Pyrethrum is a botanical insecticide produced primarily from the flowers of Tanacetum cinerariaefolium, which is a species of the chrysanthemum plant family. It is found mainly in tiny oil-containing glands on the surface of the seed case in the tightly packed flower head and is the plant’s own insecticide that has evolved to keep insects away. Pyrethrum is made up of six complex chemical esters known as pyrethrins which work in combination to repel and kill insects.

The natural pyrethrins are contact poisons which quickly penetrate the nerve system of the insect. A few minutes after application, the insect cannot move or fly away. But, a "knockdown dose" does not mean a killing dose. The natural pyrethrins are swiftly detoxified by enzymes in the insect. Thus, some pests will recover. To delay the enzyme action so a lethal dose is assured, organophosphates, carbamates, or synergists may be added to the pyrethrins.

Pyrethrin has been used effectively to control insects for decades. It decomposes rapidly in the environment, making it an excellent choice for controlling pests.

Pyrethrin is one of the safest insecticides known, as it has a very low Mammalian toxicity, and is rapidly metabolised if accidentally swallowed by warm blooded animals. Pyrethrin has been extensively studied from a toxicology viewpoint. It is low in acute toxicity to humans and other vertebrate animals, is non-carcinogenic and causes no adverse reproductive affects.

Pyrethin has flushing action, it disturbs insects so that they move out of their hiding places and expose themselves to the insecticidal spray. Pyrethrum has greater 'flushing' power than any other commercial insecticide. Against insects, it has a very rapid paralytic action, knocking them down followed by kill. Pyrethrum repellents insects at very low concentrations, a property used in the protection of stored grain, the preparations of insect resistant packaging, and in deterring mosquitoes, both by spray-on repellents and mosquito coils. In addition, the smoke from Pyrethrum-based mosquito coils will inhibit mosquitoes from biting. Pyrethrin is commonly used as a post harvest treatment for fruits and vegetables especially during shipment and is popular as a non-residual household insecticide spray.

Pyrethrum rapidly degrades by sunlight into harmless breakdown products, but at the same time it can be safely stored for years in suitable containers.

Despite its long history of use, very few cases of insects resistance to Pyrethrum have been discovered, and these have mainly arisen as a result of cross-tolerance conferred by the insects developing resistance to another insecticide. This is in strong contrast to synthetic insecticides where innumerable cases of resistance have been discovered throughout the world.

Pyrethrum is effective against a wide range of insect pests including mosquitoes, cockroaches, fleas, flies, aphids, etc. Thus a single can of a suitably formulated aerosol can be safely used against any insect found in the house or garden.