The effectiveness of different concentrations of chlorpyrifos-methyl and cyfluthrin, singly and in combination, on the mortality of stored-grain beetles and their progeny was evaluated over time in an initial experiment. Subsequently, a new product, Storcide((R)), containing both insecticides was granted tolerance for emergency exemption (US Environmental Protection Agency) and also evaluated for residual activity in preventing infestations. Treated wheat was held for various periods of time and at constant or fluctuating temperatures before adult stored-grain pests were introduced. The beetles utilised were Rhyzopertha dominica, Sitophilus oryzae and Tribolium castaneum. Bioassays for adult mortality were evaluated after a 7-day exposure and for progeny production after seven weeks. All treatments that contained cyfluthrin effectively controlled R. dominica. Sitophilus oryzae was not controlled by either cyfluthrin or chlorpyrifos-methyl when used alone. When cyfluthrin at 2 ppm was combined with chlorpyrifos-methyl at 0.5-6 ppm, mortality of S. oryzae adults was greater than 80% and only a few progeny were produced at the lowest concentrations. Tribolium castaneum adults were the most difficult to kill, even at the highest combinations. However, very few progeny were produced in any treatment. Grain treated with Storcide((R)) (3 ppm chlorpyrifos-methyl and 2 ppm cyfluthrin) had nearly 100% adult mortality for R. dominica and S. oryzae at all time periods. No R. dominica progeny were produced and there was only one S. oryzae adult produced at 48 weeks. Storcide((R))'s effectiveness on adult T castaneum decreased with increasing time but no progeny were produced. Storage at warm temperatures did not seem to affect the residual activity of the insecticides for the time periods studied. Combining insecticides at lower dosages than when used singly may be an effective strategy for controlling stored grain insects.
