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Fitness costs associated with insect resistance to insecticides can be exploited to implement resistance management programs. However, most of these studies are restricted to evaluating biological traits on artificial diets. Here, we investigated the fitness cost associated with chlorantraniliprole in Spodoptera frugiperda (J.E. Smith) feeding on corn, soybean, and cotton plants. We used a near-isogenic strain of S. frugiperda resistant to chlorantraniliprole (Iso-RR), a susceptible strain (SS), and heterozygotes strains (H1 and H2) to evaluate several biological and population growth parameters. Larval survival of the Iso-RR strain was on average 90% on corn, 65% on soybean, and 57% on cotton plants. Development time of the larval stage also differed among host plants, Iso-RR strain took on average 14, 17, and 26 days to reach the pupal stage on corn, soybean, and cotton plants respectively. Net reproductive rate, intrinsic rate of population increase, and finite rate of population increase were higher for Iso-RR strain feeding on corn plants than other host plants. The relative fitness, based on the intrinsic rate of population increase, of S. frugiperda resistant strain on corn, soybean, and cotton plants were 1.04, 0.85, and 0.88, respectively. Therefore, no fitness cost was observed for S. frugiperda feeding on corn plants, but a significant fitness cost was observed when this pest fed on soybean and cotton plants. We showed that the food source influences the fitness cost of S. frugiperda resistant to diamide. Such information may help to implement resistance management strategies based on each crop., (© The Author(s) 2022. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: Spodoptera frugiperda (J.E. Smith) is a difficult pest to manage mainly because of its resistance to insecticides and Bt proteins. We evaluated fitness costs of S. frugiperda resistant strains to diamide insecticides with different genetic backgrounds aiming to highlight the importance of using isogenic strains. We established a near-isogenic strain of S. frugiperda resistant to diamides (Iso-RR), using a chlorantraniliprole resistant strain (RR) selected from a field-collected population and a susceptible reference strain (SS). Fitness costs were assayed using strains with close-related genetic backgrounds (Iso-RR and SS) and strains with distant-related genetic backgrounds (RR and SS)., Results: No fitness cost associated with chlorantraniliprole resistance in S. frugiperda was observed using the Iso-RR strain, based on life history traits. The only parameter that differs between Iso-RR and SS strains was the mean length of a generation (T), whereas the Iso-RR strain presented T = 35.8 and SS strain showed T = 34.6. On the other hand, a significant fitness cost was detected using the RR strain. All population growth parameters differ between RR and SS strains. Based on the intrinsic rate of population increase (r m ) parameter, the relative fitness estimated was 1.02 for the Iso-RR strain and 0.64 for the RR strain., Conclusion: The genetic background of the resistant strains alters fitness cost outcomes. The RR strain showed fitness costs associated with resistance, but the Iso-RR did not. Our work supports the decision-making process of resistance management programs and adds to the growing body of research that enlightens the importance of strain genetics in fitness cost experiments., (© 2021 Society of Chemical Industry.)