Degradation and Biological Efficacy of Chlorpyrifos-Methyl on Wheat Stored at Five Temperatures and Three Moisture Contents

Abstract

Soft red winter wheat was treated with a calculated dosage of 6 ppm chlorpyrifos-methyl and stored at 15, 20, 25, 30, or 35°C and either 11.2, 12.1, or 13.7% moisture content (15 combinations). Measured residue deposition on the wheat was 4.39 ± 0.57 ppm, a 27.2% reduction from the calculated dosage. Residue degradation was asymptotic at all combinations except 15°C and 11.2 and 12.1% moisture content. Residue loss during the initial months of storage increased with increases in both temperature and moisture content within temperature. Rice weevils, Sitophilus oryzae (L.), did not survive bimonthly bioassays on wheat stored at either 15 or 20°C, but did survive on wheat stored at 20°C and 13.7% moisture content. Above 20°C, survival on treated wheat increased as residues decreased, and residues became either inactivated or diluted by increased moisture content. Numbers of F1 progeny, the percentage of insect-damaged kernels, and the amount of ground wheat flour (dockage) were positively correlated with weevil survival and negatively correlated with residue levels.