Geographical distribution of Vip3Aa20 resistance allele frequencies in Spodoptera frugiperda (Lepidoptera: Noctuidae) populations in Brazil.
BACKGROUND: The use of Bt plants has been the main strategy for controlling the fall armyworm Spodoptera frugiperda (J. E. Smith) in Brazil. However, many resistance cases were already registered. The resistance of S. frugiperda to the Vip3Aa20 protein was recently characterized under laboratory conditions but it is still efficient under field conditions. Here, resistance monitoring studies were conducted using phenotypic (purified protein and Bt maize leaves) and genotypic (F1 and F2 screen) methods to support insect resistance management (IRM) programs and preserve Vip3Aa20 technology on maize. RESULTS: Phenotypic monitoring with purified protein showed two populations significantly different from the susceptible strain on the second crop season in 2016. This number increased for the first and second crop seasons in 2017 in several regions. The genotypic monitoring estimated a mean frequency of the resistance allele of 0.0027 for the F1 screen and 0.0033 for the F2 screen. Three new resistant strains to Vip3Aa20 were selected from F2 screen assays. Complementation tests on these new resistant strains were positive with the previous resistant strain. CONCLUSION: Here we showed that the resistance allele of S. frugiperda to Vip3Aa20 protein is widely distributed in maize-producing regions in Brazil and its frequency increases throughout crop seasons.