Stability of cypermethrin and cyfluthrin on wheat in storage

Cypermethrin 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, cis I [(αR),(1R)‐cis + (αS),(1S)‐cis], cis II [(αR),(1S)‐cis + (αS),(1R)‐cis], trans III [(αR),(1R)‐trans + (αS),(1S)‐trans], and trans IV [(αR),(1S)‐trans + (αS),(1R)‐trans] for each pyrethroid were determined at five intervals during storage. For all storage conditions, the cis I isomers were the most stable, and the trans IV 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, cis I, 252, 62 and trans IV 66, 27; cyfluthrin, cis I, 114, 52 and trans IV 42, 23. The results suggested that one of the enantiomers of the cypermethrin trans IV pair was degraded faster than the other.