Application and limitations to the use of bioassays for conclusions of Bt resistance evolution in Helicoverpa zea

Field populations of Helicoverpa zea and Heliothis virescens have been established and exposed to Cry1Ac in diet incorporation and diet overlay assays at the University of Arkansas since 2002. These seven years of data provide insight into the magnitude and dynamic variability in susceptibilities of these pests to the first Bt toxin expressed in Bt cotton. Conclusions of resistance evolution on these basis of these data may not be definitive because of high variability in response prior to the commercial deployment of Bt crops, consistent high variability in response during each sample year, lack of strong collaborative relationships between bioassay response and field survival, and conflicting definitions of field resistance. We present a synopsis of our work at the University of Arkansas and relate the collective findings to reports of suspected resistance evolution.