Resistance to the CrylAc δ-Endotoxin of Bacillus thuringiensis in the cotton bollworm, Helicoverpa annigera (Lepidoptera: Noctuidae)

Abstract

Three laboratory strains of Helicocepa annigera Hübner) were established by mating of field-collected insects with an existing insecticide-susceptible laboratory strain. These strains were cultured on artificial diet containing the CrylAc protoxin of Bacillus thuringiensis using three different protocols. When no response to selection was detected after 7-11 generations of selection, the three strains were combined by controlled mating to preserve genetic diversity. The composite strain (BX) was selected on the basis of growth rate on artificial diet containing CrylAc crystals. Resistance to CrylAc was first detected after 16 generations of continuous selection. The resistance ratio (RR) peaked approximately 300-fold at generation 21, after which it declined to oscillate between 57- and 111-fold. First-instar H. annigera from generation 25 (RR = 63) were able to complete their larval development on transgenic cotton expressing CrylAc and produce fertile adults. There appeared to be a fitness cost associated with resistance on cotton and on artificial diet. The BX strain was not resistant to the commercial Bt spray formulations DiPel and XenTari, which contain multiple insecticidal crystal proteins, but was resistant to the MVP formulation, which only contains CrylAc. The strain was also resistant to CrylAb but not to Cry2Aa or Cry2Ab. Toxin binding assays showed that the resistant insects lacked the high affinity binding site that was detected in early generations of the strain. Genetic analysis confirmed that resistance in the BX strain of H. annigera is incompletely recessive.