Degradation of Phenothrin in Stored Wheat Grains

Abstract

¹⁴C-Labeled (+)-trans and (+)-cis isomers of phenothrin [3-phenoxybenzyl (±)-cis, trans-chrysanthemate] were each applied at the concentration of 4ppm to wheat grains with 11% moisture content, and stored at 15 or 30°C in the dark. Both trans and cis isomers were decomposed slowly, and 79 and 87% of the applied radiocarbon remained intact in the grains after 12-month storage at 30°C, respectively. Both isomers were majorly metabolized via hydrolysis of ester linkage, oxidation of the benzyl alcohol to the benzoic acid and methylation of the benzoic acid. The joint application with piperonyl butoxide and fenitrothion inhibited the degradation of phenothrin isomers to some extent. The phenothrin isomers and their decomposition products were mainly located at seed coat during 12-month storage, and the residue levels of both isomers in flour and bran were 0.77 and 11.4ppm, respectively. The phenothrin residues in flour somewhat decreased through the baking process, leaving 0.57ppm of phenothrin isomers in bread. These findings were compared with those of ¹⁴C-malathion which was rapidly decomposed to desmethylmalathion, malathion-monoacids, -diacid and ¹⁴CO₂, with respective half-lives of 8 and 1.1 months at 15 and 30°C.