ABSTRACT

Laboratory colonies of the cotton bollworm, Helicoverpa zea were established from field populations of larvae collected from Bt and conventional crops in Arkansas and surrounding states during 2002 and 2003. Progeny, usually those from the 1^st to 3^rd generations following colonization, were exposed to a range of Cry1Ac endotoxin protein in a standard diet incorporation assay. Concentration-response regressions were compared to those obtained with similar procedures at Mississippi State University during 1992 and 1993. Resulting LC50s for several colonies, especially those established from larvae surviving on Bt crops, were higher than those measured prior to the commercial release of Bt cotton and Bt corn. Assays with progeny from field colonies crossed and backcrossed to a laboratory susceptible colony illustrated dominant or incompletely dominant inheritance with a maternal influence. Trends in survivorship on the treated diet were confirmed in assays with larvae fed Bt cotton leaf tissue. Results suggest that field selection for Bt resistance may be measurable by collecting larvae from the field environment and conducting traditional assays of the progeny in the laboratory. Genetic variability in H. zea populations to Cry1Ac appears to be a component of field control problems, but lower LC50s in colonies from conventional crops suggest that susceptible insects from non-Bt crop hosts often dilute frequencies of Bt resistance genes in H. zea. The higher LC50s in colonies from Bt crops and a few late-season colonies from conventional cotton illustrate the genetic potential for resistance and the need to continue to manage selection pressure.