Variation in susceptibility of laboratory and field strains of three stored product insect species to insecticides recommended for empty bin treatments

Applying insecticides to empty bins prior to storing newly-harvested grain is one of the strategies for stored-product insect management. The time-dependent immediate knockdown (KD), 7-day mortality, and progeny production of adults of 16 strains of the red flour beetle (RFB), 7 strains of sawtoothed grain beetle (STGB), and 2 strains of the lesser grain borer (LGB) were evaluated by exposing them on concrete surfaces treated with β-cyfluthrin at low (0.01 g[AI]/m²) and high (0.02 g[AI]/ m²) rates and chlorpyrifos-methyl + deltamethrin (0.12 + 0.02 g[AI]/m²). All strains were recently-collected field strains and were compared with laboratory strains reared since 1999. The minimum time for ~ 100% KD and mortality of laboratory strains was established through time-response studies. Nonlinear or linear models fitted to KD and mortality data showed significant differences among species and insecticides. Mortality of all species was lower than KD because of recovery when placed on food after different exposure time intervals for different species. Chlorpyrifos-methyl + deltamethrin did not control most field strains of LGB and STGB. β-cyfluthrin was not very effective against RFB and STGB field strains as evidenced by low mortality after exposure and subsequently high  progeny production. Exposing the two least susceptible field strains of RFB and STGB to 1X - 4X the high labeled rate of β-cyfluthrin resulted in only 36-90% mortality. This reduced susceptibility in field strains could be an indication of low levels of resistance to cyfluthrin.