Broad-spectrum resistance to Bacillus thuringiensis toxins in Heliothis virescens.

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Abstract
Evolution of pest resistance to insecticidal proteins produced by Bacillus thuringiensis (Bt) would decrease our ability to control agricultural pests with genetically engineered crops designed to express genes coding for these proteins. Previous genetic and biochemical analyses of insect strains with resistance to Bt toxins indicate that (i) resistance is restricted to single groups of related Bt toxins, (ii) decreased toxin sensitivity is associated with changes in Bt-toxin binding to sites in brush-border membrane vesicles of the larval midgut, and (iii) resistance is inherited as a partially or fully recessive trait. If these three characteristics were common to all resistant insects, specific crop-variety deployment strategies could significantly diminish problems associated with resistance in field populations of pests. We present data on Bt-toxin resistance in Heliothis virescens, a major agricultural pest targeted for control with Bt-toxin-producing crops. A laboratory strain of H. virescens developed resistance in response to selection with the Bt toxin CryIA(c). In contrast to other cases of Bt-toxin resistance, this H. virescens strain exhibits cross-resistance to Bt toxins that differ significantly in structure and activity. Furthermore, the resistance in this strain is not accompanied by significant changes in toxin binding, and resistance is inherited as an additive trait when larvae are treated with high doses of CryIA(c) toxin. These findings have important implications for Bt-toxin-based pest control.