Emerald Ash Borer: The Treat or Remove Decision in Illinois

If you have an ash tree on your property in Bureau County, you have almost certainly already made this decision or you are about to. There is no version of this where doing nothing preserves the tree.

We have been caring for trees around Princeton for over fifty years, and this is the single largest change we have seen to the tree population in that time. Here is how to think about your specific tree.

What You Are Dealing With

Emerald ash borer is a small metallic green beetle whose larvae feed in the layer just under the bark, the tissue that transports water and nutrients through the tree. Enough larvae and the tree is effectively girdled from the inside.

It arrived in North America decades ago and has since moved through Illinois comprehensively. Ash was planted heavily in subdivisions and along streets through the second half of the last century, often as a replacement for elms lost to Dutch elm disease, which means many properties have several.

The relevant point for a homeowner is that this insect does not moderate. Unlike most pests, which damage a percentage of trees, EAB has killed essentially all untreated ash in areas it has moved through. There is no waiting it out.

Is Your Tree Actually an Ash?

Worth confirming before spending anything, because several trees get mistaken for ash.

Ash has opposite branching, meaning branches and buds emerge directly across from each other rather than alternating. That alone rules out most trees. It has compound leaves with typically five to nine leaflets on a central stem. Mature bark forms a distinct diamond patterned ridge.

Mountain ash is not a true ash and is not affected. Boxelder also has opposite branching and compound leaves and is frequently confused with ash, and it is not a host.

If you are unsure, that is a reasonable thing to have identified before making a decision either way.

Reading the Canopy

The canopy is the deciding factor, and September is a reasonable month to assess it while leaves are still on.

Look at the top of the tree first. EAB damage generally shows from the top down, so thinning and dieback in the upper canopy is the early signal. Compare it against what the tree looked like two or three years ago if you can.

Other signs to look for: D shaped exit holes in the bark, roughly an eighth of an inch across and distinctly flat on one side, which no other pest leaves. Splitting bark. Winding S shaped tunnels underneath bark that has come away. And epicormic sprouts, meaning shoots emerging from the trunk and base, which is the tree attempting to grow around a failing vascular system.

Heavy woodpecker activity is a useful indirect sign. Woodpeckers strip bark to reach larvae, leaving pale patches on the upper trunk and limbs, and they usually find the problem before people do.

The Threshold

Here is the honest guidance, and it is not what everyone wants to hear.

A tree that still has a full or nearly full canopy is a good candidate for treatment. Treatment works well on trees that are still healthy enough to move the product through their vascular system.

A tree that has lost roughly a third or more of its canopy is generally past the point where treatment reliably brings it back, because the very system that would distribute the treatment is what has been damaged. Money spent there frequently delays removal rather than preventing it.

A tree with major dieback, large dead limbs, or bark falling away is a removal decision and increasingly a safety one. Dead ash becomes brittle unusually quickly, which makes removal more dangerous and more expensive the longer it waits. That is a genuine reason not to defer.

What Treatment Involves

The effective approach for a valued tree is trunk injection, where product is introduced directly into the vascular system and distributed through the tree.

It is done by a licensed applicator, it is repeated on a cycle of every one to two years depending on the product used, and it needs to continue for as long as you want the tree. This is the part homeowners most often misunderstand: it is not a cure, it is protection maintained indefinitely.

Cost scales with trunk diameter, since dosage is calculated from it. A large mature ash costs meaningfully more per treatment than a smaller one.

Soil drenches sold at retail exist and are generally less effective on larger trees than professional trunk injection, and there are environmental considerations around soil applied neonicotinoids and pollinators that are worth discussing rather than ignoring.

Timing Within the Year

Trunk injection is generally done during the growing season, when the tree is actively moving fluid and will distribute the product through its own vascular system. In practice that means spring through summer for most applications, with the specific window depending on the product and the applicator's judgment.

Which raises a fair question about why we are writing this in September. The reason is that autumn is the right time to make the decision, not to perform the treatment.

Right now the leaves are still on, which means you can actually assess the canopy. In February a bare ash tells you almost nothing, and by the time leaves return in spring you are already inside the treatment window with no time to think.

Autumn is also when removal is often easier to schedule and less disruptive, since the ground is firm, the leaves are down, and there is less below to damage.

So the useful action this month is assessment. Decide which trees you intend to protect, and get them on the schedule for next season rather than discovering in May that everyone else already booked.

Making the Call

The way we frame it with customers is to weigh the ongoing treatment cost against what the tree is worth to the property.

A large, healthy, well placed ash providing shade to the house is genuinely valuable, and protecting it can compare favorably against removal plus replacement plus the twenty years it takes a new tree to do the same job.

A smaller ash, or one poorly placed, or one already declining, is usually better removed and replaced with something diverse. If you have several ash trees, treating all of them is often not sensible; treating the one or two that matter most and removing the rest is a reasonable middle path.

Whatever you replace with, plant something different. The lesson from both elm and ash is that a landscape dependent on one species is a landscape waiting for one problem.

What to Do Next

Go look at your ash trees while the leaves are still on. Assess the upper canopy specifically, and look for D shaped exit holes and woodpecker damage on the trunk and limbs.

If the canopy is largely intact, treatment is worth considering and you should understand it as an ongoing commitment. If the canopy is thin or dying back significantly, get a removal assessment rather than spending on treatment, and do not let a dead ash stand longer than necessary.

If you would like an honest assessment of a specific tree, Taylor's Way has handled lawn, tree, and pest care around Princeton, Bureau County, and north-central Illinois for over fifty years. Call 815-875-8231 or visit taylorsway.com.

Here is what to expect. We will tell you plainly whether a tree is worth treating, and if it is not, we will say so rather than sell you an injection schedule for a tree that will not recover. On properties with several ash, we will help you decide which ones are worth protecting rather than treating all of them by default.