Response estimates for assessing heliothine susceptibility to Bt toxins

Field and laboratory colonies of Helicoverpa zea (Boddie) and Heliothis virescens (F.) were exposed to CrylAc and Cry2Ab2 toxins to compare different mortality and mortality + stunting responses. Lethal concentrations for mortality (LC(50S)), molt inhibitory concentrations for mortality + surviving L1 larvae (MIC(50S)), and inhibitory concentrations for mortality + surviving L1 and L2 larvae (IC(50S) were established to describe the different responses across a range of test concentrations. MIC(50S) and IC(60(50S) at 7 d significantly correlated to LCsos at 7 d, but MIC(50S) were more closely correlated with LC(50S) than IC(50S). Experiments conducted to determine the fate of survivors (larval stage L1, L2, or L3 larvae) after 7 d of exposure of Bt toxin and transfer to untreated or Bt-treated diet revealed that significant numbers of stunted larvae recovered and survived to pupation. Percentage of survival was significantly higher for larvae fed untreated diet after 7 d on Bt-treated diet, but a few survivors also were found when larvae were fed only Bt diet. Survival to pupation of larvae classified as L1 at 7 d was significantly lower than those for L2 or L3 larvae at 7 d, indicating that many stunted larvae suffer chronic effects of the initial exposure to Bt and fail to reach the next life stage. Pupation of larvae classified as L1 at 7d was reduced and delayed in comparison with those for L2 and L3 larvae at 7d. Collectively, these data suggest that MIC50 estimates at 7 d more accurately project total larval mortality from Bt exposure than do LC50 or IC50 at 7 d.