Influence of commodity on the effect of spinosad dust againstRhyzopertha dominica(F.) (Coleoptera: Bostrychidae) andSitophilus oryzae(L.) (Coleoptera: Curculionidae)

We undertook laboratory bioassays to evaluate the insecticidal effect of spinosad dust against two major stored-grain beetle species, Rhyzopertha dominica (F.) and Sitophilus oryzae (L.), in wheat, barley and maize. Spinosad was applied at four dose rates: 20, 100, 500 and 1000 ppm of the formulation, corresponding to 0.025, 0.125, 0.625 and 1.25 ppm of active ingredient, respectively. Adult mortality was measured after 7 and 14 days of exposure in the treated substrate, while progeny production was assessed after the removal of the parental adults. For R. dominica, adult mortality was high and reached 100% at doses ≥0.125 ppm of a.i. after 14 days of exposure. In contrast, mortality of S. oryzae adults was >95% after 14 days of exposure only at 1.25 ppm of a.i. With the same exposure interval, no significant differences were noted in R. dominica mortality levels among the three grains tested, while mortality of S. oryzae was significantly higher on wheat than on barley and maize. More adult progeny were recorded for S. oryzae than for R. dominica. A simple and reliable method using high-performance liquid chromatography/mass spectrometry method (LC/MS) was developed for the precise and selective quantitative determination of spinosyns residues in wheat, barley and maize grains. Spinosad dissipation on different commodities such as maize, barley and wheat differed significantly. Accelerated degradation of spinosad was observed in maize grains 14 days after application; the concentration of spinosad was slightly decreased in barley, but in the case of wheat a negligible degradation of spinosad was observed. These findings suggest that the reduced mortality of S. oryzae in maize is probably due to the high degree of pesticide degradation.