What are these D-shaped holes in my ash tree?

D-shaped exit holes in an ash tree are the unmistakable calling card of the emerald ash borer (EAB) (Agrilus planipennis), the most destructive forest insect pest in North American history. EAB has killed hundreds of millions of ash trees since its discovery near Detroit in 2002.

Confirming EAB infestation:

• Exit holes are about 1/8 inch wide and distinctly D-shaped (flat on one side)
• Serpentine, S-shaped larval galleries beneath the bark filled with frass (sawdust)
• Canopy thinning starting at the top of the tree (top-down dieback)
• Epicormic sprouting — new shoots emerging from the trunk and major branches (the tree's stress response)
• Bark splitting that reveals galleries underneath
• Increased woodpecker activity (they feed on EAB larvae)

Treatment options:

1. If less than 50% canopy loss, treatment is worthwhile. Trunk injections of emamectin benzoate (brand name TREE-äge) provide 2-3 years of protection and are highly effective.
2. Soil drenches with imidacloprid work for smaller trees but take longer to be effective.
3. Treatment must continue for the life of the tree or until EAB populations collapse locally.
4. If more than 50% canopy loss, the tree likely cannot be saved and should be removed for safety — dead ash become extremely brittle.

For our comprehensive guide, visit Emerald Ash Borer Guide. Also see Signs of Insect Damage on Trees for identifying other wood-boring pests.