Absorption and metabolism of permethrin and cypermethrin in the cockroach and the cotton‐leafworm larvae

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Abstract
Permethrin was metabolised by attack at the ester bond, in vivo by adult cockroaches, Periplaneta americana and in vitro by esterase preparations. Metabolites retaining the ester linkage could not be detected. In all cases, the (1RS)‐trans‐isomer (transpermethrin) was more labile than the (1RS)‐cis‐isomer. Cypermethrin was metabolised at one‐fifth the rate for permethrin. In‐vitro and in‐vivo studies with synergists indicated that cleavage of the ester bond in permethrin can be both oxidative for the (1RS)‐cis‐isomer and hydrolytic for the (1RS)‐trans‐isomer. The penetration of permethrin through the cuticle of the cockroach was significantly greater than that of cypermethrin. The penetration and metabolism of permethrin and cypermethrin in sixth‐instar larvae of susceptible and resistant strains of the Egyptian cotton‐leafworm, Spodoptera littoralis, were studied as possible factors in resistance. No significant differences were found. It is suggested that the major resistance factor to permethrin in S. littoralis is probably non‐metabolic.