Chlorpyrifos-methyl degradation and biological efficacy toward maize weevils (Coleoptera: Curculionidae) on corn stored at four temperatures and three moisture contents

Corn was treated with 6 ppm chlorpyrifos-methyl and stored for 10 mo at each of four temperatures (15.0, 22.5, 30.0, and 37.5 degrees C) and three moisture contents (11.4, 12.4, and 14.4%). Every 2 mo, samples were removed from storage and analyzed for chlorpyrifos-methyl residue and infested with maize weevils, Sitophilus zeamais Motschulsky. Initial residues on the corn were 4.95 +/- 0.67 ppm. Of the residues that were lost, most were lost during the first 2 mo of storage. The loss rate increased as temperature and moisture content increased. No weevils survived on corn stored at 15.0 or 22.5 degrees C, except for 7.2% of the weevils that survived at month 10 on corn stored at 22.5 degrees C. Weevil survival increased as moisture content increased in corn stored at 30.0 and 37.5 degrees C. The threshold for weevil survival was approximately 1.22 ppm chlorpyrifos-methyl. Population growth, the percentage of insect-damaged kernels, and dockage weight were all correlated with insect survival