Methodology for Assessing Progeny Production and Grain Damage on Commodities Treated with Insecticides

In evaluating insecticides, progeny production on grain commodities can be evaluated by either exposing adults on a commodity for a given time period, then removing them and assessing mortality and progeny production, or by leaving the adults on the commodity continuously, and then assessing progeny production. Little research directly compares these methodologies. Thus, our aims were to: (1) determine residual efficacy of Diacon IGR+ (methoprene+deltamethrin) and Gravista (methoprene+deltamethrin+piperonyl butoxide) on wheat, corn, and brown rice over the course of a year, using bioassays with select stored product insects at different time intervals, and (2) directly compare the two different methods of parental adult exposure on progeny production. Adults were either exposed for 7 d, then removed and assessed for survival, and the commodities were held for 6–7 weeks to assess progeny production, or adults were continuously exposed on the commodities for 6–7 weeks. Commodities were aged and sampled every 3 months for 12 months. Afterwards, samples were examined for progeny, sample weight loss, and insect feeding damage. Each insecticide killed exposed adults and prevented progeny of Rhyzopertha dominica on wheat and brown rice, and Tribolium castaneum on corn. There was extensive survival of Sitophilus spp. on all commodities, though Gravista did initially suppress S. oryzae on wheat and S. zeamais on corn compared to Diacon IGR+. Progeny, weight loss, and insect feeding damage were positively correlated in the 7 d exposure compared with continuous parental exposure. Both insecticides will control R. dominica and externally-feeding insects, but may exhibit reduced effectiveness for Sitophilus spp., especially S. oryzae. Food managers can utilize these data to more effectively plan management programs.