ABSTRACT
Field populations of Helicoverpa zea (Boddie) and Heliothis virescens F. from the eastern half of the U. S. cotton belt were monitored (1996-1998) for tolerance to the Bt toxin CryIAc. The tolerances of field populations of H. zea increased during the three-year period. Areas producing the greatest increase in tolerance to CryIAc had a greater percentage of their acreage planted in Bt cotton. In general, tolerances of H. virescens populations did not change, with the single exception being the third generation of H. virescens collected from the MS Delta. The small changes in tolerance reported herein suggest that populations may be more tolerant of CryIAc but do not show that the current tolerance is at a level to cause control failures in the field.
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