Factors affecting spinosad toxicity to stored grain insects

Spinosad was registered as a grain protectant in the United States in 2005 at a rate of 1 mg (AI)/kg of grain. In a laboratory study conducted at 28° C and 65% relative humidity, the immediate and delayed effects of dry and liquid formulations of spinosad at labeled rate were characterized against adults of two stored grain insects, the lesser grain borer, Rhyzopertha dominica (F.), and rice weevil, Sitophilus oryzae (L.). In another study, the time-mortality relationships of R. dominica exposed to wheat having 10-100% of the kernels treated with dry and liquid formulations of spinosad at 0.1 and 1 mg (AI)/kg of grain or liquid formulation of spinosad at 1 mg (AI)/mL were investigated. Furthermore, the effects of treating germ/embryo side, endosperm side and whole kernel of wheat with liquid formulation of spinosad at 0.1 and 1 mg (AI)/mL of water on responses of R. dominica were investigated. In general, both dry and liquid formulations of spinosad were equally effective against adults of R. dominica. Spinosad exhibited greater degree of delayed toxicity against adults of R. dominica but not against S. oryzae. For highly sensitive species such as R. dominica, treating 10% of the wheat kernels at labeled rate is as good as treating 100% of kernels and 100% mortality was achieved following 7 days exposure to treated wheat. With regard to treating germ/embryo and endosperm sides of kernel, treating the germ/embryo side or endosperm side is as good as treating the whole kernel for controlling R. dominica.