Emerald Ash Borer Treatment

Is your ash tree worth saving?

An ash tree is worth treating if it has lost less than about a third of its canopy, is close enough to a known infestation to be at real risk, and is valuable enough to you to justify treating it indefinitely. Those three tests — condition, proximity, value — are the whole decision, and it is worth being unsentimental about each of them before you spend anything.

Condition. Extension programs converge on canopy loss as the practical measure. A tree with up to roughly 30 percent canopy thinning is generally considered a good candidate for a rescue treatment with trunk-injected emamectin benzoate, and often recovers. Above roughly 50 percent canopy loss, treatment is generally not recommended — too much of the conductive tissue that would carry the insecticide is already destroyed — and by 60–70 percent crown loss the tree is past saving. Different state programs state these thresholds slightly differently, so read them as a band rather than a bright line, and get an arborist's read if your tree is near the boundary.

Proximity. Treating an ash that is nowhere near an infestation is wasted money. The rule of thumb extension entomologists use is that preventive treatment becomes worthwhile once EAB is confirmed within about 15 miles of your tree; some programs widen that to 15–30 miles as an early-warning band. Below that distance, preventive treatment of a healthy tree is far more effective — and cheaper over time — than a rescue treatment later.

Value. Because protection has to be renewed every one to two years, the cost is a subscription, not a purchase. A large, structurally sound ash shading the south side of a house, or one that would cost thousands to remove and replace with anything comparable, usually justifies it. A crowded, poorly formed, or already-declining ash at the back of the lot usually does not — and the money goes further on removal plus a replacement of a species EAB does not attack.

Not sure your tree even has EAB? Check it against the signs and symptoms first — a thinning canopy has several other causes — or, if you are not certain what species the tree is, start with what's killing my tree?

How EAB treatment works

EAB treatment works by putting a systemic insecticide into the tree's own circulation, so the tree itself becomes the delivery mechanism. The product is introduced at the roots, through the bark, or directly into the trunk; the tree's water-conducting tissue carries it up and out into the canopy; and the beetles are killed where they feed — adults nibbling on leaves in late spring, and, more importantly, young larvae feeding in the phloem just under the bark.

Two consequences follow from that, and both matter. First, treatment depends on a functioning vascular system, which is exactly what an advanced infestation has been destroying — the reason a badly thinned tree cannot be rescued is partly that it can no longer distribute the product. Second, systemic insecticide does not persist indefinitely; it is metabolised and diluted by growth. Most soil-applied products need reapplying every year; professionally injected emamectin benzoate is the exception, generally holding for two years per treatment.

Timing is set by the tree's calendar, not the beetle's. Soil-applied products need weeks to move from the roots into the canopy, so they go on earliest — commonly mid-April to mid-May in the Midwest, or around bud break. Trunk injections work faster and go on slightly later, generally from bud break into June. Your local extension office publishes the window for your state; a treatment applied at the wrong time can be almost entirely wasted.

Soil drench vs trunk injection vs bark spray

The three delivery methods differ mainly in how much of the product reliably reaches the canopy, how long it lasts, and whether you can legally and practically do it yourself. Trunk injection is the most effective and the only one that buys two years at a time; soil drench is the only genuinely homeowner-friendly option; basal bark spray sits in between and is mostly a professional tool.

For a homeowner treating a modest ash themselves, the soil-applied route is the one that actually exists: a granular or liquid imidacloprid tree-and-shrub product applied as a soil drench around the root flare, plus a measuring jug so the rate matches the trunk circumference on the label rather than a guess. Research cited by extension programs found soil applications of homeowner imidacloprid products gave excellent protection for ash under about 15 inches diameter at breast height (roughly 47 inches around); on trunks much larger than that, the same product is a weaker bet and injection is the better spend.

Trunk injection is deliberately not a DIY item on this page. The equipment exists — plugs, ports, and injection kits are sold — but the products that make injection worth doing, emamectin benzoate above all, are not available to homeowners, and drilling injection ports into a tree you value is a skill worth paying for. If a page tells you otherwise, it is selling you equipment for a chemical you cannot legally buy.

When to call a professional arborist

Call a certified arborist whenever the tree is large, the tree is already symptomatic, or the right product is one you cannot buy — which covers most cases where the tree genuinely matters. Concretely:

• The trunk is more than about 15 inches in diameter at chest height (about 47 inches around). Homeowner soil products lose reliability above that size, and injection becomes the sensible option.
• The canopy is already thinning. A rescue treatment on a symptomatic tree means emamectin benzoate injection, which is professional-only. A soil drench at this stage is likely to disappoint.
• You need a treat-or-remove verdict. Estimating canopy loss from the ground is harder than it sounds, and this is the judgement call the whole decision hangs on.
• The tree is near a structure, a driveway, or a power line. If the answer turns out to be removal, a dead ash is unusually brittle and hazardous to take down — this is not a do-it-yourself removal.
• Your state restricts the product. Where imidacloprid is a restricted-use pesticide, a licensed applicator is the only legal route.

Look for an ISA Certified Arborist, ask specifically what product and method they intend to use and on what interval, and be wary of anyone who promises a permanent fix from a single visit.

What NOT to do

Skip anything that promises to cure emerald ash borer without a systemic insecticide — there is no such thing, and several popular substitutes actively waste the window in which the tree could still have been saved. Specifically:

• Cover sprays on the bark and leaves. Spraying the outside of the tree cannot reach larvae feeding under the bark, which is where essentially all the damage happens. At best it kills a few adults; it does not protect the tree.
• Home remedies and "natural" concoctions. Dish soap, vinegar, salt solutions, garlic sprays, and injected household chemicals do not control EAB, can damage the tree or the soil, and are illegal to use as pesticides. Extension services warn against these for exactly this reason.
• Fertilizing an infested tree as a treatment. Fertiliser does not deter EAB, and pushing growth on a girdled tree does not restore the conductive tissue that has been destroyed.
• Treating a tree that is already too far gone. Past roughly 50 percent canopy loss, the money is better spent on removal and replanting. Paying to inject a tree that will die anyway is the most expensive version of this mistake.
• Moving the wood. If the tree comes down, do not haul the firewood elsewhere. Long-distance EAB spread has been overwhelmingly human-assisted through firewood and untreated ash logs.

Cost and timing expectations

Expect an ongoing cost rather than a one-off, and expect the annual figure to scale with trunk diameter, because every product is dosed by tree size. A homeowner soil drench on a modest ash is the cheapest route by a wide margin but has to be repeated every year and is dosed by trunk circumference. Professional trunk injection costs substantially more per visit but covers two years, which narrows the gap on an annualised basis — and on a large tree it is often the only thing that works, which makes the comparison moot.

We deliberately do not quote dollar figures here: professional pricing varies enormously by region, tree size, and access, and any number printed on a page like this would be stale and misleading within a season. Get two or three local quotes, and ask each to price the program — what the next six years cost — rather than the next visit.

On timing: put the treatment in the spring window your extension office publishes, applying soil products first (they need lead time to move up the tree) and injections slightly later. Then plan the renewal into your calendar. The single most common way a treatment program fails is not a bad product choice — it is a year that gets skipped.

Next: read the signs and symptoms guide to judge where your tree sits on the canopy-loss scale, go back to the emerald ash borer overview for the biology behind all of this, or start at what's killing my tree? if you are not yet sure EAB is the culprit.