Countering multiple resistance with modified Bt toxins

Evolution of resistance in pests threatens the long-term efficacy of insecticidal proteins from Bacillus thuringiensis (Bt) used in sprays and transgenic crops.  For example, field-evolved resistance to Bt cotton producing the single Bt toxin Cry1Ac has been documented for pink bollworm (Pectinophora gossypiella) in western India.  In laboratory diet bioassays, we found that the genetically modified Bt toxins Cry1AbMod and Cry1AcMod effectively countered pink bollworm resistance to the native Bt toxins Cry2Ab, Cry1Ab and Cry1Ac.  Resistance ratios based on the concentration of toxin killing 50% of larvae for a lab-selected resistant strain relative to a susceptible strain were 210 for Cry2Ab, 270 for Cry1Ab, and 310 for Cry1Ac, but only 1.6 for Cry1AbMod and 2.1 for Cry1AcMod.  For both the resistant and susceptible strains, the results show slight but significant synergism between Cry1AbMod and Cry2Ab, whereas the other combinations of toxins tested did not show consistent synergism or antagonism.  The results suggest that the modified toxins may be useful for managing populations of pink bollworm resistant to Cry1Ac, Cry2Ab, or both.