The pyrethrins are a pair of natural organic compounds that have potent insecticidal activity. Pyrethrin I and pyrethrin II are structurally related esters with a cyclopropane core. They differ by the oxidation state of one carbon. They are viscous liquids that oxidize to become inactivated. They are non-persistent, being biodegradable, and break down on exposure to light or oxygen. The chemical structure of pyrethrins is the basis for a variety of synthetic insecticides called pyrethroids such as permethrin and cypermethrin.
The pyrethrins are contained in the seed cases of the perennial plant pyrethrum (Chrysanthemum cinerariaefolium), which is grown commercially to supply the insecticide. Although extracts of the plant were already used as insecticide, the structure was first published by Hermann Staudinger and Lavoslav Ružička in 1924. Pyrethrins are neurotoxins that attack the nervous systems of all insects. When present in amounts not fatal to insects, they still appear to have an insect repellent effect. Pyrethrins are gradually replacing organophosphates and organochlorides as the pesticide of first choice.
Pyrethrins are harmful to fish, birds, and mammals, including humans. In humans, pyrethrin irritates the eyes, skin, and respiratory systems, and it may cause other harmful effects. One study suggested a link between maternal pyrethrin use and autism in children. The study indicated that mothers of autistic children were twice as likely to have washed a pet dog with a flea shampoo containing pyrethrin while they were pregnant.