Insecticidal effect of spinetoram and thiamethoxam applied alone or in combination for the control of major stored-product beetle species

Laboratory bioassays were conducted to evaluate the efficacy of two insecticides, spinetoram and thiamethoxam, applied alone or in combination to wheat against adults of Sitophilus granarius, S. oryzae, Oryzaephilus surinamensis, Tribolium castaneum and Cryptolestes ferrugineus, as well as adults and larvae of T. confusum. Spinetoram was applied at two (0.25 and 0.5 ppm) and thiamethoxam at three (1, 2 and 5 ppm) dose rates, whereas all binary combinations of the insecticide concentrations were also tested. Insect mortality was determined after 1, 2, 7, 14 and 21 d of exposure, while 65 d later progeny production was evaluated. In the case of T. confusum larvae, adult emergence was estimated 4 weeks after the initiation of the bioassays. Among the species examined, S. granarius and S. oryzae were the most susceptible to spinetoram in terms of parental mortality, however progeny production was significantly suppressed only in the case of S. granarius. In contrast, O. surinamensis, T. confusum and T. castaneum were the least susceptible to the specific dose rates of spinetoram tested. Thiamethoxam was effective against all species tested providing high levels of parental mortality and progeny production suppression. However, the combined application of thiamethoxam and spinetoram at the specific dose rates tested did not exhibit any additive or synergistic effect against the stored-product beetle species examined. Still, the combined application of these active ingredients did not exhibit any antagonistic effect.