Imidacloprid, the active ingredient works by killing adults when they feed in the summer before they lay eggs. It slowly kills the two youngest stages of grubs that feed beneath the bark. The later and larger two stages are not killed. Material applied in the fall does not start killing beetles til spring. It takes twice the dose in the fall to get the same effect as a spring application. Trees with a trunk diameter of >20 inches at 4.5 ft above the ground can’t be controlled with imidacloprid.
So if your trees are starting to die I would suggest you skip the fall application of imidacloprid and switch to a professional injection of emamectin benzoate. See Protecting Ash Trees with Insecticides, Purdue Extension Emerald Ash Borer, for more information.
Cliff Sadof, Coordinator of Extension
Purdue University Department of Entomology
What to do about emerald ash borer, Got Nature?, Purdue Extension-FNR
Emerald Ash Borer, Purdue Extension-Entomology
EAB research: Saving trees early less costly than replacing them, Purdue Agriculture News