Evaluating the Long-Term Efficacy of Four Active Ingredients Against Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) and Sitophilus oryzae (L.) (Coleoptera: Curculionidae) on Stored Sorghum in the United States

Grain protectants are insecticide formulations applied directly to raw grain to prevent and control insect infestations in bulk storage; however, their efficacy depends on insect species, commodity, and grain quality. The objective of this study was to determine the residual efficacy of four commercially available grain protectants, Gravista® (deltamethrin + methoprene + PBO), Diacon® IGR (methoprene), SensatTM (spinosad), and EverGreen® (pyrethrin), applied to sorghum and held for 28 weeks, against Rhyzopertha dominica (F.) and Sitophilus oryzae (L.). Subsamples were collected every four weeks and infested with 10 adults of either species. Adult mortality was assessed after seven days and progeny, frass, and insect-damaged kernels were evaluated after eight weeks. Rhyzopertha dominica was more susceptible to all insecticides compared to S. oryzae, and had fewer progeny compared to control sorghum. Spinosad-treated sorghum resulted in 100% adult mortality in R. dominica compared to 30–39% in S. oryzae. There was an overall decline in kernel moisture content, which may have impacted progeny of both species in the control and insecticide-treated sorghum. These findings highlight the influence of insecticide formulation, insect species, and grain moisture content on the long-term efficacy of grain protectants applied to sorghum.