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 ۲۰ and ۳۰ ± ۱ °C by adults of Rhyzopertha dominica (F.)(Coleoptera: Bostrichidae). Samples were taken after zero, ۲, ۳, ۵, ۷, ۱۵, ۳۰, ۶۰, ۹۰ and ۱۸۰ days of insecticides application. Results of median lethal concentrations (LC۵۰s) 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 ۹۹.۰۲ and ۶۹.۳۲ days at ۲۰ ± ۱ °C, respectively. These were shortened at ۳۰ ± ۱ °C to ۴۹.۵۱ and ۴۶.۲۱ 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.

Keywords:

Insecticidal activity , median lethal concentrations , degradation , Rhyzopertha dominica