Efficacy of Chlorpyrifos-Methyl for Control of Maize Weevils (Coleoptera: Curculionidae) and Red Flour Beetles (Tenebrionidae) in Mixtures of Treated and Untreated Corn

After 0, 2, 4, 6, 8, and 10 mo of storage, corn treated at a calculated rate of 6 ppm chlorpyrifos-methyl was either mixed with or layered on the top surface of untreated corn. Before mixing at each storage interval, treated corn was analyzed for residue, and mixed samples were bioassayed by exposing maize weevils, Sitophilus zeamais Motschulsky, and red flour beetles, Tribolium castaneum (Herbst), to separate lots. As the percentage of treated corn in the mixtures increased, insect populations, damaged kernels, and dockage decreased. A ratio of 2:3 treated/untreated corn controlled maize weevils and red flour beetles for 6 and IO mo, respectively. The surface treatments were not as effective as the mixture treatments. Damaged kernels and dockage weights were strongly positively correlated with insect populations (r = 0.88-0.99).