Emerald ash borer adults are emerging from Illinois to Ohio and points northward. They'll continue to do so for several weeks and it means now is the time to treat for them in hopes of saving your Ash tree. This should most definitely be done if the emerald ash borer has been found within fifteen miles of your tree says University of Illinois Extension Entomologist Phil Nixon.
Again, you can save your Ash tree, but it'll take about twenty years to do that, if the experience in the state of Michigan is any guide. You or a professional can do that by making an insecticide application.
Call your local Extension office for more details about the process and how best to go about treating for the Emerald Ash Borer.