The influence of transgenic cottons expressing one or two Bacillus thuringiensis proteins against Helicoverpa zea (Lepidoptera: Noctuidae) and factors affecting Bacillus thuringiensis resistance in Helicoverpa zea

Because control of bollworm with transgenic Bacillus thuringiensis var. kurstaki cottons that express the Cry1Ac protein has not been absolute compared to that of tobacco budworm, transgenic cottons expressing two B. t. proteins (Cry1Ac + Cry2Ab) have been developed to provide greater control of bollworm. To substantiate this, various field, greenhouse, and laboratory studies were designed to investigate the influence of single-gene and dual-gene B. t. cottons, Bollgard RTM and Bollgard II, respectively, against bollworm and to determine the elements affecting resistance evolution to the Cry1Ac and Cry2Ab delta-endotoxins. Bollgard II cottons exhibited an increased field efficacy against bollworm by reducing larval survival and associated damage compared to Bollgard cottons. Seed cotton yields of Bollgard II cottons were comparable to those of Bollgard cottons under pyrethroid-treated and non-treated conditions. In greenhouse studies, Bollgard II cottons exhibited an increased efficacy against both a feral and Cry1Ac-selected strain of bollworm compared to that of Bollgard cottons. The Cry1Ac-selected strain exhibited increased survival and damage potential compared to the feral strain on both Bollgard and Bollgard II cottons, which suggests that Bollgard II cottons should be commercialized before field level resistance occurs to Bollgard cottons. In field studies, Bollgard II cottons offered the best B. t. resistance management strategy available for bollworm since bollworm adult production was much lower in Bollgard II cottons than in Bollgard cottons. Pyrethroid oversprays may also serve as an effective resistance management tool by reducing bollworm larval production and consequently, adult production. Laboratory studies indicated that no measurable shift has occurred in the frequency of major resistance genes for Cry1Ac or Cry2A proteins. Reciprocal cross data suggested that inheritance of resistance to Cry1Ac and Cry2Ab proteins is dominant or semi-dominant. Results demonstrated that there is heritable variation for minor resistance genes that could decrease the efficacy of Bollgard cottons.