Pyrethrum powder is an extract of the Pyrethrum plant, containing several chemicals known as pyrethrins. These chemical compounds are neurotoxic to all insects and paralyze them quickly on contact. The compunds have been widely used as insecticides since the beginning of the 20th century.
As insecticides, pyrethrins have a number of desirable qualities. They have been called "probably the safest of all insecticides for use in a food plant" by the United States Department of Agriculture in 1972. They are also biodegradable and do not persist in the environment, breaking down after exposure to light and oxygen. For these reasons, pyrethrins are becoming the number one choice for insecticides, replacing organophosphates and organochlorides.