Efficacy of spinosad in layer-treated wheat against five stored-product insect species

The biological insecticide spinosad was evaluated in laboratory bioassays as a surface treatment for wheat to control adult Rhyzopertha dominica, Sitophilus oryzae and three psocid species, Liposcelis paeta, L. bostrychophila, and Lepinotus reticulatus. Spinosad was applied at 1 ppm to 35 g of wheat placed in a vial or to the upper one half, one fourth, or one eighth layer of the wheat; insects were either added to the vials before or after the wheat. When R. dominica were introduced into the vials after the wheat, mortality was 100% except for 83% mortality in the one eighth layer treatment. In contrast, when adults were placed in the vials before the wheat, mortality was 100% only when all of the wheat was treated. Mortality of S. oryzae was lower compared to R. dominica but there was some evidence of upward movement into the treated layers. Mortality of L. paeta and L. bostrychophila was <50% when the entire quantity was treated, in contrast to 100% mortality of L. reticulatus. However, for all psocid species, overall mortality decreased with decreasing depth of the treated layer. The results of this laboratory study show that while spinosad has some effectiveness as a layer treatment on a column of wheat, efficacy will be dependent on the target species, the depth of the treated layer, and the upward or downward mobility of the insect species.