Protect Your Valuable Ash Trees Against Emerald Ash Borer

By Bill McNee, DNR Forest Health Specialist, Oshkosh 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 pest is currently the most damaging threat to trees in Wisconsin, killing more than 99% of the ash trees it infests.

Ash tree branch in West Allis has been damaged by woodpeckers. Sections of bark are missing or have been flecked away.

This ash tree branch in West Allis has been damaged (“flecked”) by woodpeckers feeding on EAB larvae under the bark.
Photo: Bill McNee, Wisconsin DNR

A common first sign of EAB infestation is woodpecker damage that is created when birds feed on EAB larvae beneath the bark of ash trees. Treatment of infested ash trees is more likely to succeed if the trees have low or moderate levels of woodpecker damage.

Now is a good time to consider insecticide protection, because the treatments are typically done 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.

EAB has become so widespread that treatments are worth considering anywhere in Wisconsin. The highest risk of infestation is in communities already known to be infested or within 15 miles of a known infestation. The risk is lower elsewhere, but it is widely believed that there are additional undetected EAB infestations. More EAB detection information can be found at

Location isn’t the only thing to consider when deciding to treat. For example, the treatments are not economically practical for woodlot ash trees and need to be repeated every one to three years (frequency of treatments will depend on the product and method used).

A Wisconsin map indicates where EAB has been found as of March 2022.

Communities and tribal lands known to have EAB as of March 1, 2022 are shown in green and blue.
Photo: Wisconsin Department of Agriculture, Trade and Consumer Protection

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. Additionally, you can search for a certified arborist at the Wisconsin Arborist Association’s website. Other businesses that also conduct EAB treatments may be found online or in a phone book.

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, product used and whether a professional is hired. Some products are applied annually, while 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.
Ash bark lies on the snow beneath heavily “flecked” ash trees at Sheboygan Marsh County Park.

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

Signs And Symptoms Of An 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 (one-eighth 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.

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