Ash trees face a handful of insect pests, but one dominates the conversation: the emerald ash borer, an invasive beetle that has killed hundreds of millions of ash trees across North America. Beyond that headline threat, ash trees also attract plant bugs, lace bugs, aphids, and native borers that cause less dramatic but still visible damage. Knowing which pest you’re dealing with determines whether you need urgent action or can simply wait it out.
Emerald Ash Borer: The Most Destructive Threat
The emerald ash borer is a metallic green beetle, originally from Asia, that targets every North American ash species. The adults are small, only about half an inch long, and feed on leaves without doing much harm. The real destruction happens out of sight. Females lay eggs in bark crevices, and the larvae hatch and tunnel into the inner bark, feeding on the tissue that transports water and nutrients. Over several years, the feeding intensifies until the tree is essentially girdled, cutting off its own supply lines. Death follows.
The beetle was first detected in Michigan in 2002 and has since spread across dozens of states. It was found in Oregon in July 2022, expanding its reach to the Pacific Northwest. Once it arrives in an area, unprotected ash trees rarely survive.
How to Spot an Emerald Ash Borer Infestation
Early detection makes the difference between saving a tree and losing it. Look for these signs, roughly in order of how they appear:
- Thinning canopy. The tree’s crown starts losing leaves from the top down. Branches that once leafed out fully now look sparse, especially in upper sections.
- Epicormic sprouting. Stressed trees try to compensate by pushing new growth from the base of the trunk or along major branches. Clusters of small shoots in unusual locations signal trouble.
- Bark splitting. As larvae tunnel beneath the bark, the tree produces callus tissue that causes vertical splits. Peeling back loose bark reveals the telltale S-shaped galleries packed with fine sawdust-like material called frass.
- D-shaped exit holes. Adult beetles chew their way out between June and July, leaving exit holes about 1/8 inch wide with a distinctive flat side, forming a “D” shape. These can face any direction on the bark and are one of the strongest confirmations of an infestation.
- Woodpecker activity. Woodpeckers feed heavily on the larvae, so unusual flaking or stripping of bark by birds can be an indirect clue.
Ash Plant Bugs
Ash plant bugs are a much less serious pest, but they’re common enough that you’ll likely notice their damage before you ever see the insects themselves. These small bugs use needle-like mouthparts to pierce leaves and feed on sap. The feeding creates tiny yellow spots that merge together over time, giving leaves a blotchy, washed-out appearance. In heavier infestations, leaves wilt, turn brown, and look scorched along the edges. They can also twist and become visibly deformed.
A reliable clue is the underside of affected leaves: both nymphs and adults leave small, black, sticky droppings there. Damaged leaves typically stay on the tree through the fall season rather than dropping early. While the cosmetic damage can look alarming, ash plant bugs don’t kill trees or cause lasting structural harm. Healthy trees tolerate the feeding without intervention.
Other Common Ash Tree Insects
Several other insects feed on ash trees, and most are native species that cause limited damage:
- Lace bugs. These feed on leaf undersides and cause stippling similar to plant bugs. You can identify them by the lace-like pattern on their wings. Damage is cosmetic.
- Aphids. Ash trees attract several aphid species that cluster on new growth and produce sticky honeydew. Heavy infestations cause leaf curling and can encourage sooty mold, a dark fungal coating on leaves and branches below the colony.
- Native borers. Several wood-boring insects, including the lilac/ash borer (a type of clearwing moth), attack stressed ash trees. Their exit holes are round, not D-shaped, which is the key distinction from emerald ash borer. Native borers generally target trees already weakened by drought, injury, or poor growing conditions, and they rarely kill otherwise healthy trees.
The round-versus-D-shaped exit hole distinction matters enormously. If you see round holes in your ash tree, the problem is likely a native borer and far less urgent. D-shaped holes mean emerald ash borer, and the clock is ticking.
Protecting Ash Trees From Emerald Ash Borer
If emerald ash borer has been detected within 15 miles of your property, preventive treatment is worth considering for any ash tree you want to keep. Treatment works best while the tree is still healthy. Trees that have lost up to 50% of their canopy can sometimes be saved with insecticides, but success drops sharply beyond that point, and nearly all treatments are most effective before significant damage occurs.
Two main approaches are used. Soil treatments involve drenching the base of the tree with a systemic insecticide that the roots absorb and distribute through the trunk. This method works well for smaller trees (under about six inches in trunk diameter) in the first year. Larger trees often need two consecutive years of soil treatment before they’re fully protected. Trunk injections, performed by a certified arborist, deliver the insecticide directly into the tree’s vascular system. These are generally more effective for larger trees and provide protection for two to three years per treatment.
Spring applications consistently outperform fall treatments in university research, so timing matters. The treatment window for most products runs from mid-April through early summer, aligning with the period when trees are actively moving water upward through their trunks.
Biological Control Efforts
Because chemical treatment isn’t practical for wild forests or entire urban canopies, the USDA has released tiny parasitic wasps that specifically target emerald ash borer larvae. One species, roughly 5 to 7 millimeters long, lays its eggs directly on the borer larvae beneath the bark. It poses no danger to people, pets, or other animals. Three years after its initial release at study sites, this wasp established a self-sustaining population, spread more than eight miles from the original release points, and was parasitizing 35 to 78 percent of emerald ash borer populations in those areas.
This won’t save every tree, but it represents a long-term strategy to reduce borer populations across landscapes where tree-by-tree treatment isn’t feasible. For homeowners, biocontrol is a background benefit rather than a substitute for direct treatment of valued trees.
Treatment vs. Removal
The decision to treat or remove an ash tree comes down to a few practical factors. If your tree is healthy, well-placed in your landscape, and large enough to provide significant shade, ongoing treatment is often cheaper than removal and replanting. A mature ash tree can cost thousands of dollars to remove, while preventive treatments typically run a few hundred dollars every two to three years.
If the tree has already lost more than half its canopy, treatment is unlikely to restore it. Dead ash trees also become brittle and hazardous surprisingly quickly, so delaying removal of a heavily infested tree creates safety risks. An arborist can assess canopy loss and help you decide whether the math favors treatment or a new tree.