Toxicity of three organophosphorus compounds and pyrethrins to malathion-resistant Tribolium castaneum (Herbst) (Coleoptera, Tenebrionidae)

Malathion-resistant red flour beetles, Tribolium castaneum (Herbst), were tested for susceptibility to Bay 77488 (o,o-diethyl phosphorothioate o-ester with phenylglyoxylonitrile oxime), Bay 78182 (o,o-diethyl phosphorothioate o-ester with (o-chlorophenyl)glyoxylonitrile oxime), diazinon, and pyrethrins by topical applications. The malathion-resistant beetles showed 13·3-fold resistance to pyrethrins, 19·7-fold resistance to Bay 78182, 7·7- and 10·7-fold resistance to diazinon, and only 4·5- and 9·5-fold resistance to Bay 77488. Only about one-quarter as much Bay 77488 as malathion was required for the ₅₀ in the resistant beetles. About one-third as much Bay 78182 as malathion was required. Diazinon and pyrethrins were less toxic than malathion to the resistant insects.