BETA-CYPERMETHRIN

Beta-cypermethrin is used to control a wide range of insects, especially Lepidoptera and Coleoptera, in alfalfa, cotton, cereals, fruit, vines, vegetables, beans, beets, tobacco, oilseed rape and other crops. It is also used for insect control in public health and for ectoparasite control on animals.

Beta-cypermethrin is composed of two of the four cis-cypermethrin isomers (1RcisaS and 1ScisaR isomers) [i.e. alpha-cypermethrin] and two of the four trans-cypermethrin isomers (1RtransaS and 1StransaR) [i.e. thetacypermethrin]. The fate of these four isomers (separately) has been reported for soils. Metabolite analysis was conducted in enough detail to indicate that the fate of the components was very similar to their fate when presented to biological systems as part of cypermethrin. Thus reference can be made to the cypermethrin entry for details. The structure and Scheme numbering used below refers to the cypermethrin entry.

Beta-cypermethrin is stable as a solid but it is epimerised in weak base solution and readily hydrolysed at higher pH. Its half-lives at pH values 3, 7 and 9 (25 °C) were 50,40 and 15 days (PM). By analogy with cypermethrin, the rate-determining step in dilute solution is nucleophilic attack by OH^-. Half-lives of cis-cypermethrin in river water and sea water at 25 °C were 21 and 24 days, respectively; the trans-isomer would be expected to be more rapidly degraded. Major products were 3-(2,2- dichlorovinyl)-2,2-dimethylcyclopropanecarboxylic acid (2, DCVA), 3- phenoxybenzaldehyde (9, 3PBAl) and a-carbamoyl-3-phenoxybenzyl 3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate( the amide 3); minor products were the a-carboxy analogue of 3 (4) and 3- phenoxybenzoic acid (10,3PBA) (see cypermethrin, Schemes la and lb).
Photodecomposition would be expected to be similar to that of cypermethrin.