The dispersal ability of the invasive larger grain borer and the cosmopolitan maize weevil after brief exposure to a newer insecticide formulation

Abstract

The larger grain borer (Prostephanus truncatus) and the maize weevil (Sitophilus zeamais) are major pests of stored maize. Given the threat of these insects to our maize supply, the increasing insecticide resistance to currently used chemical control tactics such as phosphine, and the removal of effective chemistries from the market (methyl-bromide), there is a need for alternative integrated pest management tools. Here, we present a more realistic test of the effectiveness of a novel insecticide formulation (methoprene + deltamethrin + piperonyl butoxide, PBO; tradename, Gravista ®) in comparison to past conventional insecticide evaluations at reducing the horizontal dispersal capacity to food patches of both stored product species in comparison to an older formulation (methoprene + deltamethrin; tradename, Diacon IGR+®). For these experiments, we used a dispersal apparatus (consisting of 30 or 70 cm PVC pipe modified with a baited pitfall trap at one end) where we introduced cohorts of adults after exposure on concrete arenas at labeled rates and gave them 48 h to disperse to a novel food source consisting of 15 g of maize. The dispersal capacity for P. truncatus, measured as the percentage of adults dispersing to the novel food source, was significantly reduced by 90–97% after exposure to Gravista or Diacon IGR+ compared to the control. By contrast, the dispersal capacity for S. zeamais was only reduced by 5–25% after exposure to either insecticide formulation compared to the control. Both formulations performed equally well for each individual species, but dispersal of S. zeamais was not strongly impacted. Overall, our study highlights the importance of using realistic behavioral assays in comprehensive insecticide testing to provide a full picture of insecticide efficacy.