Laboratory studies were performed to characterize the lepidopteran toxicity of cotton plants expressing two different toxin proteins from Bacillus thuringiensis (Bt), in order to assess insect resistance management implications of a commercial, two‐toxin transgenic cotton. An independent and additive interactive effect of two Bt δ‐endotoxins expressed by the transgenic cotton variety 15985 was demonstrated by examining the responses of Heliothis virescens (F.), Helicoverpa zea (Boddie), and Spodoptera frugiperda (J.E. Smith) larvae to field‐ or greenhouse‐grown tissue from genetic near‐isolines, which expressed Cry1A only, Cry2Ab only, or both toxins. In all cases, the Cry2Ab component was the larger contributor to total toxicity in the two‐toxin isoline. Toxin‐specific, quantitative enzyme‐linked immunosorbent assay (ELISA) tests confirmed that the levels of each toxin in tissues of the two‐toxin isoline were not statistically different (P > 0.05) from the levels found in the corresponding tissues of the respective single‐toxin isoline. Resistance management considerations were discussed. Considering the additive interaction of toxins, a relatively simple insect resistance‐monitoring procedure was proposed for the monitoring of commercial cotton varieties expressing both toxins.

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