Emerald Ash Borer: Symptoms and Treatments

June 16, 2023

Ash trees are susceptible to a number of pests and diseases, including anthracnose, yellows, and rust, but the Emerald Ash Borer is by far the most destructive. The EAB is a serious threat that essentially strangles the tree, causing rapid decline and death. Young or stressed trees may succumb in as little as a season or two, while even robust, mature trees may die within four to five years. Over the last few decades, EAB has caused extensive damage to the woodland systems throughout the northeast. It is important that businesses and homeowners have their trees inspected regularly for signs of EAB. Most infestations, when caught early, are treatable and reversible. However, many homeowners end up being too late.

What are the warning signs of the EAB?

A close up of a tree trunk with worms carved into it.

Canopy thinning and crown dieback – as larvae feed beneath the bark, they damage tissues responsible for transporting water and nutrients. Initial damage appears as thinning in the upper canopy of the tree. Over time whole branches and leads can die.  Other conditions (disease, soil compaction, etc.) can cause canopy die-back in ash trees, so canopy thinning alone does not definitively indicate EAB.


Epicormic Sprouting – when trees are stressed, they may grow new branches and leaves wherever they still can (suckering). Stressed ash trees may have new growth at the base of the trunk or on main branches.  As with canopy thinning, other sources of stress can cause suckers around the base of ash trees, though this symptom does not specifically indicate EAB.

Woodpecker Damage – trees infested with EAB are often sought out and attacked by woodpeckers.  This damage is quite noticeable because when woodpeckers remove the outer bark, the lighter-colored inner bark is revealed, resulting in what is known as “blonding” (after a while the exposed inner bark turns gray due to weathering). Although outer bark can fall off for other reasons, when it has been flaked off by woodpeckers foraging on larvae, the inner bark will also show numerous, irregular holes where woodpeckers have extracted prey.  However, woodpeckers will attack any tree full of insect larvae. Woodpecker activity can be an indicator of EAB, but it does not specifically confirm an EAB infestation. 

D-Shaped Exit Holes – as they emerge from ash trees in June and July, adult Emerald Ash Borers leave behind distinct D-shaped exit holes. These holes are approximately 1/8” wide and can be oriented in any direction. These are a strong indicator of EAB.  Exit holes of native borers will be round or oval and can vary in size.


S-Shaped Galleries & Splitting Bark – trees infested with EAB have a distinct S-shaped tunnels beneath the bark.  As EAB larvae feed, trees attempt to create callus tissue around larval galleries, which can cause the bark to split vertically.  The S-shaped can often be seen beneath the split bark.  No other insect makes looping galleries such as these, which makes this evidence indicative of EAB infestation. 

 

In addition to the D-shaped exit holes and the S-shaped tunnels, finding EAB larvae, pupae, or adults will confirm an EAB infestation.  No other insect larvae in ash trees have white, bell shaped body segments like EAB. 

Emerald Ash Borer Treatment Options

There is little to be done to save an infected tree, however, control measures can be taken to reduce damage to infested trees and prevent infestation from spreading.


The following are the most effective Emerald Ash Borer treatment and control methods:


Trunk injection –

  • Who applies treatment: Licensed pesticide applicators (LPAs)
  • Treatment process: An LPA will drill through the bark and into the outer sapwood of the ash tree, where they will inject the insecticide.
  • Frequency: 1-3 years, depending on the selected product
  • Effectiveness: When properly administered by an LPA, some trunk injections have shown to be 90 percent effective at controlling EAB, making this treatment the most effective method by far
  • Drawbacks: Cost and potential drilling wounds. While the most effective, trunk injections that last multiple years are typically more expensive than other EAB treatments.


Soil Drench –

  • Who applies treatment: LPAs, or property owners (if ash tree is under 12” diameter at chest height)
  • Treatment process: Though soil drenches vary, they’re typically applied in the form of liquid or as granules that are then watered into the bare soil around the, thus allowing the product to take hold in the roots of the tree
  • Frequency: The ideal frequency varies by product and the size of the ash tree being treated. However, soil drench treatments should not be applied more than once a year.
  • Effectiveness: Inconsistent. In some trials, EAB control was excellent, while others yielded poor results. That said, soil drenches administered by tree professionals proved to be more effective.
  • Drawbacks: Effectiveness and environmental impact. While they proved to be better EAB control than no treatment at all, soil drenches are significantly more likely to fail than trunk injections. They’re also more likely to impact other plants, insects, and animals that may be near the tree.


Bark Spray –

  • Who applies treatment: LPAs
  • Treatment process: This formulated insecticide is sprayed on the bottom five to six feet of the ash tree using a common garden sprayer. That insecticide then penetrates the bark and is transported systemically throughout the tree.
  • Frequency: Once annually
  • Effectiveness: Inconsistent. The results of effectiveness testing on bark sprays have yielded similar results when compared to the testing on soil drenches.
  • Drawbacks: Effectiveness and environmental impact. Pesticides can blow onto adjacent plants, potentially impacting plants, insects, and animals nearby.


Remove and Replace –

  • Who applies treatment: Licensed tree care professionals
  • Treatment process: While some property owners may be savvy enough to plant a tree on their own, removing a tree is a complicated process that requires a tree professional – oftentimes for permitting or safety reasons
  • Frequency: Just once
  • Effectiveness: 100 percent. While getting a new tree to take hold presents its own challenges, removing and properly disposing of an ash tree is 100 percent effective at controlling the spread of EAB.
  • Drawbacks: Losing a tree that may have intrinsic value to the property owner



If you have ash trees, it is very important that you have your trees inspected on a regular basis and treated to prevent infestations. The impact of EAB has been significant, but ongoing efforts in research, management, and public participation aim to mitigate the damage and protect ash tree populations for future generations.

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