feed. The tiny caterpillars mine out the needles of tamarack and use them as “cases” to protect themselves, thus their name.

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First Populations Of Invasive Pale Swallow-wort Confirmed In Wisconsin

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Closeup of pale swallow-wort flower cluster.

Pale swallow-wort flowers along the stem at the leaf axils. / Photo Credit: Chris Evans, University of Illinois, Bugwood.org

By Mackenzie Manicki, DNR Natural Resources Research Scientist

The Wisconsin Department of Natural Resources has confirmed that the state’s first populations of the invasive vine pale swallow-wort (Vincetoxicum rossicum) have been found.

The populations were first detected in Oconto and Shawano counties, within the outskirts of Krakow along the shared county line.

These are the first known populations in the state, making rapid response efforts critical in preventing the establishment of this species in Wisconsin.

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Forest Health Staffers Plan Appearances

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Two workers inspecting a tree's leaves.

Mike Hillstrom and Linda Williams of the Wisconsin Department of Natural Resources Forest Health team look over a tree during a walking tour put on by the Cambridge Tree Project on July 10, 2025. / Photo Credit: Wisconsin DNR

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

Mike Hillstrom, Invasive Forest Pest Coordinator for the Wisconsin DNR Forest Health Team, will make a presentation on forest health to the Heartland National Christmas Tree Organizations Convention, scheduled for Friday, Aug. 7, at Hann’s Christmas Farm, 848 Tipperary Road, Oregon, WI 53575.

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Watch For Spongy Moth Caterpillars And Tree Damage This Summer

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

Close-up of caterpillar feeding on a leaf.

A spongy moth caterpillar feeds on a leaf. / Photo Credit: Wisconsin DNR

Property owners are encouraged to monitor their trees for spongy moth larvae and damage this summer to determine whether control measures are needed.

Property owners who have oak, birch, crabapple, aspen, willow and linden (basswood) trees should be particularly watchful because the caterpillars of this invasive insect prefer these species.

However, they will feed on many other kinds of trees and shrubs.

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Spruce Budworm Outbreak Continues In 2026

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

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

The spruce budworm outbreak is continuing for yet another year in some areas of northern Wisconsin.

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Forest Tent Caterpillar Outlook

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Closeup of caterpillars on a tree.

Forest tent caterpillar larvae displaying gregarious, clustering behavior on a tree stem. / Photo Credit: Wisconsin DNR

By Paul Cigan, DNR Forest Health Specialist, Hayward
dnr.wisconsin.gov/topic/foresthealth/staff, 715-416-4920

Populations of forest tent caterpillar (FTC) are likely to remain low throughout northern Wisconsin this growing season, continuing a record 23-year statewide trend.

Spring egg mass surveys showed a low presence of overwintering egg masses, suggesting that any defoliation this season is likely to be localized. Overwintering egg masses present on the twigs of deciduous twigs and branch tips are counted during surveys to forecast FTC population density and the associated risk of heavy defoliation going into spring leaf-out.

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