An ash tree next to a park trail in Winnebago County shows damage from woodpeckers feeding on EAB larvae beneath the bark. / Photo Credit: Bill McNee, Wisconsin DNR

By Bill McNee, Forest Health Specialist
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.

Now is a good time to consider insecticide protection, because the treatments typically take place between mid-April and mid-May. Ash (in the genus Fraxinus) is the only type of tree that needs protection against EAB. Mountain ash and prickly ash do not need protection because the insect does not attack them.

A January 2025 map of Wisconsin showing emerald ash borer infestations. / Map Credit: Wisconsin Department of Agriculture, Trade and Consumer Protection

EAB has now been detected in all of Wisconsin’s 72 counties, and treatments are worth considering anywhere in the state.

The southern half of Wisconsin is heavily infested, and the vast majority of large ash trees there have already died. In northern counties, EAB is being found at an increasing number of locations and tree mortality is not as widespread.

Location isn’t the only consideration when making a decision about insecticide treatments. For example, the treatments are not economically practical for woodlot ash trees; they need to be repeated every one to three years, or the trees will become heavily infested and die. The frequency of treatments will depend on the product and method used.

What You Should Do

Some insecticide products can be applied by homeowners, and others must be applied by a certified professional. Review the available options before selecting an insecticide and treatment method. Visit the Wisconsin EAB website and EAB Information Network website for more information about insecticides.

Arranging for insecticide treatments ahead of time can make it easier to secure an applicator, avoiding a last-minute rush when pesticide application businesses may already be very busy. Resources for finding a qualified pesticide applicator include the Wisconsin Arborist Association, the International Society of Arboriculture and an online directory.

Ash bark lies on the snow beneath a heavily “flecked” ash tree at Sheboygan Marsh County Park. / Photo Credit: Bill McNee, Wisconsin DNR

Consider the following:

• Determine whether the tree is worth treating. Some ash trees are too heavily infested to save or have structural or health problems that make them unlikely candidates for insecticide treatment. Property owners may save money by removing an untreated ash tree before it becomes heavily infested.
• Trees with abundant woodpecker damage may be too infested for successful insecticide protection. Consult an arborist for a professional opinion.
• Landscape trees can improve views, increase property values, provide shade and cooling and contribute to the quality of life in a neighborhood. Weigh these benefits against the expense of insecticide treatments.
• Consider the cost of removing or replacing trees. You will often be able to treat your ash tree for a decade and spend less money than it would cost to remove that tree. Meanwhile, you get the benefits the tree provides.
• The cost of an insecticide treatment will depend on the tree size, the product used and whether a professional is hired. Some products are applied annually; others are applied every two or three years.
• Check the credentials and pesticide applicator certification of any business you hire to treat your ash trees.
• Unprotected ash that are dead or declining from EAB are often structurally weakened and present a safety hazard. These trees are also more hazardous to remove. Consult a tree care professional.

Signs And Symptoms Of Infestation

Stay informed and be on the lookout for EAB. Know where the pest has already been found and look for the signs and symptoms of EAB infestation. Watch ash trees for the following:

• Woodpecker damage, called “flecking,” where pieces of bark have been removed while feeding on EAB larvae beneath the bark. It usually starts up in the canopy and progresses down the tree over the next few years if the tree is not treated.
• Sprouts growing from the base or trunk of the tree.
• Thinning canopy with smaller, pale leaves.
• Small (1/8-inch), D-shaped exit holes in the bark.
• Green beetles crawling on the trunk of ash trees during the summer.

Visit the DNR EAB webpage for more information.