Mortality of 4-day old O. nubilalis larvae [± SE] exposed to Bt toxins. The Bt toxins were either from microbial solutions (activated Cry1Ab toxin) sprayed on non-Bt, isogenic maize leaves or Bt-maize plant material. Exposure times were either continuous for 7 days (Bt continuous = Bt maize, Toxin continuous = Bt toxin-sprayed isogenic maize) or for 24 h only (Bt 24, Toxin 24) followed by Bt-free, isogenic maize for 6 days, respectively. Data represent the mean of 4 replications (n = 16). Different letters above the bars (a, b and c) indicate significant differences between treatments.