Tom is right for a large garden area. The grub killer treatment and some natural treatments work for the larger areas, mostly in prevention, as once they are into the rhizome it is harder to kill them these ways. For small gardens you can examine the clumps and see if "frass" is there, the little drops of excrement, a sure sign of borer activity, then use the "squish" method when they are in the foliage. If in the rhizome already you can dig it out, clean the wound letting it dry in the sun and replant. I have also left the roots in the ground exposing the rhizome and doing the same. When wound is dry recover with soil. If it is a large clump just toss the rhizome into the trash to save the time and work.
A chemical "cure" is the Bayer Long Season Grub Control or something similar as Merit. For a more natural approach you could try bacillus thuringiensis [BT] but follow the instructions as it may need 2 treatments due to weather.
