Field studies were conducted in 1999 and 2000 to evaluate field efficacy of cotton (Gossypium hirsutum L.) expressing two insecticidal proteins of Bacillus thuringiensis Berliner (Bt) on various lepidopteran pests. Near-isogenic varieties including non-transgenic, single-toxin, and dual-toxin cotton were compared for heliothine numbers, numbers of occasional lepidopteran pests, damaged terminals, bolls, and squares, and seed cotton yield. Relative to the non-Bt cotton cultivar, both single and dual-toxin cotton caused a significant reduction in the numbers of heliothine larvae, damaged terminals, and damaged squares and bolls found during visual sampling. Generally, fewer heliothines were found in MON15985 than in DPL50B or DPL50 (Table 1, Figure 1). Additionally, fewer heliothines were found in unsprayed subplots with cotton expressing both Cry1Ac and Cry2Ab than non-Bt subplots treated with insecticide. There was no significant difference in damaged terminals, squares, or bolls (predominantly due to bollworm) between single and dual-toxin varieties, but both Bt cultivars exhibited less damage than non-Bt cotton. Cotton expressing one and two insecticidal proteins reduced cumulative bollworm populations 79% and 96% for both years, respectively. Although single-toxin cotton only incited a 14% cumulative reduction of fall armyworms, populations were reduced by 96% in cotton expressing two Bt toxins compared with non-Bt cotton and cotton expressing Cry1Ac alone. Dual-toxin Bt cotton caused a 94% cumulative reduction in soybean loopers, when compared to non-Bt cotton. More soybean loopers were found in the single-toxin Bt cotton than the non-Bt cultivar. Although both single and dual toxin improved seed cotton yield over non-Bt cotton, there was no significant difference between the two varieties.

These data suggest that the insertion of the Cry2Ab gene will provide substantially better control of a wide range of lepidopteran pests when compared to cotton expressing Cry1Ac alone. Since armyworms and loopers are not as affected by the single-toxin cultivar as heliothines, the true benefit of this second toxin-producing gene will likely be more apparent for these occasional pests. However, control of bollworms should also be improved.