The high-dose/refuge strategy is considered as the main strategy for delaying resistance in target pests to genetically modified crops that produce insecticidal proteins derived from Bacillus thuringiensis Berliner. This strategy is based on a key assumption that resistance alleles are initially rare (<10−3). To test this assumption, we used an F2 screen on natural populations of Sesamia nonagrioides Lefebvre (Lepidoptera: Noctuidae) from Greece and Spain. In total, 75 lines from Greece and 85 lines from Spain were screened for survival of F2 larvae on Cry1Ab corn, Zea mays L., leaves. No major resistance alleles were found. The frequency of resistance alleles in the Greek population was <9.7 × 10−3 with 95% probability, which was very similar to that of the Spanish population (<8.6 × 10−3 with 95% probability), and the expected frequencies were 3.2 × 10−3 (0–0.0097) and 2.9 × 10−3 (0–0.0086) in Greece and Spain (pooled 1.5 × 10−3). The experiment-wise detection probability of resistance was 94.0 and 97.5% for the Greek and the Spanish population, respectively. Evidence of alleles conferring partial resistance to Cry1Ab was found only for the Greek population. The frequency of alleles for partial resistance was estimated as 6.5 × 10−3 with a 95% credibility interval between 8 × 10−4 and 17.8 × 10−3 and a detection probability of 94%. Our results suggest that the frequency of alleles conferring resistance to Cry1Ab, regarding the population of S. nonagrioides, may be rare enough so that the high-dose/refuge strategy could be applied with success for resistance management.
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Vol. 100 • No. 1