Why do F1 screens estimate higher frequencies of Cry2Ab resistance in Helicoverpa armigera (Lepidoptera: Noctuidae) than do F2 screens?

F2 and F1 tests to detect resistance to Cry2Ab in Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) were performed during the 2007-2008 summer. F2 tests indicated a resistance frequency of 0.006, which is similar to the published resistance frequencies for this species during the summers spanning 2002-2006. In contrast F1 tests indicated a resistance frequency of 0.033. Thus, F1 tests isolated Cry2Ab resistance alleles almost six-fold more frequently than the F2 method. A discrepancy might be expected if the F2 tests detected resistance conferred by more than one locus because F1 tests identify only the form of resistance present in the tester resistant colony. However, if so, F2 tests would detect more, not fewer, cases of resistance. In addition, complementation tests on 10 separate isolates indicate that there is only one common form of resistance. We hypothesized that some "resistance alleles" are homozygous lethal if autozygous (as generated in F2 tests) but not as allozygous homozygotes (as generated in F2 tests). The hypothesis was extended to accommodate the possibility that alleles at linked loci may be homozygous lethal. Neither of two tests of the hypothesis provided evidence that any alleles that confer resistance are associated with severe fitness costs. Thus we are presently unable to explain the basis of the difference in frequencies between the methods. Because of the simplicity of the F1 tests, it is difficult to imagine that it overestimates the frequency of resistance and we therefore accept that this test should provide a more robust method to estimate the frequency of Cry2Ab resistance in H. armigera.