Abstract
(+)-Cyanofenphos was at least more than 25-fold as toxic to rice stem borer larvae as the (-)-isomer. In addition, (+)-cyanofenphos-oxon was 36-fold more potent inhibitor than the (-)-oxon toward rice stem borer larva acetylcholinesterase. The (+)-, (-)- and racemic forms of cyanofenphos were metabolized in rice stem borer larvae at almost equal rates. However, (+)-cyanofenphos produced 4- to 9-fold larger amounts of cyanofenphos-oxon in the insect body than the (-)-isomer. On the other hand, (-)-cyanofenphos-oxon was meta-bolized to 4-cyanophenol and its conjugate at a faster rate compared to the (+)-oxon. The difference in toxicity to rice stem borer larvae exhibited by the optical isomers of cyanofenphos is attributable to the difference in the amount and persistence of cyanofenphos-oxon isomers formed in the insect body as well as in anti-acetylcholinesterase activity of the oxon isomers.
