The only practical treatment for a large or mature ash tree is to have a certified pesticide applicator inject the tree (or possibly soak the roots) with systemic insecticide, which will likely entail using a neonicotinoid (like imidacloprid or one of several other ingredients in that chemical group). There isn't a DIY approach that works well or for very long that a person without an applicator's license (or the ability to inject pesticide into wood) can use.

During the egg-laying period when adult beetles are looking for host trees around mid-spring, a trunk spray of a residual insecticide like a pyrethroid-class ingredient might work, but that approach will also harm or kill a variety of other non-pest insects, including predators of this and other pests. There is no ingredient specific to Emerald Ash Borer that won't potentially also harm other species if they are exposed to application residues or any ingredient the tree soaks up which contaminates its flower pollen. Different systemics (even within the neonicotinoid group) last for different amounts of time after application, but none should be used before bloom.

There are some examples and sample videos of DIY treatment applications in the linked resources on our Emerald Ash Borer web page, but they all refer to using neonicotinoids like imidacloprid, so in Maryland, any such approach needs to be done by a MDA-certified pesticide applicator.

Miri