CHLORPYRIFOS METHYL DEGRADATION AND BIOLOGICAL EFFICACY TOWARD MAIZE WEEVILS (COLEOPTERA, CURCULIONIDAE) ON CORN STORED AT 4 TEMPERATURES AND 3 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 almost-equal-to 1.22 ppm chlorpyrifos-methyl. Population growth, the percentage of insect-damaged kernels, and dockage weight were all correlated with insect survival.