Baseline susceptibility of European corn borer (Lepidoptera : Crambidae) to Bacillus thuringiensis toxins

Susceptibility to CrylAb and CrylAc toxins from Bacillus thuringiensis was determined for 11 populations of neonate European corn borer,Ostrinia nubil alis (Hubner), from the United States and 1 from northern Italy. Corn borer larvae were exposed to artificial diet treated with increasing B. thuringi ensis concentrations, and mortality and growth inhibition were evaluated af ter 7 d. The range of variation in B. thuringiensis susceptibility indicate d by growth inhibition was very similar to that indicated by mortality. Alt hough interpopulation variation insusceptibility to both proteins was obser ved, the magnitude of the differences was small (less than or equal to 4-fo ld) and comparable to the variability observed among generations within a p articular population (less than or equal to 3-fold). Additionally, there wa s no indication that B. thuringiensis susceptibility was influenced by pher omone race, voltine ecotype, or geographic location. These results suggest that the observed susceptibility differences reflect natural variation in B . thuringiensis susceptibility among corn borer populations rather than var iation caused by prior exposure to selection pressures. Therefore, European corn borers apparently are susceptible to B. thuringiensis toxins among po pulations across most of their geographic range.