Fitness costs associated with spinetoram resistance in Spodoptera frugiperda is driven by host plants

Insecticide resistance is usually associated with fitness costs. The magnitude of fitness costs is affected by environmental and ecological factors. Here, we explored how host plants could affect fitness costs associated with insecticide resistance. Initially, spinetoram-resistant (RR) and susceptible (SS) strains of Spodoptera frugiperda were selected using F2 screen from a population collected in São Desidério, Bahia State, Brazil in 2018. Besides de RR and SS strains, fitness costs were also assessed for a heterozygous strain (RS). Life-history traits were evaluated to estimate population growth parameters of neonate larvae of each strain fed on corn, soybean and cotton plants. Compared to the SS strain, the relative fitness of the RR strain, based on intrinsic rate of population increase, was 1.06, 0.84 and 0.67 on plants of corn, soybean and cotton respectively. The relative fitness of the RS strain was similar to the SS strain regardless the host plant, suggesting a recessive fitness cost. No differences were found between the strains fed on corn plants. The larval development time was greater for RR strain fed on soybean and cotton plants compared to RS and SS strain. Low survival rate and fecundity of the RR strain were found when larvae fed on plants of soybean and cotton. The results of this study demonstrated that fitness costs of spinetoram resistance in S. frugiperda depend strongly on the host plants that S. frugiperda larvae fed on. Such information can be used to design resistance management strategies considering the host plants of the agricultural landscape. Key messages ### Competing Interest Statement The authors have declared no competing interest.