Tobacco budworm and bollworm populations were exposed to laboratory selection for several generations. Assays were conducted on larvae of different ages of larvae from the selection populations to determine concentration-response regressions, and a range of diagnostic doses were used to assay field populations of mixed species and age structure. Progeny of the selected colonies were exposed to Bt cotton in a plant assay. Resulting data are being used to study factors influencing field selection for resistance.