Pyrethrin  The pyrethrins are a class of organic compounds normally derived from "Chrysanthemum cinerariifolium" that have potent insecticidal activity by targeting the nervous systems of insects. Pyrethrin naturally occurs in chrysanthemum flowers and is often considered an organic insecticide when it is not combined with piperonyl butoxide or other synthetic adjuvants. Their insecticidal and insect-repellent properties have been known and used for thousands of years. Pyrethrins are gradually replacing organophosphates and organochlorides as the pesticides of choice as the latter compounds have been shown to have significant and persistent toxic effects to humans. Because they are biodegradable pyrethrins are widely preferred to pyrethroids, which are synthetic analogues of pyrethrin that accumulate in the environment. The pyrethrins occur in the seed cases of the perennial plant pyrethrum ("Chrysanthemum cinerariaefolium"), which has long been grown commercially to supply the insecticide. Pyrethrins have been used as an insecticide for thousands of years. It is believed that the Chinese crushed chrysanthemum plants and used the powder as an insecticide as early as 1000 BC. It was widely known that the Chou Dynasty in China widely used pyrethrin for its insecticide properties. For centuries, crushed Chrysanthemum flowers have been used in Iran to produce Persian Powder, an insecticide for household use. Pyrethrins were identified as the potent chemical in the Chrysanthemum plants responsible for the insecticidal properties in the crushed flowers around 1800 in Asia. In the Napoleonic Wars, French soldiers used the flowers to keep away fleas and body lice. Well after their use as insecticides began, their chemical structures were determined by Hermann Staudinger and Lavoslav Ružička in 1924. Pyrethrin I (CnH28O3) and pyrethrin II (CnH28O5) are structurally related esters with a cyclopropane core. Pyrethrin I is a derivative of (+)-"trans"-chrysanthemic acid. Pyrethrin II is closely related, but one methyl group is oxidized to a carboxymethyl group, the resulting core being called pyrethric acid. Knowledge of their structures opened the way for the production of synthetic analogues, which are called pyrethroids. Pyrethrins are classified as terpenoids.