The emerald ash borer has been present in Ohio since at least 2003 and has now spread to all 88 counties in the state. This invasive beetle, native to Asia, has killed tens of millions of ash trees across North America, and Ohio was one of the earliest and hardest-hit states. If you have ash trees on your property, understanding how to spot an infestation and weigh your options is essential.
How the Emerald Ash Borer Kills Trees
The adult beetles are small, metallic green insects about half an inch long. But the real damage comes from the larvae. Female beetles lay eggs in bark crevices, and once the larvae hatch, they tunnel into the layer just beneath the bark where the tree transports water and nutrients. These tunnels, called galleries, have a distinctive S-shaped or serpentine pattern and are about 1/8 inch wide, packed with a fine mixture of sawdust and insect waste.
As the larvae feed, they effectively cut off the tree’s circulatory system. A healthy ash tree can survive an initial wave of feeding, but repeated years of infestation destroy enough of this critical layer that the tree starves. Most untreated ash trees die within three to five years of the first signs of infestation.
Signs Your Ash Tree Is Infested
The earliest visible symptom is thinning in the upper canopy. Branches near the top of the tree begin dying back, and the crown looks increasingly sparse. Canopy thinning alone doesn’t confirm the emerald ash borer, though, since other problems like disease and soil compaction cause similar symptoms.
The most reliable sign is the D-shaped exit holes adult beetles leave when they emerge from the tree in June and July. These holes are roughly 1/8 inch wide, noticeably smaller and flatter than the round holes left by native wood-boring insects. The flat side of the “D” can face any direction. If you peel back loose bark, you may also see the S-shaped galleries underneath, sometimes with cream-colored larvae still inside.
Another telltale sign is vertical bark splitting. As larvae tunnel beneath the surface, the tree tries to heal by producing callus tissue around the galleries. This response causes the bark to crack and split lengthwise, often revealing the tunnels underneath. You may also notice a sudden increase in woodpecker activity on the trunk. Woodpeckers feed heavily on the larvae and can strip bark in patches, giving the trunk a blotchy, lighter appearance.
Treatment Options for Ash Trees
If your ash tree is still relatively healthy, with less than roughly 30 to 50 percent canopy loss, treatment can be effective. The most widely recommended approach is a professional trunk injection using emamectin benzoate, which can protect a tree for two to three years per treatment. This is the active ingredient with the strongest track record against the emerald ash borer. Treatments can be applied from April through September.
Other treatment methods include soil drenches and trunk sprays using different active ingredients. Soil-applied treatments are typically done in spring or fall and need to be repeated annually. Bark sprays target adult beetles before they lay eggs and may require multiple applications during the emergence season. These approaches generally offer less consistent protection than trunk injections but can work for smaller trees or as part of a broader management plan.
Professional treatment typically costs $200 or more per application, depending on the size of the tree, the product used, and your location. Since trunk injections with the most effective products last two to three years, the ongoing cost of keeping a large ash tree alive works out to roughly $100 or more per year. That’s a meaningful expense, but it’s worth comparing to the alternative.
Treatment Versus Removal
Removing a small ash tree (under 25 feet tall) typically costs several hundred dollars when done by a professional arborist. Larger trees often run $1,000 or more, and the price climbs significantly if the tree is near power lines, buildings, or other structures. Stump grinding adds to the total. Dead ash trees become brittle quickly, which makes them more dangerous and more expensive to remove the longer you wait.
The decision comes down to the tree’s current health and its value to your property. A large, structurally sound ash tree that provides significant shade and adds to property value is often worth treating. A tree that has already lost most of its canopy, shows extensive bark splitting, and has heavy woodpecker damage is unlikely to recover even with treatment. In that case, removal sooner rather than later is the safer and more cost-effective choice, since dead ash trees shed large branches unpredictably.
Biological Control Efforts in Ohio
Ohio is one of several states where federal scientists have released tiny parasitic wasps that attack the emerald ash borer. These wasps, originally collected from the beetle’s native range in northeast China, are so small they’re essentially invisible to people and do not sting. Three species were approved for release in the United States starting in 2007, each targeting a different life stage of the beetle. One species attacks the eggs, while the other two parasitize the larvae inside the tree.
The results have been genuinely encouraging. At study sites in Michigan, researchers from the USDA documented a 90 percent decline in live emerald ash borer larvae over a five-year period. Egg parasitism by one of the introduced wasps increased from less than 5 percent in the early years to nearly 30 percent by 2013. Both egg-attacking and larva-attacking wasps have been confirmed as established and spreading in Ohio, along with several other states.
These biocontrol agents won’t save individual trees fast enough to replace chemical treatment for a valued yard tree. Their role is broader: gradually reducing emerald ash borer populations across the landscape so that younger ash trees and new seedlings have a better chance of surviving. Over decades, this could allow ash species to persist in Ohio’s forests rather than being effectively eliminated.
Which Trees Are at Risk
All native North American ash species are vulnerable to the emerald ash borer. In Ohio, the most common species are white ash, green ash, and blue ash. Green ash is particularly widespread because it was heavily planted as a street and yard tree for decades due to its hardiness and fast growth. If you aren’t sure whether a tree on your property is an ash, look for compound leaves with five to nine leaflets arranged in opposite pairs along the stem, with one leaflet at the tip. The bark on mature trees has a distinctive diamond-shaped ridge pattern.
Mountain ash, despite the name, is not a true ash and is not affected by the emerald ash borer. The same goes for other trees with “ash” in their common name but belonging to different plant families.