Environmental impact of genetically modified crops.

Chapter 5 (Page no: 74)

Insect resistance to genetically modified crops.

Crops genetically modified to produce crystal (Cry) toxins from Bacillus thuringiensis (Bt) for insect control can reduce reliance on conventional insecticides. Evolution of resistance to Bt toxins by insect populations is the primary threat to the continued success of this approach. Resistance of lepidopteran insects to Bt toxins in the Cry1A family commonly entails recessive inheritance and reduced toxin binding to midgut membrane target sites. Analysis of more than a decade of data from studies monitoring resistance to Bt maize and Bt cotton shows that field-evolved resistance was detected in some US populations of Helicoverpa zea, but not in populations of five other major lepidopteran pests from Australia, China, Spain and the USA: Helicoverpa armigera, Heliothis virescens, Ostrinia nubilalis, Pectinophora gossypiella and Sesamia nonagrioides. The resistance of H. zea to the Cry1Ac toxin in Bt cotton has not caused widespread crop failures, in part because insecticide sprays and two-toxin cotton producing Cry2Ab and Cry1Ac have been used to control this pest. Field-evolved resistance also has been reported recently to Bt corn producing Cry1Ab in Busseola fusca in South Africa and to Bt corn producing Cry1P in Spodoptera frugiperda in Puerto Rico.The documented field outcomes are consistent with projections from modelling based on the population genetic principles underlying the refuge strategy. In particular, H. zea was expected to evolve resistance faster than other pests because it has non-recessive inheritance of resistance to Cry1Ac. In other words, the concentration of Cry1Ac is not sufficient to kill a high percentage of hybrid progeny from matings between resistant and susceptible moths. The results suggest that refuges of non-Bt host plants have helped to delay resistance.

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