Cyhalothrin is the ISO common name for an organic compound that, in specific isomeric forms, is used as a pesticide. It is a pyrethroid, a class of synthetic insecticides that mimic the structure and properties of the naturally occurring insecticide pyrethrin which is present in the flowers of Chrysanthemum cinerariifolium. Pyrethroids such as cyhalothrin are often preferred as an active ingredient in agricultural insecticides because they are more cost-effective and longer acting than natural pyrethrins. λ-and γ-cyhalothrin are now used to control insects and spider mites in crops including cotton, cereals, potatoes and vegetables. Structure and stereochemistry Gamma-cyhalothrin and lambda-cyhalothrin are the active ingredients in the current commercial products based on cyhalothrin. Both are cyanohydrin esters of cis-3-[(Z)-2-chloro-3,3,3-trifluoropropenyl]-2,2-dimethylcyclopropanecarboxylic acid. All of the insecticidal activity is due to the proportion of absolute stereochemistry (1R) in the mixture. The active isomer of deltamethrin, (1R)-cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarboxylic acid, has the same stereochemistry. γ-cyhalothrin (a single chiral isomer) is indeed twice as active as λ-cyhalothrin on a weight-for-weight basis. The latter is racemic and contains the (1R) and inactive (1S) isomers in equal amounts. History By 1974, a team of Rothamsted Research scientists had discovered three pyrethroids (MoA 3a), suitable for use in agriculture, namely permethrin, cypermethrin and deltamethrin. These compounds were subsequently licensed by the NRDC, as NRDC 143, 149 and 161 respectively, to companies which could then develop them for sale in defined territories. Imperial Chemical Industries (ICI) obtained licenses to permethrin and cypermethrin but their agreement with the NRDC did not allow worldwide sales. Also, it was clear to ICI's own researchers at Jealott's Hill that future competition in the marketplace might be difficult owing to the greater potency of deltamethrin compared to the other compounds. For that reason, in the period 1974–1977, chemists there sought patentable analogues which might have advantages compared to the Rothamsted insecticides by having wider spectrum or greater cost-benefit. The first breakthrough was made when a trifluoromethyl group was used to replace one of the chlorines in cypermethrin, especially when the double bond was in its Z form. The resulting material was found to be more potent than cypermethrin, to which it is most closely related, but also with good activity against the spider mite Tetranychus urticae, which added to its attractiveness as a potential new product. The second breakthrough occurred when ICI process chemists developed a practical manufacturing process for the Z-cis acid, by controlling the stereochemistry