Abstract
The Ebro Valley (Spain) is the only hotspot area in Europe where resistance evolution of target pests to Cry1Ab protein is most likely, owing to the high and regular adoption of Bt maize (>60%). The high-dose/refuge (HDR) strategy was implemented to delay resistance evolution, and to be effective it requires the frequency of resistance alleles to be very low (<0.001). An F2 screen was performed in 2016 to estimate the frequency of resistance alleles in Sesamia nonagrioides from this area and to evaluate if the HDR strategy is still working effectively. Out of the 137 isofemale lines screened on Cry1Ab maize leaf tissue, molted larvae and extensive feeding were observed for two consecutive generations in one line, indicating this line carried a resistance allele. The frequency of resistance alleles in 2016 was 0.0036 (CI 95% 0.0004–0.0100), higher but not statistically different from the value obtained in 2004–2005. Resistance does not seem to be evolving faster than predicted by a S. nonagrioides resistance evolution model, but the frequency of resistance is now triple the value recommended for an effective implementation of the HDR strategy. Owing to this, complementary measures should be considered to further delay resistance evolution in the Ebro Valley.
Highlights
The commercial use of genetically engineered (GE) crops in Europe has been controversial
A total of 1,327 fifth and sixth instar larvae of S. nonagrioides were collected in non-Bt maize fields at four regions of the Ebro Valley in September and October of 2016 (Table 1)
For two consecutive generations larvae of line P350 molted to second instar and caused substantial feeding damage after 5 days on Bt leaf tissue, indicating that this line carried a resistance allele
Summary
The commercial use of genetically engineered (GE) crops in Europe has been controversial. For the HDR approach to be effective, mating should be random within fields, so that resistant individuals emerging in a Bt field will most likely mate with individuals from the larger susceptible refuge population, resulting in heterozygous offspring that are susceptible and killed by Bt plants Other requirements for this strategy to work effectively are recessive inheritance of resistance and a very low frequency of resistance alleles, ideally
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