BACKGROUND Fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), a major lepidopteran pest in Latin and North America, has very recently invaded the continents of Africa and Asia. FAW has evolved resistance to different insecticides and transgenic corn expressing Bacillus thuringiensis (Bt) toxins. Here, we investigated the extent and mechanisms of resistance to diamide insecticides in a Brazilian field‐collected FAW strain selected using chlorantraniliprole. RESULTS Continuous laboratory selection of a field‐collected FAW strain with chlorantraniliprole resulted in resistance ratios of 225‐fold and > 5400‐fold against chlorantraniliprole and flubendiamide, respectively, when compared with a susceptible strain. Pre‐exposure to different synergists known to inhibit detoxification enzymes did not result in significantly increased larval toxicity, suggesting a minor role for metabolic resistance. Sequencing of the FAW ryanodine receptor (RyR) C‐terminal domains II to VI revealed a single nucleotide polymorphism, resulting in a I4734M mutation recently said to confer target‐site resistance to diamides in lepidopteran pests. Genotyping by pyrosequencing of field‐collected FAW larvae sampled in the 2018 crop season suggests a low resistance allele frequency. Furthermore, we developed a fluorescent polymerase chain reaction (PCR)‐based allelic discrimination assay for rapid genotyping of field‐collected FAW samples, because diamides are increasingly used in Bt−/non‐Bt corn. CONCLUSIONS Recently, the identified RyR mutation has been shown to confer field resistance in other lepidopteran pests such as diamondback moth, tomato leafminer and striped rice stem borer. The developed PCR‐based allelic discrimination assay will help to monitor the frequency and future spread of diamide resistance allele in FAW field populations and help to implement appropriate resistance management measures. © 2019 Society of Chemical Industry

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. © 2019 Society of Chemical Industry