DE will help kill just about every exoskeletal organism, by exoskeletal, they mean the skeleton is on the Outside like an ant, earwig, potato bug, beetles. What it does is pierce or cut the skeleton, this allows dirt and other things to get into teh cut and infect it, in most cases causing death. They make insect powder that is 95% de and 5% some sort of toxin, this is supposed to ensure the cut gets infected by means of the toxin. It costs a LOT more than just DE, and supposedly, from people who have tried both ways, it really doesn't do much more, likely, the cut plus the dirt environment pretty much guarantee that infection sets in anyways. I had no problems getting rid of an anthill under my hose reel this summer wiuth straight DE.
As for grubs, because they also have a "skeletal type skin" protecting them on the outside, DE will also be effective - actually, grub killer is the same as ant killer etc.

DE does to bugs what broken glass would do to you.