An Insect Update from a Forest Health Specialist

By Denise Thornton

For those managing woodlands in Wisconsin, insect infestations may not always be a primary concern. But according to Bill McNee, Wisconsin DNR Forest Health Specialist for southeast Wisconsin, it is crucial to monitor the destructive insects already in the state, as well as some that are heading our way.  Here’s an overview of what to be alert for and how to respond.

Spongy Moth, Twolined Chestnut Borer, and Drought Threaten Oaks

After seeing several years of high spongy moth populations in parts of Wisconsin, the state finally experienced a wetter-than-normal spring in 2024, causing heavy die-off of spongy moth caterpillars from the Entomophaga fungus. “2025 may be the last year of this outbreak of spongy moth caterpillars, and then it should be quiet for a few years,” says McNee. “When we get another period of mild winters and warm, dry springs and summers, the population will rise again.”

Oaks that have been stressed by recent droughty summers need time — and some rain — to recover.  “Heavily defoliated oaks may linger for 1-3 years and then finally die,” says McNee, “so land owners should be watching their woods for tree decline and mortality over the next few summers. Oaks are at risk from the Armillaria root disease fungus and the twolined chestnut borer (TLCB) beetle, which thrive when oak trees are suffering from spongy moth and other defoliating insects, drought, or other stresses.”  Consider harvesting dead timber promptly once most of the mortality occurs.  However, this may not be easy to accomplish in practice and the economic value of the dead trees generally continues to decline.”

Typically, central Marinette County, which is very dry and sandy, is ground zero for infestations. “Severe outbreaks of the caterpillars have been relatively regular there on a five-year cycle, except when we had the polar vortex and the record-setting precipitation back in the late 2010s,” McNee says. “That kept a foot on the brake a few years longer than normal, but the foot came off the brake when we had warm, dry weather again.”

“When I flew over my southeast Wisconsin region last summer, I could already see that we had some areas of heavy oak mortality where the oak trees had endured two consecutive summers of both spongy moth caterpillars and drought. Oak mortality is probably going to become more prominent in other parts of the state as well this summer, especially in Marinette County and parts of southern Wisconsin.”

Spongy moth has been present in Wisconsin for nearly 40 years, but it is still spreading into the western part of the state, and those landowners may not yet be familiar with this insect. “We encourage landowners in the western counties to talk with a forester on how to manage their woods for the arrival of this insect,” says McNee. “They still have time to prepare for it, if they wish to do so.” It hasn’t built up in most of the Driftless Area, but at some point, a buildup can be expected. 

McNee predicts that as spongy moth continues to spread into the Driftless Area, a lot of defoliation and oak mortality will be seen there. The crooked trees, rocky ridge tops and steep south- and west-facing slopes are very favorable habitats for spongy moth.

As is the case with many insect pests, the first-time spongy moth invades an area, it tends to reach higher numbers and be more destructive than in future cycles of the insect. “Because most of the Driftless Area has not had that first blow-up of the population, the first impact could be high or even severe,” McNee warns. “If a forest is prepared in advance, the landowner can reduce the impact and have a more mixed forest in the future, knowing that the species diversification will also help a stand be more resilient to the insects and diseases that are going to keep arriving.”

Preparing woods usually means improving the vigor of trees through stand thinning, removing the less healthy trees that are going to die anyway, and favoring less desirable tree species (for feeding caterpillars) so that the overall impact in the woods is reduced. Oak and aspen are very desirable to spongy moth caterpillars. “There are other trees like hickory, pine, spruce and maple that the caterpillars will feed on but do not prefer,” says McNee. “If you increase the proportion of these other species, the overall impact in your woods will be reduced even if the preferred species do get heavily hit.

In many stands the rule-of-thumb is to aim for a reduced but healthier oak component. Depending on the management plan of the property, replacing economically or ecologically desirable oaks with less desirable tree species may or may not make sense. Oak is very valuable for wildlife and these benefits may outweigh reducing the oak component to reduce the spongy moth impacts. It depends on each property and the manager’s objectives.”

The lack of sufficient natural oak regeneration is another common issue for landowners, even in stands that haven’t been impacted by spongy moth. Regeneration following spongy moth outbreaks is usually dominated by non-oak species, most notably maple. and it is important to work with a forester to make sure that your woods will have the desired amount of oak in the future.

Planting new trees is a long-term investment. Some common present-day trees may not grow very well in areas of Wisconsin in the future, and a different mix of species may do better down the road. A forester can help, by sharing their best predictions for a future altered by a changing climate. For advice on what trees are expected to thrive in different areas of Wisconsin over the next century, check out the USDA Climate Change Field Guide for Southern Wisconsin Forests or the USDA Climate Change Field Guide for Northern Wisconsin Forests.

Emerald Ash Borer (EAB) Continues to Spread North

“EAB is the grim reaper of the ash forest,” McNee says. “We found it in Burnett County — the 72nd and final county — last summer, so every county in the state is known to have it.” Learn more at Wisconsin’s Emerald Ash Borer Information Source . and the WDNR Emerald Ash Borer website. In the southern half of the state, the vast majority of large ash trees are already dead. “They have often lost their value as forest products and might only be useful as firewood at this point,” says McNee. “Most dead ash isn’t even usable for pulpwood after it has been dead awhile. After an ash tree dies from EAB, there are only 2-3 years of economic value left. The dead trees get dry and light. Because pulp wood is sold by the ton, dead ash are often a money loser to haul out of the woods.”

McNee advises that landowners in northern Wisconsin who have a significant amount of ash should also talk to their forester. EAB is expected to kill more than 99 percent of the bigger ash trees, even the healthiest ones. It can infest small trees, but those 5-6” and up are first killed. “It is going to have far more of a future impact in the north because EAB is not yet widespread there and many of the swamps and other wet sites have a high proportion of ash. When you have most of your wet site trees dying, you get problems with invasive plants coming in and water tables rising. The forested site may become wet to the point that other tree species can’t grow there anymore, and the site converts to brush. We have seen some of this in southern Wisconsin counties already.”

“EAB is going to be a slow creep through the north woods over the next decade or so,” says McNee. Most of the ash in the state are growing in low swampy areas in the north where relatively few people see them. “We encourage landowners to be proactive and manage these sites before there are EAB impacts.  You will frequently have more options, have fewer invasive plants to deal with, and spend less money.”  Some sites will sell, and others will not attract interest from a logger.  Talk to a DNR or consulting forester about your woods and the best plan for them.  The DNR’s EAB Silviculture Guidelines are a good place to start planning.

“In the southern counties, I have been noticing that young ash an inch in diameter or smaller are still abundant on the landscape and producing seed. It is hopeful that ash will not go extinct. The larger ash will be temporary habitat for birds after they die, but they will mostly be gone like elm is now. We still have elms around after Dutch Elm Disease, but by and large, they are not big trees.” McNee predicts that as most ash will get to be 5-6” in diameter, EAB will re-infest and kill them.

Ash Hazard Tree Issue

With ash, there is an extra concern. A study conducted at the University of Minnesota confirms that several very aggressive species of decay fungi quickly produce serious loss of structural strength in dead ash. “These trees are dry and weak, and they have a habit of snapping unpredictably. We have had injuries and at least two fatalities in the state from dead ash trees. These trees can behave very differently than what landowners are used to,” says McNee and suggests that tree services have ways to remove ornamental ash trees without people getting hurt.

Beech Threats

Another insect to be on the alert for is “beech scale” (an aphid-like insect) that contributes to beech bark disease, which results when the scale and a fungus (Neonectria) interact. The scale makes wounds that allow several species of fungus to get into the tree, and the fungus causes the tree to decline. Many of the large beech trees are likely to eventually die from beech bark disease.

“We know that beech bark disease has been in Door County for at least 15 years. We are seeing the scale insect build up in other parts of the state and are monitoring beech to see if they start to get the disease,” says McNee.

Another threat to beech trees looms as well.  “Beech leaf disease” is caused by a tiny nematode (a “roundworm”) and it is as close as southern Michigan.  Birds appear to be involved in spreading the nematode.  The buds of beech trees are physically damaged and then the leaves are damaged when they leaf out.  Small beech trees seem to be particularly impacted and mortality of these trees has been seen in other states.

New Insects to Watch For

Hemlock Wooly Adelgid

Hemlock is ecologically important because it provides shade along streams, and deer like to eat it and rest under it in the winter.  A slow killer of hemlock trees is looming on the horizon because the hemlock woolly adelgid (HWA) is already in western lower Michigan. An adelgid is a little aphid-like insect that drinks the sugars out of a tree and slowly starves the tree over 5-10 years. Recent forest inventory data estimates that Wisconsin has about 39 million hemlock trees over five inches in diameter.

“HWA is well established in western lower Michigan and has probably been there for many years,” reports McNee. “It is one windstorm, one infested bird, or one infested piece of camping equipment away from arriving in Wisconsin. In Michigan, HWA is moving north, and they are finding it in the Traverse City area, which is across from Green Bay.  At some point it will be found in the U.P., and then it is just a progression of forest to Wisconsin.”

“We want to spot it as early as we can when it gets here,” McNee says. “We have a lot of hemlock, especially in the north. Landowners with a small amount of hemlock can treat them with insecticide and keep them alive. Those insecticides last 5-7 years and might be more feasible than treating ash trees. They are not practical for the tens of millions of hemlock trees that we have.” WDNR staff are working on a plan to respond to the arrival of HWA in Wisconsin.

Balsam Wooly Adelgid

WDNR is also watching the news about balsam wooly adelgid (BWA) over in Michigan. This is another tree-killing, aphid-like insect that drinks the sugars out of a fir tree and causes a swelling known as “gouting.” BWA was introduced from Asia, and infestations have been found in several places in Michigan as well as many states in the eastern and western U.S.  Affected trees include Balsam fir, Fraser fir, and White fir.

“We keep buying products from overseas, and other parts of the world buy from us,” says McNee. “We are unlikely to stop all movement of destructive insects, but we can prepare our forests to reduce future impacts.  We encourage landowners to talk to a forester about protecting their woodlands.”  Find a forester near you at this link.