Emerald Ash Borer in Southeastern Wisconsin: Treat Your Ash Tree or Remove It?

If you have an ash tree in your yard in southeastern Wisconsin, the question is no longer whether emerald ash borer (EAB) is nearby. As of 2024, according to UW-Madison Extension, all 72 of Wisconsin’s counties have confirmed EAB infestations. The question is what to do about your tree, and the answer depends on how healthy it is right now.

What emerald ash borer does to an ash tree

EAB is an invasive, wood-boring beetle that kills ash trees by eating the tissues under the bark. The Wisconsin DNR reports it was first detected in the United States in 2002 and arrived in Wisconsin in 2008. Wisconsin has been under a statewide EAB quarantine since 2018, which mostly matters for moving firewood and ash material around.

The DNR also notes that mountain ash and prickly ash are not true ash trees and are not attacked by EAB. If you are not sure what you have, that is an easy thing for us to confirm on site.

Signs your ash may be infested

These are the signs the DNR lists:

• Thinning leaves at the top of the tree, usually within one to three years of infestation, followed by dying branches.
• New sprouts at the base or along the trunk, often just below where the larvae are feeding.
• Vertical splits in the bark over the larval tunnels.
• Woodpecker damage, where woodpeckers pick at the outer bark looking for larvae.
• D-shaped exit holes about 1/8 inch across, and winding tunnels packed with sawdust-like frass under the bark.

Thinning in the crown can have other causes too, such as disease or other insects, so a thin ash is not automatically an EAB ash. That is one reason to have someone look at it.

The deciding question: how much canopy is left?

UW-Madison Extension puts it plainly: insecticide treatments are significantly more effective on infested ash trees with less than 50 percent canopy thinning. Trees with more thinning than that should be removed rather than treated.

So the practical rule looks like this:

• Mostly full canopy, sound structure, good location: treatment is worth considering.
• Half the crown or more already thin: treatment is unlikely to pay off. Plan the removal.
• Poor shape, structural damage, or a bad spot, such as near a power line: Extension classes these as lower value trees. Treatment money is usually better spent elsewhere.

What treating actually involves

Treatment does not cure a tree and walk away. Extension says that most insecticides registered for EAB require yearly applications, for example soil treatments with imidacloprid or basal bark treatments with dinotefuran. Products containing emamectin benzoate are labeled for two years of protection, and the trunk injected versions are intended for professional insecticide applicators. Another Extension page notes that treatment may be required for the rest of the tree’s life.

That is the real trade: you are choosing an ongoing commitment to keep a tree you value. Treatments should always be applied by licensed applicators who follow the product label directions, so ask who is doing the work and which product they plan to use.

Why waiting on a doomed ash is the expensive choice

Extension warns that ash trees are known to become brittle and hazardous relatively quickly, which can increase removal costs. The DNR likewise advises removing ash before EAB has killed the tree, to avoid the safety hazards of EAB killed trees. Extension adds that removing recently killed trees while they are still structurally sound can be safer and more cost effective.

A dead ash over a driveway, play area, roof or power line is not a tree to put off. If the canopy is past the point where treatment helps, scheduling the removal sooner generally means a safer job.

If the ash is in the strip between the sidewalk and the street, it may belong to your municipality. Extension suggests contacting your local town, village or city first.

Treat or remove: a quick way to decide

1. Is it really an ash? Confirm the species first.
2. How much of the crown is thin? Under 50 percent leaves treatment on the table. Over 50 percent points to removal.
3. Is the tree worth the long haul? Good health, good shape and a good location are what make a tree worth yearly or every other year treatment.
4. What is under it? The closer it stands to people and property, the less time you have to decide.

Our guide on saving vs. removing a tree explains how we weigh risk, health, location and cost for any tree, ash included.

Get your ash looked at

You do not have to guess. Johannes Tree Care can assess your ash, tell you what we see in the canopy and structure, and talk through whether treatment or removal makes more sense for your yard. Request a free estimate or ash assessment and we will take a look.

Johannes Tree Care, certified arborists, southeastern Wisconsin.