Influence of grain type on the efficacy of spinetoram for the control of Rhyzopertha dominica, Sitophilus granarius and Sitophilus oryzae

The efficacy of spinetoram against adults of three major stored grain beetle species, the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), the granary weevil, Sitophilus granarius (L.) (Coleoptera: Curculionidae) and the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) in seven grain commodities was evaluated under laboratory conditions. Spinetoram residues in each grain were quantified using high-performance liquid chromatography (HPLC) with UV detection. The tested grains were hard wheat, soft wheat, oats, rye, triticale, paddy rice and maize. Spinetoram was applied at three dose rates: 0.1, 1 and 10 ppm (mg of active ingredient per kg of grain) and bioassays were carried out at 25 °C and 60% r.h. Mortality of the exposed adults was assessed after 7 and 14 d of exposure while progeny emergence was recorded 65 d after the last exposure interval (14 d). Among the tested species, R. dominica was highly susceptible to spinetoram for all commodities tested, since 100% mortality was noted after 14 d of exposure at 1 and 10 ppm. From the two Sitophilus species, S. granarius was more susceptible to spinetoram than S. oryzae, while for both species 0.1 ppm was not effective. For these two species, spinetoram efficacy differed among the tested commodities. In general, mortality was higher in hard wheat for both Sitophilus species in comparison with the other grains, while the lowest mortality levels were recorded in oats and soft wheat for S. granarius and in maize and soft wheat for S. oryzae. Progeny production was low in the case of R. dominica, and F1 adult emergence was recorded mostly at 0.1 ppm. For both S. granarius and S. oryzae, the highest progeny production was noted in soft wheat. Spinetoram residue determination in each grain at the dose rate of 10 ppm, did not show any differences among the treated commodities.