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Unfortunately methoxychlor is not as persistent as

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Unformatted text preview: ia -carrying mosquitoes and it is also more expensive than DDT. 3.3.3 The pyrethroids: a case of too little persistence NATURAL PYRETHROIDS These compounds are extracted from crush ed flower heads of Chrysanthemum cineriaefolium (daisy mums) and consist of 6 esters of 2 acids (chrysanthe mate and pyrethrate) and 3 alcohols (pyrethrilone, cinerolone and jasmolone). The flowers contain 0.7 - 3.0% active ingredient (AI) with an average of 1.3%. It was in 1828 that the insecticidal action of these compounds was first recognized. In 1858 ―pyrethrum powders‖ were first introduced to the U.S. and, in 1924, the structure of pyrethrin I (Figure 3-25) was identified. In 1942, Kenya became the chief producer of pyrethrins. At the present time, world con sumption of natural pyreth rins is ca. 21,000 tonnes/year, most of which is used in household and pest control products for domestic animals. Natural pyrethrins are non-persistent, contact nerve poisons and have a negative tempera ture coefficient (become more toxic at low tempera tures). They are metabolized easily by insect and mammalian mixed function oxidases (MFOs, Chapters 1 and 2), are rapidly photolyzed by light and are hydrolyzed in water ( Figure 3-25). They have a greater mammalian selectivity ratio (AOLD50 in rats of 900 -1,500 mg/kg). However, they do have major practical problems: they are relatively expensive; crop failures may threatened the supply; and they are too non -persist ent and therefore require frequent re -applica tion in order to be effective. Their usefulness in the protection of agricultural crops is thus restricted. SYNTHETIC PYRETHROIDS The natural pyrethroids have been modified and, through the development of synthetic analogs, new compounds have been produced (i.e., synthetic pyrethroids) . The structures of these were based on pyrethrin I but, by the use of halo genated substituents on the acid moiety and/ or replacement of the furan on the alcohol moiety with phenyl rings, environ mental and meta bolic stability has been increased. This sequence started in 1924 when the structure of pyrethrin I was discovered. In 1949, allethrin was developed in the U.S. a nd in 1967 resmethrin was synthesized in the UK In 1973 permethrin () was developed in the UK by Michael Elliot. This was the first truly persistent pyrethroid and therefore suitable for agricultural uses. Since 1977 a wide range of newer synthetic pyrethroids have been developed (deltamethrin, cypermethrin, fenvalerate and fluvalinate). These pyrethroids are widely used today and have significant health Pyrethrin benefits for humans both in terms of crop C H3 production and protection against diseases O H3 C C H2 CH C H2 such as malaria. Permethrin -impregnated CH CH C CH C O bed nets will protect against mosquito bites H3 C C H for 5 or more years and are inexpensive to O CH manufacture. Hydrolysis C H3 C Ultra violet C H3 Oxidation Permethrin O H3 C C CH C O O H3 C C H CH C Cl Cl Figure 3-251 Pyrethrin I and p...
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This document was uploaded on 04/07/2014.

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