We discuss assay approaches for monitoring the sensitivity of Lepidoptera to Bacillus thuringiensis (Bt) insecticidal proteins and compare the relative sensitivity of larval feeding bioassays in which, respectively, mortality or growth inhibition were scored. Heliothis virescens (F.) and Helicoverpa zea (Boddie), major lepidopteran pests targeted for control by transgenic cotton, were used for assay comparison. Larval growth inhibition assays using sublethal CryIA(c) protein concentrations were considerably more sensitive than dose-response mortality assays. Growth inhibition assays were easy to set-up and read, and could readily deliver a diagnostic dose allowing for visual discrimination of resistant from susceptible phenotypes. The ability of a larval growth assay, combined with a diagnostic dose, to unambiguously separate resistant from susceptible insects was validated using a CryIA(c) protein resistant strain of H. virescens and F 1 hybrids derived by crossing the resistant strain to a susceptible H. virescens strain.
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