BACKGROUND Understanding the genetic basis of insect resistance to insecticides can help to implement insecticide resistance management (IRM) strategies. In this study, we selected a strain of Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) resistant to chlorantraniliprole using the F2 screen method, characterized the inheritance of resistance and evaluated patterns of cross‐resistance to other diamide insecticides. RESULTS The chlorantraniliprole‐resistant strain (Chlorant‐R) was selected from a field‐collected population with an estimated allele frequency of 0.1316. The estimated median lethal concentration (LC50) values were 0.011 and 2.610 µg a.i. cm–2 for the susceptible (Sus) and Chlorant‐R strains, respectively, resulting in a resistance ratio (RR) of 237‐fold. The LC50 values of the reciprocal crosses were 0.155 and 0.164 µg a.i. cm–2, indicating that resistance is autosomally inherited. Resistance was characterized as incompletely recessive and monogenic at concentrations close to the recommended field rates of chlorantraniliprole. Survival of the resistant strain and heterozygous larvae in maize plants treated at the field rate was ∼ 60 (similar to the susceptible strain on untreated maize) and 15%, respectively. The Chlorant‐R strain showed low cross‐resistance to cyantraniliprole (RR ∼ 27‐fold) and very high cross‐resistance to flubendiamide (RR > 42 000‐fold). CONCLUSIONS The frequency of chlorantraniliprole resistance allele was moderately high in a field‐collected population of S. frugiperda. The inheritance of chlorantraniliprole resistance was characterized as autosomal, incompletely recessive and monogenic. S. frugiperda showed cross‐resistance to other diamide insecticides. These results provide essential information for the implementation of IRM programs to preserve the useful life of diamide insecticides for controlling S. frugiperda in Brazil. © 2019 Society of Chemical Industry

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.

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Simple Summary Lepidopteran species commonly interact in the same niches in multiple crops. Interspecific competition has been neglected as a pressure selection agent in insecticide resistance studies. Our results showed that competition may act as an agent to speed up the evolution of diamide resistance in Spodoptera frugiperda and Helicoverpa armigera . Abstract Intra- and interspecific competition is considered a fundamental phenomenon in ecology. It acts as one of the most powerful selective forces that drives ecological diversity, the spatiotemporal distribution of organisms, fitness, and evolutionary aspects. Spodoptera frugiperda and Helicoverpa armigera are devastating pests and can co-occur in systems consisting of multiple agricultural crops and compete for food resources. Insecticide resistance in populations of these species has been a major threat to the sustainability of agroecosystems. No study to date has shown the effect of intra- and interspecific competition as a selective pressure agent on the evolution of insecticide resistance in lepidopteran pests in an experimental and theoretical way. Our study developed a parameterized computational model with experimental results for S. frugiperda and H. armigera competition. We simulated the behavior of heterozygous individuals with a competition capacity 100% equal to homozygous individuals resistant (100 RR) or susceptible to insecticides (00 RR), and intermediate between them (50 RR). Competition involving strains of these insect species can accelerate the evolution of their resistance to insecticides in agricultural crops. We found that competitive processes can result in a high probability of competitive exclusion for individuals with the susceptibility allele of these lepidopteran species. The results of this study are of paramount importance for understanding the impact of ecological factor competition on the evolution of insecticide resistance in lepidopteran pests, which until now has been neglected in these types of evolutionary dynamics studies.

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.