Efficacy of β-cyfluthrin and chlorpyrifos-methyl plus deltamethrin applied to concrete surfaces against field strains of three stored-grain insect species
Abstract: The insecticides β-cyfluthrin and chlorpyrifos-methyl plus deltamethrin are approved in the United States for empty bin treatments prior to storing newly-harvested wheat. The susceptibility of adults of 16 field strains of the red flour beetle, Tribolium castaneum (Herbst); seven strains of sawtoothed grain beetle, Oryzaephilus surinamensis (L.); and two strains of the lesser grain borer, Rhyzopertha dominica (F.), collected mainly from farm-stored grain in Kansas, USA, to commercial formulations of the two insecticides at labeled rates was evaluated on concrete surfaces. Concrete-poured 9-cm diameter plastic Petri dishes were used to simulate the concrete floor of empty bins. Adults of laboratory strains of the three species were first exposed to insecticide-treated concrete surfaces for 1 to 24 h to standardize the exposure times for field strains. Based on the time required for ~ 100% mortality of laboratory strains, the adults of T. castaneum and O. surinamensis field strains were exposed to β-cyfluthrin for 24 h while R. dominica were exposed for 2 h. Adults of all species were exposed for 8 h to chlorpyrifos-methyl plus deltamethrin-treated concrete. Chlorpyrifos-methyl plus deltamethrin did not control all R. dominica and most T. castaneum and O. surinamensis field strains. β-cyfluthrin was extremely effective against R. dominica but ineffective against T. castaneum and iO. surinamensis field strains as evidenced by low mortality and high progeny production. Exposing the two least susceptible field strains of O. surinamensis and three of T. castaneum to one to four times the high labeled rate of β-cyfluthrin resulted in only 36 to 90% mortality. Reduced susceptibility in field strains to β-cyfluthrin may be due to inherent formulation deficiency or low levels of tolerance or resistance.