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Bt cotton plants, with bollworm resistant trait, have been widely adopted since introduced in India in 2002. As a proactive measure to prevent evolution of resistance to Bt protein(s), it was approved for commercial cultivation with a stipulation of structured refuge of minimum 20% of the Bt area, constituted by plants that do not produce Bt toxins and thus allow growth and development of population of Bt susceptible pests. Though the refuge cotton seeds are provided in a separate pouch within each Bt cotton seed packet, Indian farmers have been reluctant to sow refuge. To find out a method for ensured refuge planting, we experimented with 'built-in-refuge' (BIR) in which non-Bt cotton seeds were blended with Bt cotton seeds in various proportions ie 5,10, 50% along with 20% recommended structured refuge and each treatment was evaluated in terms of fruiting body damage by major cotton lepidopterans, relative to the regulator stipulated 20% non-Bt structured refuge at five locations during cotton growing season of 2012–13. A major outcome from this study was that treatments with 5 and 10% non-Bt blends with bollga0rd-II were as effective as the 20% structured refuge, based on fruiting body damage due to bollworms, a key factor in managing Bt resistance without any compromise to the seed cotton yield. It can be concluded that the seed blend technology is a good option to impose refugia compliance to delay resistance in Bt cotton to bollworms in a country having lesser alternate hosts available for the target bollworms during the season and voluntary compliance of refuge is absent. [ABSTRACT FROM AUTHOR]

Abstract: The geographical variability in Helicoverpa armigera susceptibility levels to Cry2Ab toxin expressed by transgenic maize (not currently commercialized in India) was determined through log dose probit assays conducted on populations collected during 2004–05 (from 25 districts) and 2007–08 (from 22 districts) across India. While 2004–05 is the year prior to commercialization of dual gene Bollgard-II-Bt cotton that expresses Cry2Ab in addition to Cry1Ac, 2007–08 is two years after its commercial release. LC50 values ranged from 6.0 to 28.6 μg Cry2Ab/ml of diet in 2004–05 and from 2.46 to 34.7 μg Cry2Ab/ml of diet in 2007–08. The IC50 range in 2004–05 and 2007–08 was from 0.31 to 2.3 μg/ml and 0.10–3.4 μg/ml of diet, respectively. The probit analysis data of 2004–05 can be used as baseline indices to monitor for changes in the H. armigera susceptibility to Cry2Ab, subsequent to the introduction of Bollgard-II cotton in India. The probit analysis data of 2007–08 showed that the baseline has not undergone any significant changes two years after cultivation of Bollgard-II in India. The relatively low toxicity of Cry2Ab and its higher expression late in the cotton season compared with Cry2Ab, compounded with its presence in Bt hybrids on a separate linkage group from Cry1Ac suggest problems of resistance in the future. [Copyright &y& Elsevier]