Phosphine gas (PH3) is one of the most commonly used fumigants for controlling stored-grain pests worldwide. We estimated the discriminating dose for Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) adult insects using a laboratory susceptible strain. This discriminating dose was then used to determine presence or absence of PH3 resistance (resistance frequencies) in 19 field-collected populations of C. ferrugineus from Oklahoma, United States. The discriminating dose was estimated as 56.2 ppm of PH3 over a 20-h exposure period at 25 °C. Discriminating dose bioassay results showed that PH3 resistance was present in all 19 populations of C. ferrugineus tested. However, five populations-Stillwater (Stil), Enid Terminal 1 (ET-1), Enid Terminal 2 (ET-2), Johnson-Enid population (JE), and DK Farm 20 population (DK Farm 20) had ≥90% resistance frequencies. LC99 values estimated by probit analyses of dose-response mortality data for the laboratory susceptible strain (Lab-S), JE, and DK Farm 20 were 7.3, 636.4, and 968.6 ppm, respectively, over a 3-d exposure period. The level of resistance in DK Farm 20, the most resistant population, was 133.5 times that of the susceptible laboratory strain. This study shows that PH3 resistance in C. ferrugineus may be widespread in Oklahoma. Based on this study, there is a need for a wider PH3 resistance survey in grain-growing regions of Oklahoma and United States. Furthermore, results show there is a need to develop PH3 resistance management strategies for C. ferrugineus and other stored-product insect pest species to combat resistance and ensure continued effective future use of PH3.

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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 findings necessitate development of a phosphine resistance management strategy to ensure continued effective use of phosphine. Therefore, we investigated the efficacies 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 spinosad- or 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 83-100% 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 93-99% mortality. However, chlorpyrifos-methyl + deltamethrin was 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.