Efficacy of partial treatment of wheat with spinosad against adults of the lesser grain borer, Rhyzopertha dominica (F.)

The efficacy of partial treatment of wheat with spinosad against adults of the lesser grain borer, Rhyzopertha dominica (F.), was evaluated by mixing spinosad-treated and untreated kernels in varying proportions. Spinosad was applied to wheat kernels by dipping in 1 mg (AI)/ml spinosad solution for 1 min, and by admixing with dry and liquid formulations at 0.1 and 1 mg (AI)/kg of wheat. In kernel dipping method, the percentage of treated kernels was increased from 10 to 100 in 10% increments keeping the total number of kernels at 10 (in Petri dishes) or 100 (in vials). The mortality of introduced adults was observed over time (1- 209 h) after adult introduction.  In admixture method, treated and untreated wheat (50 g) was combined in various proportions. Adult mortality was examined after 1, 3, 5, and 7 d. In the kernel dipping method, lethal times for 50 (LT₅₀) and 95% (LT₉₅) mortality of R. dominica adults decreased as the percentage of treated kernels increased. The magnitude of decrease was greater at LT₉₅ values than at LT₅₀ values when the number of kernels was 10, while it was more or less similar for LT₅₀ and LT₉₅ values when the number of kernels was 100. In the admixture method, adult mortality increased with an increase in percentage of treated kernels, spinosad rate, and exposure time. At labeled rate (1 mg(AI)/kg), treating 30 to 90% of the kernels with spinosad was as good as treating 100% kernels in controlling R. dominica adults after 3 d of exposure.