Diatomaceous earth (DE) has many uses, but one of the most popular is as an insect killer. DE is not a poison, but a naturally occurring product that works through mechanical (physical) means to kill pests. Unlike many chemical products, bugs will never become resistant to it.
Diatomaceous earth is the fossilized remains of a type of algae known as a diatom. The shells of these diatoms are made mostly of silica, and when crushed up, they form a soft, white powder. Diatomaceous earth has microscopic, razor-sharp edges that are harmless to humans and other animals, but deadly to insects.
When insects crawl through the diatomaceous earth, the microscopic sharp edges work like shards of glass, slicing open their exoskeletons. As soon as the waxy outer covering of the bug is pierced, the insect will slowly begin to dry out. DE is also excellent at absorbing moisture, so the more the insect contacts the diatomaceous earth, the faster it will dehydrate. Thus, the actual means by which diatomaceous earth kills bugs is through desiccation.
Diatomaceous earth will work on pretty much any insect, but has been used extensively on things like bed bugs, fleas, ants, carpet beetles, and cockroaches. It can be used indoors or outdoors and will remain effective as long as it remains dry.