Research Article Received: 24 January 2016 Revised: 1 May 2016 Accepted article published: 5 May 2016 Published online in Wiley Online Library: (wileyonlinelibrary.com) DOI 10.1002/ps.4312 Fitness costs and stability of Cry1Fa resistance in Brazilian populations of Spodoptera frugiperda Oscar F Santos-Amaya, a Clébson S Tavares, a João Victor C Rodrigues, a Silverio O Campos, a Raul Narciso C Guedes, a Analiza P Alves b and Eliseu José G Pereira a,c* Abstract BACKGROUND: The presence of fitness costs of resistance to Bacillus thuringiensis (Bt) insecticidal proteins in insect populations may delay or even reverse the local selection of insect resistance to Bt transgenic crops, and deserves rigorous investigation. Here we assessed the fitness costs associated with Cry1Fa resistance in two strains of fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), derived from field collections in different Brazilian regions and further selected in the laboratory for high levels of resistance to Cry1Fa using leaves of TC1507 corn. RESULTS: Fitness components were compared using paired resistant and susceptible strains with similar genetic backgrounds and F 1 generations from reciprocal crosses, all of them reared on non-transgenic corn leaves. No apparent life history costs in the larval stage were observed in the Bt-resistant strains. Moreover, the resistance remained stable for seven generations in the absence of selection, with no decrease in the proportion of resistant individuals. Larval respiration rates were also similar between resistant and susceptible homozygotes, and heterozygotes displayed respiration rates and demographic performance equal or superior to those of susceptible homozygotes. CONCLUSION: In combination, these results indicate the lack of strong fitness costs associated with resistance to Cry1Fa in the fall armyworm strains studied. These findings suggest that Cry1Fa resistance in S. frugiperda populations is unlikely to be counterselected in Cry1Fa-free environments. © 2016 Society of Chemical Industry Supporting information may be found in the online version of this article. Keywords: Bt resistance; pleiotropic effects; fall armyworm; isogenic strains; life-history traits; intrinsic rate of population increase; resistance management 1 INTRODUCTION Acquisition of adaptation to a new environment, such as insec- ticide resistance, may result in phenotypic changes deleterious to the organism in the absence of the xenobiotic. 1 – 3 Costs asso- ciated with insecticide resistance are not always detected, and in some instances the fitness of resistant individuals is actually enhanced in the absence of the insecticide. 1,3 For resistance to Bacillus thuringiensis (Bt) insecticidal proteins produced in trans- genic plants, fitness costs may play an important role in the rate of resistance evolution in environments free of Bt proteins (i.e. refuge areas), 4,5 although the occurrence and magnitude of trade-offs between Bt resistance and insect fitness vary widely. 6 When developing pest-specific insect resistance management strategies, it is valuable to understand the potential for fitness costs to delay or potentially reverse evolution of local resistance in pest populations. 4,7,8 When present, fitness costs reduce the coefficient of selection, favoring susceptible individuals relative to resistant individuals in refuge areas. 8 Under these conditions, refuge and other environments without Bt proteins select against resistance, 6 which helps to decrease the spread and frequency of resistance alleles in field populations. 6,7,9 Another factor that modulates resistance evolution is the dominance level of fitness costs, which can be recessive, affecting only homozygous resistant individuals, or non-recessive (e.g. additive or dominant), affecting both homozygous resistant and heterozygous genotypes. 4,7,8,10 As the majority of resistance alleles are carried by heterozygous indi- viduals when resistance first evolves in a population, non-recessive ∗ Correspondence to: EJG Pereira, Departamento de Entomologia, Universidade Federal de Viçosa, Viçosa 36570–000, MG, Brazil. E-mail: eliseu.pereira@ufv.br a Departamento de Entomologia, Universidade Federal de Viçosa, Viçosa, MG, Brazil b DuPont Pioneer, Johnston, IA, USA c Instituto Nacional de Ciência e Tecnologia em Interações Planta-Praga, Univer- sidade Federal de Viçosa, Viçosa, MG, Brazil Pest Manag Sci (2016) www.soci.org © 2016 Society of Chemical Industry