Understanding evolution of resistance to pyramided Bt crops in Helicoverpa zea

To delay evolution of pest resistance to transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt), the "pyramid" strategy uses plants that produce two or more toxins that kill the same pest.  One key assumption favoring success of two-toxin plants is that they kill insects resistant to one toxin, which is called "redundant killing."  In Helicoverpa zea, selection with Cry1Ac increased survival on two-toxin cotton, which contradicts the assumption of redundant killing.  Three factors may undermine redundant killing in H. zea: low inherent efficacy of toxins, declines in toxin concentrations during the growing season, and cross-resistance between toxins.  Simulation models show that factors decreasing redundant killing can significantly reduce durability of the pyramid strategy for pests with low susceptibility to Bt toxins like H. zea.