Treat Your Valuable Ash Trees Against Emerald Ash Borer

Bill McNee, Forest Health Specialist, Oshkosh. or 920-360-0942

Property owners with healthy, valuable ash trees should consider treating 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.

Woodpecker damage during the winter is often the first sign that an ash tree is infested. Now is an excellent time to consider insecticide protection because the treatments are typically done between mid-April and mid-May once leaves begin to return.

Treatments on already-infested ash trees are more likely to be successful if the trees have low or moderate levels of woodpecker damage.

An ash tree branch with bark missing after woodpeckers attacked it while looking for larvae to eat.

This ash tree branch in West Allis has been attacked by woodpeckers looking for larvae to eat.

Ash (in the genus Fraxinus) are the only trees that need to be protected against EAB. Mountain ash and prickly ash do not need protection because the insect does not attack them.

Emerald ash borer 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. Elsewhere the risk is lower, but it is widely believed that there are additional, undetected EAB infestations. Visit the Wisconsin EAB website for a map of known EAB infestations.

Location isn’t the only consideration 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).

What You Should Do

If EAB has been found in your local area or you see any of the signs or symptoms of an infestation in your ash trees, look for information online or seek advice from a tree care professional. 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.

Some of the insecticide products can be applied by anyone, 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 on insecticides.

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.
  • Trees that have large amounts of visible woodpecker damage may be too infested to be saved by insecticides. 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 more than a decade and spend less money than it would cost to remove that tree.
  • The cost of an insecticide treatment will depend on the tree size and product to be used. 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.

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 looking for EAB larvae to eat. 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.
  • Very small (1/8”), D-shaped exit holes in the bark.
  • Adult emerald ash borer beetles present during the summer.


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