When insects crawl over soil treated with diatomaceous earth, they get tiny cuts from the sharp edges of the substance. The bugs then die from dehydration as their body fluids leak out. Diatomaceous earth also absorbs fats from the outer layers of the bugs' skins or exoskeletons, also causing dehydration.
Diatomaceous earth kills many different insects, including ants, aphids, centipedes, cutworms, crickets, Japanese beetles, millipedes, slugs, snails, sow bugs and squashbugs.
Diatomaceous earth can also kill beneficial insects such as honeybees and ladybugs. Cover treated plants with a sheet or burlap to keep the plants out of reach from beneficial bugs and bees for a day or two. Then, uncover the plants and wash off the diatomaceous earth residue with a hose.
Natural diatomaceous earth comes from crumbled sedimentary rock that contains the fossilized bones of organisms called phytoplankton. Although it is not toxic to humans, dogs or cats, the product can cause irritation or damage to lungs and nasal passages if it's inhaled. Another kind of diatomaceous earth that is used in swimming pool filters contains crystals of silica and can cause severe damage to people and pets, so avoid that type completely in your garden.