The "high dose/refuge" strategy using a single Bt gene was the only tactic available to delay insect resistance to Bt transgenic crops prior to late 2002. Theoretical models and our previous experimental studies suggest that plants containing two dissimilar Bt toxin genes in the same plant (i.e. "stacked" or "pyramided") have the potential to delay resistance more effectively than single toxin plants used sequentially or in mosaics. Regulatory applications for stacked cotton plants with two genes derived from Bt (Cry1Ac and Cry2Ab2) were approved for commercial use in Australia and the U.S. in 2002. However, there may be concern that insects will evolve resistance more rapidly to both toxins if plants containing stacked genes are utilized in conjunction with plants expressing a single Bt gene. We conducted greenhouse studies using a unique model system composed of broccoli plants transformed to express different Cry toxins (Cry1Ac, Cry1C or both) combined with populations of diamondback moth, Plutella xylostella, carrying resistance to each Bt toxin. Our results indicate that the concurrent use of one- (cry1Ac) and two-gene (cry1Ac and cry1C) plants will select for resistance more rapidly than using two-gene plants alone, especially when the frequency of initial resistance alleles is high (> 0.1) for monogenic resistance such as Cry1A-resistance.
Species 1: Lepidoptera Plutellidae Plutella xylostella
Keywords: Bacillus thuringiensis, resistance
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