The fate and insecticidal activity of pirimiphos-methyl in stored wheat grain

The degradation of ¹⁴C]pirimiphos-methyl was investigated on a 1-kg bulk of grain stored in the laboratory at 15°C for 28 weeks. Breakdown of pirimiphos-methyl was slow, leaving 77% of the applied dose unchanged at the end of the experiment. The major degradation product was the hydroxypyrimidine (up to 12.5% of the applied dose) with the remainder of the residue (up to 12.1%) remaining within the grain tissues after solvent extraction. The efficacy of the treatment was assessed in a parallel experiment by bioassay of a 5-kg bulk of grain dosed with unlabelled pirimiphos-methyl. 100% Mortality of an organophosphateresistant strain of Oryzaephilus surinamensis L. was maintained throughout the experiment whereas some survival of an organophosphate-resistant strain of Tribolium castanewm (Herbst) occurred after 12 weeks of storage. This increased to give a 20% survival rate at the end of the experiment.