Frequency of resistance to Bacillus thuringiensis toxin Cry1Ab in an Iowa population of European corn borer (Lepidoptera : Crambidae)

The refuge plus high-dose strategy for resistance management assumes that t he frequency of resistance alleles is low. We used an F-2 screen to estimat e the frequency of resistance to transgenic corn that produces Bacillus thu ringiensis Berliner Cry1Ab toxin (Bt corn) in an Iowa population of Europea n corn borer, Ostrinia nubilalis (Hubner). We also proposed a modification to the statistical analysis of the F-2 screen that extends its application for nonuniform prior distributions and for repeated sampling of a single po pulation. Based on a sample of 188 isofemale lines derived from females cau ght at light traps during the 2nd flight of 1997, we show with 95% confiden ce that the frequency of resistance to Bt corn was <3.9 x 10(-3) in this Io wa population. These results provide weak evidence that the refuge plus hig h-dose strategy may be effective for managing resistance in O. nubilalis to Bt corn. Partial resistance to Cry1Ab toxin was found commonly. The 95% CI for the frequency of partial resistance were [8.2 x 10(-4), 9.4 x 10(-3)] for the Iowa population. Variable costs of the method were $14.90 per isofe male line. which was a reduction of 25% compared with our initial estimate.