AbstractCypermethrin and cyfluthrin were applied to wheat, which was stored for 52 weeks at 25 or 35°C, and either 12 or 15 moisture content. Total residues and the proportions of the four pairs of enantiomers,cisI (αR),(1R)‐cis+ (αS),(1S)‐cis,cisII (αR),(1S)‐cis+ (αS),(1R)‐cis,transIII (αR),(1R)‐trans+ (αS),(1S)‐trans, andtransIV (αR),(1S)‐trans+ (αS),(1R)‐trans for each pyrethroid were determined at five intervals during storage. For all storage conditions, thecisI isomers were the most stable, and thetransIV isomers were the least stable. Calculated half‐lives (weeks) for the pairs of enantiomers at 25°C (12 moisture) and 35°C (15 moisture) were: cypermethrin,cisI, 252, 62 andtransIV 66, 27; cyfluthrin,cisI, 114, 52 andtransIV 42, 23. The results suggested that one of the enantiomers of the cypermethrintransIV pair was de
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