What Are Emerald Ash Borers?

Emerald ash borers (Agrilus planipennis) are invasive beetles that were likely transported from Asia to North America in wooden shipping crates around 2002. These pests target ash trees, including white, black, and green ash, as well as fringe trees and olive trees. However, European mountain ash, American mountain ash, showy mountain ash, and prickly ash are usually immune to ash borer damage.1

Emerald ash borer adults measure about a third of an inch long so they can be hard to spot. They are capable of flying short distances, although they usually stay near ash trees. Adults are most active in late May to early July when they emerge from D-shaped holes in ash tree bark and feed on ash tree leaves. After they feed, the borers mate, lay eggs in the bark crevices of ash trees, and die just two to three weeks after emerging.

When the borer eggs hatch about 7 to 10 days later, the inch-long, cream-colored larvae tunnel inside ash trees until they find the tree’s phloem and xylem—the parts of the tree that transport nutrients and water between the tree’s roots and leaves. The larvae feed on the tree through winter, carving serpentine tunnels that ultimately interfere with the tree’s ability to feed itself. When spring arrives, the larvae pupate into adults, emerge from the tree, and the 1-year life cycle repeats.

Identifying Emerald Ash Borer Damage

Emerald ash borers don’t just harm ash trees. They also affect all the species that depend on ash trees for food and shelter, including native insects and songbirds. If you notice any of these signs of ash borer damage, take action as soon as you can to limit the spread of these invasive pests and help protect native ash trees and the wildlife that depend on them.

• Increased woodpecker activity is usually the first sign of emerald ash borers, and it’s a good idea to inspect any trees that woodpeckers are drawn to. That said, woodpeckers are attracted to other types of wood-boring insects, and woodpecker activity doesn’t always indicate that borers are present.
• Canopy thinning, crown death, branch dieback, and wilting leaves occur when borer larvae feed on the vascular tissues of ash trees and interrupt the flow of water and nutrients from the trees’ roots. These issues can also occur due to drought, winter damage, and other stressors. If multiple ash trees in a small area are showing these signs of stress, borers may be to blame.
• D-shaped holes in ash bark are clear indicators that emerald ash borers are around. These ⅛-inch wide holes are left behind when adult borers emerge after winter.
• Bark splitting might occur as borer infestations progress. If you look inside these cracks, you may notice that the interior of the tree is riddled with squiggly, S-shaped tunnels or “larval galleries” where borer larvae have fed.
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• Suckering can begin as ash trees become stressed by borers and produce small leafy stems near their base. However, like canopy thinning and woodpecker activity, suckering can occur due to other stressors, and it’s not a clear indicator of borers.
• Visible borer adults, pupae, or larvae, and sawdust-like borer frass can sometimes be spotted near affected trees or in the larval galleries beneath ash bark. Just be sure you don’t confuse emerald ash borers with lookalikes, like the six-spotted tiger beetle, the dogbane beetle, and the native two-lined chestnut borer.
• Tree death can occur in just two to five years after borers invade ash trees. These pests can also transmit certain fungi,2 which accelerate rot in affected trees and increase the risk of limb falls and other hazards.

Before diagnosing an ash borer problem, make sure you’re dealing with an ash tree or related species that the borers attack. Investing in a good tree ID book can help you identify the trees in your garden.

How to Prevent and Treat Ash Borer Infestations

Even if you don’t see any signs of emerald ash borers, there a few things you can do to avoid an infestation and limit the spread of these destructive pests. If your ash trees are showing symptoms, here’s how to figure out the best action to take.

Prevention

If you live within a 15-mile radius of known borer infestations, protect your healthy ash trees with a systemic pesticide applied every couple of years to eliminate any borers before they can cause lethal damage. These treatments are best done by a professional arborist, especially if you’re working with a large tree. It’s also good practice to never transport firewood from one location to another and to purchase nursery plants locally to limit the spread of potential pests.

Management

Early detection is essential for successfully controlling borers with pesticides applied to the tree through injections or soil drenches before the insects can cause much damage. As soon as you suspect borers are present, have an arborist confirm the diagnosis and develop a treatment plan to save the tree. It’s also important to report any borer sightings or sickly ash trees to your state’s forestry service or a similar entity, especially if borers haven’t been found in your area before.

Removal

If ash borer problems progress and treating trees is no longer an option, the best solution is to have affected trees removed. This is usually performed from November to April when ash borers are dormant and less likely to spread to a new location. Once the tree is down, follow proper safety protocols when disposing of the debris or ask if the tree company can handle the debris for you.

Avoidance

While there is no need to preventively remove healthy ash trees, you should avoid planting new ash trees if you can help it. Instead, plant native trees that aren’t affected by borers, such as native oaks, silver maples, and river birches. These trees have a similar look and feel to ashes, and they support local wildlife without attracting borers.