They affect fleas, cockroaches, flies, and mosquitos too, so their use on a regular basis may help in more battles than just this one. Applied as a very fine dust, the fossilized remains of tiny crustaceans are sharp and damaging to the outer skeleton and the soft-bodied larvae of the fleas. Mortally wounded, fleas exposed to diatomaceous earth die. It's like using concertina wire everywhere. Diatomaceous Earth can be a great help when used to treat dog houses, kennels, and areas under shrubs or bushes where you pets repose for their afternoon naps and where the flea eggs fall off, hatch, and begin to change into soft-bodied larvae.
Diatomaceous earth is, however, difficult to use to treat larger areas effectively.
Also, be sure to use an insecticidal grade of diatomaceous earth. Swimming pool grade is not adequate to kill fleas. If you can keep a thriving army of nematodes alive and well in your outdoor environment, fleas will dwindle. However, nematodes do well only if kept damp. Hot, dry summer is not conducive to this type of treatment unless you want your water bill to reflect the damages.