SUSCEPTIBILITY OF FIELD POPULATIONS OF THE EUROPEAN CORN-BORER, OSTRINIA-NUBILALIS (HUBNER) (LEPIDOPTERA, PYRALIDAE), TO BACILLUS-THURINGIENSIS (BERLINER)

Susceptibility to the CryIA(b) protein from Bacillus thuringiensis (Be rliner) was determined for five field-collected populations and one la boratory colony of the European corn borer, Ostrinia nubilalis (Hubner ). Field-collected larvae were reared for two generations in the labor atory, and susceptibility of neonate larvae was determined with feedin g bioassays where increasing concentrations of the CryIA(b) toxin were applied to the surface of artificial diet. In general, the response t o B. thuringiensis among the populations was similar based on the slop es of the dose mortality curves. However, significant differences in s usceptibility (greater than 5 fold) based on LC(50) values were observ ed. Repeated and prolonged exposure to B. thuringiensis is not known t o have occurred in the populations tested indicating that the observed tolerance was due simply to natural variability among geographically distinct populations and unrelated to selective pressures associated w ith insecticide exposure.