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 (Berliner) was determined for five field-collected populations and one laboratory colony of the European corn borer, Ostrinia nubilalis (Hubner). Field-collected larvae were reared for two generations in the laboratory, and susceptibility of neonate larvae was determined with feeding bioassays where increasing concentrations of the CryIA(b) toxin were applied to the surface of artificial diet. In general, the response to B. thuringiensis among the populations was similar based on the slopes of the dose mortality curves. However, significant differences in susceptibility (greater than 5 fold) based on LC(50) values were observed. Repeated and prolonged exposure to B. thuringiensis is not known to 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 with insecticide exposure.