Rubens Hideo Kanno

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.

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 F ₂ 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: The presence of fitness costs associated with resistance can be exploited in resistance management strategies. Host plant influences the fitness costs associated with spinetoram resistance in S. frugiperda . Information considering the host plants in an agricultural landscape is essential to design effective resistance management programs.

Spodoptera frugiperda (J. E. Smith) is a serious and widespread agricultural pest with several records of resistance to different insecticides and Bt proteins, including the neurotoxic insecticides chlorpyrifos (organophosphate) and lambda-cyhalothrin (pyrethroid). In this study, we (i) characterized and monitored the susceptibility of field populations of S. frugiperda to chlorpyrifos (194 populations) and lambda-cyhalothrin (197 populations) collected from major maize-growing regions of Brazil from 2003 to 2016, and (ii) compared gene expression levels of chlorpyrifos- and lambda-cyhalothrin-resistant strains to a susceptible reference strain ( Sf-ss ) of S. frugiperda . Laboratory-guided assays to monitor larval susceptibility detected average survival ranging from 29.3% to 36.0% to chlorpyrifos, and 23.1% to 68.0% to lambda-cyhalothrin at diagnostic concentration, based on LC ₉₉ of the susceptible reference strain of each insecticide. The resistance ratio of the chlorpyrifos-resistant strain ( Clo-rr ) was 25.4-fold and of the lambda-cyhalothrin-resistant strain ( Lam-rr ) was 217-fold. Differential gene expression analyses between resistant vs susceptible strains identified 1,098 differentially expressed genes (DEGs) between Clo-rr and Sf-ss , and 303 DEGs between Lam-rr and Sf-ss . Functional analyses of the DEGs revealed the up-regulation of several detoxification enzymes, mainly cytochrome P450 belonging to the CYP3 and CYP6 clans. Genes associated with regulatory processes, such as the forkhead box O (FoxO) were also up-regulated. Our data points that the resistance mechanisms of Clo-rr and Lam-rr strains of S. frugiperda to chlorpyrifos and lambda-cyhalothrin are mainly mediated by enzyme detoxification.