INSECTICIDE RESISTANCE AND RESISTANCE MANAGEMENT Efficacies of Spinosad and a Combination of Chlorpyrifos-Methyl and Deltamethrin Against Phosphine-Resistant Rhyzopertha dominica (Coleoptera: Bostrichidae) and Tribolium castaneum (Coleoptera: Tenebrionidae) on Wheat

Highly phosphine-resistant populations of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) have recently been found in Oklahoma grain storage facilities. These Þndings necessitate development of a phosphine resistance management strategy to ensure continued effective use of phosphine. Therefore, we investigated the efÞcacies of two grain insecticides, namely, spinosad applied at label rate of 1 ppm and a mixture of chlorpyrifos-methyl and deltamethrin applied at label rates of 3 and 0.5 ppm, respectively, against highly phosphine-resistant R. dominica and T. castaneum. Adult mortality and progeny production suppression of spinosador chlorpyrifos-methyl deltamethrin mixture-treated wheat that had been stored for 2, 84, 168, 252, and 336 d posttreatment were assessed. We found that both spinosad and chlorpyrifos-methyl deltamethrin were effective against phosphine-resistant R. dominica and caused 83Ð100% mortality and also caused total progeny production suppression for all storage periods. Spinosad was not effective against phosphine-resistant T. castaneum; the highest mortality observed was only 3% for all the storage periods. Chlorpyrifos-methyl deltamethrin was effective against phosphine-resistant T. castaneum only in treated wheat stored for 2 and 84 d, where it caused 93Ð99%mortality. However, chlorpyrifos-methyl deltamethrinwas effective and achieved total suppression of progeny production in T. castaneum for all the storage periods. Spinosad was not as effective as chlorpyrifos-methyl deltamethrin mixture at suppressing progeny production of phosphine-resistant T. castaneum. These two insecticides can be used in a phosphine resistance management strategy for R. dominica and T. castaneum in the United States.