Pyrethrin

  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 ^{[1] [2]}. They differ by the oxidation state of one carbon. They are viscous liquids that oxidize readily to become inactivated.

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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.^[3] 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. They are harmful to fish, but are far less toxic to mammals and birds than many synthetic insecticides. They are non-persistent, being biodegradable, and also break down easily on exposure to light or oxygen. They are considered to be amongst the safest insecticides for use around food. For humans, pyrethrin is known to irritate eyes, skin, and respiratory systems and is particularly harmful to aquatic life.

Due to its relatively inert properties toward mammals, pyrethrins are gradually replacing Organophosphates and Organochlorides as the pesticide of first choice.

The chemical structure of pyrethrins is the basis for a variety of synthetic insecticides called pyrethroids such as permethrin and cypermethrin.

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