Estimating the frequency of Cry1F resistance in field populations of the European corn borer (Lepidoptera: Crambidae)

BACKGROUND: Transgenic corn hybrids that express toxins from Bacillus thuringiensis (Bt) have suppressed European corn borer populations and reduced the pest status of this insect throughout much of the US corn belt. A major assumption of the high-dose/refuge strategy proposed for insect resistance management and Bt corn is that the frequency of resistance alleles is low so that resistant pests surviving exposure to Bt corn will be rare. RESULTS: The frequency of resistance to the Cry1F Bt toxin was estimated using two different screening tools and compared with annual susceptibility monitoring based on diagnostic bioassays and LC₅₀ and EC₅₀ determinations. An F₁ screening approach where field-collected individuals were mated to a resistant laboratory strain and progeny were assayed to determine genotype revealed that resistance alleles could be recovered even during the first year of commercially available Cry1F corn (2003). Estimates of frequency from 2003-2005 and 2006-2008 indicated that, although allele frequency was higher than theoretical assumptions (0.0286 and 0.0253 respectively), there was no indication that the frequency was increasing. Similar estimates in 2008 and 2009 using an F₂ screening approach confirmed the presence of non-rare resistance alleles (frequency≈0.0093 and 0.0142 for 2008 and 2009, respectively). The results of both screening methods were in general agreement with the observed mortality in diagnostic bioassays and LC₅₀ and EC₅₀ determinations. CONCLUSIONS: These results are consistent with previous modeling results, suggesting that the high-dose/refuge strategy that is in place for Bt corn may be effective in delaying resistance evolution even when a relatively high frequency of resistance alleles exists.