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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]

The baseline toxicity of Cry1A δ-endotoxins (derived from Bacillus thruringiensis (Kurstaki)) on field populations of the cotton bollworm, Helicoverpa armigera (Hub.), was determined through log dose probit analysis. All the three Cry1A proteins were found to be toxic to the bollworm larvae. Cry1Ac was found to be the most toxic followed by Cry1Aa andCry1Ab. LC[sub 50] values ranged from 0.07 to 0.99 μg/ml(14- fold) for Cry1Aa, 0.69 to 9.94 μg/ml (14-fold) for Cry1Ab and 0.01 to 0.67 μg/ml of diet (67-fold) for Cry1Ac. The LC[sub 50] values deduced from the cumulative log dose probit response of the data pooled from all assays, were 0.62 μg/ml for Cry1Aa, 4.43 μg/ml for Cry1Ab and 0.100 μg/ml of diet for Cry1Ac. The respective LC[sub 50] values represent the baseline susceptibility indices for resistance monitoring through the conventional log dose probit assays. The LC[sub 99] values derived from the cumulative data were 515 μg/ml for Cry1Aa, 13385 μg/ml for Cry1Ab and 75 μg/ml of diet for Cry1Ac. These values represent the diagnostic doses for routine monitoring of resistance to the respective toxins through discriminating dose assays after the introduction of Bt transgenic crops. [ABSTRACT FROM AUTHOR]