Eight strains of red flour beetle, Tribolium castaneum (Herbst), and 21 of the lesser grain borer, Rhyzopertha dominica (F.), collected from wheat stored on farms in Oklahoma were tested for resistance to malathion, chlorpyrifos-methyI, and phosphine. Results of discriminating dose tests of the red flour beetle indicated that all strains were resistant to malathion, one strain was resistant to phosphine, and none was resistant to chlorpyrifos-methyl. Similar discriminating dose tests for the lesser grain borer indicated that all strains were resistant to malathion, dichlorovs, and to chlorpyrifos-methyl, and 8 of 21 strains were resistant to phosphine.
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Zettler et al. (1990) studied this question.