Geographical distribution of Vip3Aa20 resistance allele frequencies in Spodoptera frugiperda (Lepidoptera: Noctuidae) populations in Brazil Running title: Resistance of S. frugiperda to the Vip3Aa20 protein Fernando S. de A. e Amaral 1 , Aline S. Guidolin¹, Eloisa Salmeron¹, Rubens H. Kanno¹, Fernando E. O. Padovez¹, Júlio C. Fatoretto 2 , Celso Omoto¹* Abstract BACKGROUND: The use of Bt plants has been the main strategy for controlling the fall armyworm Spodoptera frugiperda (J. E. Smith) in Brazil. Field-evolved resistance of this insect pest to some Cry1 proteins has already been documented. 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 (F 1 and F 2 screen) methods to support Insect Resistance Management (IRM) programs and preserve Vip3Aa20 technology on maize. RESULTS: The phenotypic monitoring with purified protein showed two populations significantly different from the susceptible strain on the 2 nd crop season/2016. This number increased for the 1 st and 2 nd crop seasons/2017 on several regions. The genotypic monitoring estimated a mean frequency of the resistance allele of 0.0027 for the F 1 screen and 0.0033 for the F 2 screen. Three new resistant strains to Vip3Aa20 1 Department of Entomology and Acarology, University of Sao Paulo, Luiz de Queiroz College of Agriculture (USP / ESALQ), Piracicaba, SP, 13418-900, Brazil 2 Syngenta Crop Protection, Av. Nações Unidas 18001, São Paulo, SP, Brazil * Correspondence to: celso.omoto@usp.br This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/ps.5490 Accepted Article This article is protected by copyright. All rights reserved.