Beneficial nematodes! I've used them and they work well. There are many sources from which to order them online. They should be applied shortly after you see the adults flying around. Beneficial nematodes kill the grubs at any of their life stages, but do it more efficiently when the grubs are young.

I caution against the use insecticides as they indiscrimintly kill beneficial insects and bees, and would be little to curb the beetles. There are many insects that prey on other insects and keep their populations in control.

If not familiar with beneficial nematoes do some research on them. They are live organisms and need to be applied shortly after your receive them and the ground kept moist to give them a chance to move into the soil. They won't kill this years beetles which are likely to be in their pupa stage about now, but will keep next year's population down. The nematoes continue to live in the soil from year to year if it's not too dry.

Here is a link that might be useful: Google search for beneficial nematodes ...

Thanks roselee! I'll try the nematodes. Your information was very helpful!