A laboratory strain of the tobacco budworm, Heliothis virescens, was selected every generation by larval dip using Denim^® (emamectin benzoate). A moderate level of resistance was detected after selection for seven successive generations. The LC₅₀ for the parental strain by dip was 0.8 PPM while that for the selected strain (generation 8) was 6.1 PPM, a resistance ratio of 7.6-fold. Using this resistant strain, we developed a feeding disruption assay for the diagnosis of Denim^® resistance in neonates of the tobacco budworm. The presence of blue feces on a white background is a marker for resistance. The kit is in the commercialization phase of development.