I think my ash tree has that beetle — what should I do?

Ash trees only, plus the occasional white fringetree. Thinning starts at the very top and works down while the lower canopy still looks normal. Sprouts erupt from the trunk and root flare. Vertical splits open in the bark. Peel loose bark and you find S-shaped tunnels packed with fine frass. Crisp D-shaped exit holes about an eighth of an inch across, flat on one side. Pale patches where woodpeckers stripped bark to reach the larvae. Larvae feed in the cambium just under the bark, severing the tree's plumbing from the inside. Once enough galleries overlap, the crown above starves regardless of soil, water, or fertilizer. Untreated infested ash die — this is not a decline trees recover from on their own. The line between the ones you can save and the ones you cannot is almost entirely how much canopy is still alive the day you first look.