The pyrethrins are a class of organic compounds normally derived from Chrysanthemum cinerariifolium that have potent insecticidal activity by disrupting 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. They first appeared on markets in the 1900s and have been continually used since then in products such as bug bombs, building insect sprays, and to spray animals so that they do not get infectious diseases.
Chemistry
History The pyrethrins occur in the seed cases of the perennial plant pyrethrum (Tanacetum cinerariifolium), which has long been grown commercially to supply the insecticide. In the 1880s, pyrethrum cultivation began in Japan when Ueyama grew the flowers in Wakayama and promoted their use across the country, primarily for lice control. Inspired by this, his wife Yuki conceptualized the mosquito coil, which became an effective and globally used tool against mosquitoes. Scientific interest followed, with biologist Fujitani publishing the first study on pyrethrum's insecticidal properties in 1909, sparking international chemical research. In 1923, Yamamoto identified that the active compounds in pyrethrum contained a cyclopropane ring, building on interest from Umetaro Suzuki. Later, Staudinger and Ruzicka analyzed the compounds in depth, and in 1944, LaForge and Barthel finally confirmed the full structures of pyrethrins I and II, along with cinerins I and II.
Biosynthesis
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. The key step in the biosynthesis of the naturally occurring pyrethrins involves two molecules of dimethylallyl pyrophosphate, which join to form a cyclopropane ring by the action of the enzyme chrysanthemyl diphosphate synthase.
Production
Commercial pyrethrin production mainly takes place in mountainous equatorial zones. The commercial cultivation of the Dalmatian chrysanthemum (C. cinerariifolium) takes place at an altitude of 1600 to 3000 meters above sea level. This is done because pyrethrin concentration has been shown to increase as elevation increases to this level. Growing these plants does not require much water because semiarid conditions and a cool winter deliver optimal pyrethrin production. The Persian chrysanthemum C. coccineum also produces pyrethrins but at a much lower level. Both may be planted in low-altitude zones in dry soil, but the pyrethrin level is lower. Pyrethrum extracted of the Persian chrysanthemum (painted daisy) was already imported to central Europe from Georgia in the middle of the 19th century. Most of the world's supply of pyrethrin and C. cinerariaefolium today comes from Kenya, which produces the most potent flowers. Other countries include Croatia (in Dalmatia) and Japan. The flower was first introduced into Kenya and the highlands of Eastern Africa during the late 1920s. Since the 2000s, Kenya has produced about 70% of the world's supply of pyrethrum. A substantial amount of the flowers are cultivated by small-scale farmers who depend on it as a source of income. It is a major source of export income for Kenya and source of over 3,500 additional jobs. About 23,000 tons were harvested in 1975. The active ingredients are extracted with organic solvents to give a concentrate containing the six types of pyrethrins: pyrethrin I, pyrethrin II, cinerin I, cinerin II, jasmolin I, and jasmolin II. Processing the flowers to cultivate the pyrethrin is often a lengthy process, and one that varies from area to area. For instance, in Japan, the flowers are hung upside down to dry which increases pyrethrin concentration slightly. To process pyrethrin, the flowers must be crushed. The degree to which the flower is crushed has an effect on both the longevity of the pyrethrin usage and the quality. The finer powder produced is better suited for use as an insecticide than the more coarsely crushed flowers. However, the more coarsely crushed flowers have a longer shelf life and deteriorate less.
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