In the years surrounding the 1996 commercial introduction of Bollgard® cotton (Bollgard® is the gene that encodes the CryIAc insecticidal toxin from the soil microbe Bacillus thuringiensis var. kurstaki.), studies have been performed to explore several factors which may influence survival of the cotton bollworm (Helicoverpa zea) in plantings of B.t. transgenic cotton. These factors include: H. zea susceptibility to the B.t. toxin, levels of B.t. toxin expressed in the plant and its temporal and spatial distribution, and the behavioral responses of H. zea larvae to B.t. toxin. Previous LC50 comparisons indicate that H. zea neonate larvae are 4 to 60 times less susceptible to CryIAc than are Heliothis virescens, and that geographically diverse populations of H. zea can exhibit LC50 values that vary as much as 16-fold. CryIAc expression studies show that toxin present in economically important fruiting positions varies from about 10-15 mg/g fresh weight (fw) at 40 days after planting (DAP) to 1-2 mg/g fw at 120 DAP; CryIAc levels in terminal foliage change similarly over time from around 20 mg/g fw to around 5 mg/g fw at 40 and 120 DAP, respectively. Furthermore, evaluation of component parts of blooms show that expression of CryIAc is not uniform; very little toxin is apparent in the pollen. Laboratory studies suggest that H. zea larvae can avoid diet with CryIAc and will preferentially feed upon non-toxic alternatives including untreated diet and, at higher population densities, other larvae. These data and others suggest scenarios whereby H. zea populations, inherently less susceptible to CryIAc than H. virescens and highly variable in their susceptibility, may be able to take advantage of several factors to survive in numbers and produce economic damage to Bollgard® cotton plantings. These factors include: lower levels of CryIAc later in the growing season, non-uniform expression of toxin, especially in blooms, larval avoidance of toxin, and cannibalism.