Susceptibility of field strains of three stored product insect species to spinosad and chlorpyrifos-methyl + deltamethrin

The effectiveness of spinosad and chlorpyrifos-methyl + deltamethrin  at the labeled rates was evaluated against two field strains of the lesser grain borer (LGB); 11 strains of the red flour beetle (RFB); and 6 strains of the saw toothed grain beetle (STGB),  collected from farm bins in Kansas. Twenty-five adults of each species were exposed to 50 g of wheat treated with spinosad at 1.0 ppm and chlorpyrifos-methyl + deltamethrin at 3.0 + 0.5 ppm, to determine susceptibility of the field and corresponding insecticide-susceptible laboratory strains. Mortality of adults was assessed after 7 and 14 days, and progeny production after 42 days. There were significant differences in susceptibility to spinosad among field strains of RFB and STGB (mortality ranging from 2.4 – 58.4%) but not among LGB strains. Chlorpyrifos-methyl + deltamethrin was extremely effective against field strains of all the three species (~ 100% mortality). The two least susceptible strains of RFB and STGB and the two strains of LGB were selected for dose-response studies with spinosad. The LD₅₀ values for the two RFB field strains (5.31- 6.92 ppm) were ~ 2 times greater than that of the laboratory strain (3.17 ppm). Corresponding values for STGB field strains were ~ 4 times and for LGB field strains were ~ 1 time greater than the corresponding laboratory strains. The results indicate variable susceptibility in field populations. Baseline data on the susceptibility of the three insect species to spinosad will be useful for monitoring resistance development and making recommendations to grain managers.