Key Takeaways

• Emerald ash borer (EAB) has killed the vast majority of untreated ash trees across the greater St. Louis region since it was first confirmed in Missouri, and infested trees that go untreated typically die within two to four years.
• Early-stage infestations, generally those affecting less than 30 to 50 percent of the canopy, respond best to systemic insecticide treatment, while heavily infested trees usually don’t recover regardless of treatment.
• Warning signs include canopy dieback starting at the top, D-shaped exit holes in the bark, increased woodpecker activity, and bark splitting that reveals S-shaped larval galleries underneath.
• Treatment is an ongoing commitment, not a one-time fix. Most systemic treatments need to be reapplied every one to three years for the life of the tree.
• Dead or heavily infested ash trees become structurally unsafe quickly, since EAB damage weakens wood well before visible decay sets in, making prompt removal important once treatment is no longer viable.

Few pests have reshaped the tree canopy of a region as dramatically as emerald ash borer has reshaped St. Louis. Since this invasive beetle was first confirmed in Missouri, it has moved through the metro area and surrounding counties with devastating consistency, and untreated ash trees have died at rates well above 99% in areas where the infestation has run its course. For homeowners across Hazelwood, Florissant, St. Charles, and the broader region who still have ash trees on their property, the question isn’t really whether EAB is a threat. It’s whether a specific tree is still a good candidate for treatment, or whether removal has become the safer, more practical choice.

How Emerald Ash Borer Actually Kills a Tree

EAB is a small, metallic green beetle whose larvae do the real damage. Adult females lay eggs in bark crevices, and the larvae that hatch tunnel into the tree, feeding on the layer just beneath the bark that transports water and nutrients throughout the tree. As larval populations build inside a tree over successive years, these feeding galleries cut off more and more of the tree’s ability to move water and nutrients between the roots and the canopy, essentially starving the tree from the inside out.

This is why ash tree decline from EAB tends to follow a recognizable pattern: dieback starting at the top of the canopy and working downward, since the uppermost branches are furthest from the roots and the first to lose adequate water and nutrient flow. By the time dieback is obvious throughout the canopy, the tree has usually already lost a significant portion of its internal vascular system, which is the point at which treatment options become far less effective.

Recognizing an Infestation Early

Catching EAB early dramatically improves the odds that a tree can be saved. A few signs are worth watching for consistently, especially on ash trees in yards with any known EAB activity nearby.

• Canopy thinning or dieback that starts at the top of the tree and progresses downward over one or more growing seasons is often the first visible sign, though by the time it’s noticeable, the infestation has usually been present for a year or more already.

• D-shaped exit holes in the bark, roughly the size of a grain of rice, mark where adult beetles have emerged after completing their larval stage inside the tree.

• Increased woodpecker activity, sometimes called “flecking,” where woodpeckers strip away sections of outer bark to feed on larvae underneath, is one of the more reliable early indicators, since birds often find an infestation before it’s visible from the ground.

• Bark splitting can reveal the S-shaped tunnels, or galleries, that larvae carve as they feed, which is one of the clearest confirmations of EAB once bark has opened up enough to expose them.

• Epicormic sprouting, where a stressed tree sends out clusters of small shoots directly from the trunk or major limbs, is a stress response that often appears as the tree tries to compensate for lost canopy function.

Any ash tree showing early signs like woodpecker flecking or minor canopy thinning is a strong candidate for a professional evaluation before deciding on a course of action.

When Treatment Actually Works

Systemic insecticide treatment, typically applied as a trunk injection or soil application, can be genuinely effective, but timing and tree condition matter enormously. 

• Treatment tends to work best on trees where less than 30 to 50 percent of the canopy shows dieback, since this indicates the tree still has enough functional vascular tissue to distribute the treatment and continue supporting itself while the insecticide controls the larval population.
• For those trees, ongoing systemic treatment can protect the tree for years, sometimes indefinitely, as long as treatment continues on schedule. This isn’t a one-time application. Most effective systemic treatments need to be reapplied on a one-to-three-year cycle for the life of the tree, since the protection doesn’t persist indefinitely and new larvae can still attempt to infest an unprotected tree.
• Treatment tends to make the most sense for high-value ash trees, mature specimens providing significant shade or screening, or trees with sentimental or landscape value where the ongoing cost of treatment is worth preserving what the tree provides.

When Removal Is the Safer Choice

• Once canopy dieback exceeds roughly 50 percent, the odds of a tree recovering with treatment drop significantly, and in many cases removal becomes both the more practical and the safer option. This isn’t only about whether the tree can theoretically survive. It’s about what happens structurally while a heavily infested tree is dying.
• EAB-damaged wood becomes brittle and prone to sudden limb or trunk failure well before it looks obviously dead from the ground, since the larval galleries compromise the wood’s structural integrity long before visible rot sets in. 
• A dead or dying ash tree near a home, driveway, or area where people regularly walk represents a real and growing safety risk the longer it’s left standing, and ash wood in particular is known for becoming unpredictably brittle as it dries out after death, sometimes shedding large limbs with little warning.
• For trees that have already lost the majority of their canopy, removal, followed by stump grinding and, where appropriate, replanting with a resistant species, is generally the recommended path forward.

Making the Decision

The right call depends on a combination of factors best evaluated in person: 

• how much of the canopy is already affected
• how many EAB indicators are present
• the tree’s overall structural condition, its proximity to structures or high-traffic areas, and how much ongoing treatment cost the property owner is willing to commit to. 

An experienced arborist with EAB in the St. Louis, Missouri can assess all of these factors together and provide a clear recommendation, rather than leaving a homeowner to guess based on how the tree looks from the yard.  Mathias Precision Tree Service helps St. Louis and Hazelwood homeowners manage emerald ash borer threats with professional tree evaluations. Our ISA-Certified Arborists assess tree health, structural risks, and treatment options. When removal is necessary, we provide safe tree removal solutions to protect your property. We also offer pruning, stump grinding, and ongoing tree care recommendations to support long-term landscape safety. With over 20 years of Missouri experience, we provide honest assessments and reliable solutions.

Frequently Asked Questions About Emerald Ash Borer in St. Louis

Is it too late to treat my ash tree if I’m already seeing some dieback?

Not necessarily. Trees with less than roughly 30 to 50 percent canopy dieback are often still good candidates for treatment, though the sooner treatment starts relative to when symptoms appear, the better the odds of success.

How much does ongoing EAB treatment typically cost compared to removal?

Costs vary by tree size and treatment method, but ongoing treatment over a tree’s remaining lifespan is an extended financial commitment, which is part of why the decision often comes down to how much value the specific tree provides.

Can a completely dead ash tree still be treated?

No. Once a tree is dead, insecticide treatment can’t reverse the damage, and a dead ash tree should be evaluated for removal rather than treatment.

Are all ash trees in the St. Louis area at risk, even if I haven’t seen EAB nearby?

Given how thoroughly EAB has spread through the region, any untreated ash tree in the greater St. Louis area should be considered at risk, even without confirmed local sightings.

What should I plant if I have to remove an ash tree?

A certified arborist can recommend species well-suited to the site conditions and resistant to the pest pressures common in the region, which helps avoid repeating a similar loss with a different pest or disease down the road.

Summary Checklist: EAB Treatment vs. Removal Decision

Final Advice

Emerald ash borer doesn’t leave much room for a wait-and-see approach. The window where treatment can genuinely save a tree is real, but it narrows quickly once canopy dieback sets in, and a heavily infested ash tree left standing becomes a structural hazard well before it looks obviously dead. Getting an ash tree evaluated as soon as any warning signs appear gives homeowners the best chance at an informed decision, whether that’s ongoing treatment or safe, timely removal.

If you have an ash tree showing signs of decline, Mathias Precision Tree Service can evaluate its condition and give you a clear recommendation on treatment versus removal.