Development of diagnostic concentrations for monitoring Bacillus thuringiensis resistance in European corn borer (Lepidoptera : Crambidae)

Two candidate diagnostic concentrations of the Cry1Ab and Cry1Ac toxins fro m Bacillus thuringiensis corresponding to the LC99 and EC99 (effective conc entration that causes 99% growth inhibition) for European corn borer, Ostri nia nubilalis (Hubner), were determined based on previously obtained baseli ne data. Validation experiments using field-collected European corn borer p opulations from across North America showed that for Cry1Ab, a concentratio n corresponding to the upper limit of the 95% confidence interval of the LC 99, produced mortality >99% for all populations tested. However, for Cry1Ac , adjustments and further validation are probably necessary. Development of B. thuringiensis resistance monitoring programs that rely on diagnostic te chniques are discussed.