Bioactivity and stability of spinetoram and spinosad on stored wheat as determined by adults of Rhyzopertha dominica (Coleoptera: Bostrichidae) bioassay

To examine the differences between spinetoram and spinosad in their insecticidal activity and stability on the stored wheat, bioassay tests were carried out for six consecutive months at 20 and 30 ± 1 °C by adults of Rhyzopertha dominica (F.)(Coleoptera: Bostrichidae). Samples were taken after zero, 2, 3, 5, 7, 15, 30, 60, 90 and 180 days of insecticides application. Results of median lethal concentrations (LC50s) determined after three days of treatment indicated that spinosad was significantly more toxic than spinetoram against the adults. Results also revealed that mortality decline of R. dominica was correlated with temperature and the gradual degradation of the two insecticides over time during storage period. The half-life of spinosad and spinetoram on stored wheat were 99.02 and 69.32 days at 20 ± 1 °C, respectively. These were shortened at 30 ± 1 °C to 49.51 and 46.21 days, respectively. However spinosad displayed more bioactivity and stability than spinetoram. It can be concluded that spinosad and spinetoram do not remain persistent and even a slow degradation of these insecticides is likely to occur during wheat storage. Residues of spinosad and spinetoram persist on the wheat grains at levels well below the established tolerance levels.