I live in Iowa and have grubs also. Once you put down the stuff (We use the Bayer brand) it takes time to kill the little buggers! Than you have to reseed the area they killed. By putting down the stuff early in June (maybe earlier where you are ) you can kill the larvae before they hatch. I've been told they come from the June bug eggs.(?).
BTW: we have used Grub X but it only works if it goes down before the problem starts. I think I 'll stick with the Bayer brand.