Assessing organic chemical management strategies for Sitophilus zeamais (Coleoptera: Curculionidae) and Prostephanus truncatus (Coleoptera: Bostrichidae) at food facilities

There is an increasing demand for organically certified practices throughout the postharvest supply chain. Yet, evaluating mortality and sublethal effects from organic chemical control options have received less attention. Thus, the aims of this study were to evaluate the direct mortality and changes in mobility of Prostephanus truncatus and Sitophilus zeamais when using (i) fresh residues or (ii) 7-mo-aged residues on maize from a suite of organically approved grain protectants (3 formulations of 5% pyrethrins, or 8.66% spinosad) compared to a conventional insecticide (4.75% deltamethrin) and water-treated negative control in a grain bin environment. Adults were exposed to residues continuously for 24 or 168 h. There were 92% to 99% and 78% to 88% fewer dead S. zeamais and P. truncatus, respectively, after exposure to maize with pyrethrins or H2O compared to deltamethrin, whereas there were 16% fewer to 1.4% more dead S. zeamais and P. truncatus after exposure to maize treated with spinosad. Exposure to maize treated with pyrethrins, spinosad, or deltamethrin reduced progeny production by 43% to 99% for S. zeamais and by 27% to 100% for P. truncatus compared to H2O-treated grain. There were 2.4-fold and 4.1-fold more progeny produced by S. zeamais and P. truncatus, respectively, on maize aged 7 mo compared to initially treated with a grain protectant. We found consistent sublethal reductions in movement by S. zeamais after exposure to maize treated with both organic and conventional insecticides. Overall, our work helps raise the profile of the importance in assessing available organic options for management of insects after harvest.