A grub is the larvae of a few different kinds of beetle, such as Japanese beetles and European Chafer. Before they reach adulthood, they sit below the surface of the soil and munch on the roots of your grass and plants, which can cause brown spots and dead zones. They also attract moles and raccoons who eat them. Moles will leave tunnels which kill the turf above them (and create an injury hazard in your lawn when they settle) and raccoons will tear the turf apart to get to them.
If you have a lot of beetles around your property in the early summer, there's a chance you have grubs. The two species I mentioned in the previous section are common in Michigan. A good way to check if you have grubs is to pull back a section of sod about 1 square foot and check the soil about 3-4in down, right around the root zone. If you see more than 1-3 grubs you're okay but any more than that and you have a problem.
Grub control, like fertilizer, is most efficiently done by a licensed applicator. They know exactly how to mix their pesticides, where to spray, and when to spray. Usually these applications are done in the early part of the Summer, either just before or right as the grubs hatch. Most companies that offer comprehensive fertilization programs will have a grub control option. If you prefer to do it yourself, there are plenty of consumer grade options available at your local home improvement store.