Large-Scale Monitoring of the Frequency of Ryanodine Receptor Target-Site Mutations Conferring Diamide Resistance in Brazilian Field Populations of Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

SIMPLE SUMMARY: Fall armyworm (FAW), Spodoptera frugiperda, is a destructive moth pest species on various crops, particularly corn. It is native to the tropical regions of the Western Hemisphere such as Brazil, but recently invaded Africa, India, China, and Australia. Its larval stages damage crops by feeding, and to keep them under damage thresholds, insecticide applications are common. Due to frequent insecticide applications, FAW evolved resistance to different chemical classes of insecticides, including diamides. Field relevant levels of diamide resistance are usually conferred by ryanodine receptor (RyR) mutations and compromising recommended label rates. Diamide resistance in FAW so far remained restricted to laboratory-selected strains. Here, we investigated the frequency of specific resistance mutations in field-collected Brazilian populations of FAW by an F2 screen, selected two populations (BA-R and TF-R) for high levels of diamide resistance, deciphered the genetics of resistance, and employed a molecular genotyping assay to correlate resistance levels with the presence of RyR mutations. Crossin studies indicated that resistance is autosomal and (incompletely) recessive in both strains. F1 backcrosses suggested monogenic resistance, supported by the identification of an I4734M/K target-site mutation in the RyR. Our results will help to sustainably manage diamide resistance in FAW in Brazil. ABSTRACT: Fall armyworm (FAW), Spodoptera frugiperda, is an important lepidopteran pest in the Americas, and recently invaded the Eastern Hemisphere. In Brazil, FAW is considered the most destructive pest of corn and cotton. FAW has evolved resistance to many insecticides and Bacillus thuringiensis (Bt) proteins. Here, a large-scale monitoring was performed between 2019 and 2021 to assess diamide insecticide susceptibility in more than 65 FAW populations sampled in corn and cotton. We did not detect a significant shift in FAW susceptibility to flubendiamide, but a few populations were less affected by a discriminating rate. F2 screen results of 31 selected FAW populations across regions confirmed that the frequency of diamide resistance alleles remained rather stable. Two laboratory-selected strains exhibited high resistance ratios against flubendiamide, and cross-resistance to anthranilic diamides. Reciprocal crosses indicated that resistance is autosomal and (incompletely) recessive in both strains. F1 backcrosses suggested monogenic resistance, supported by the identification of an I4734M/K target-site mutation in the ryanodine receptor (RyR). Subsequent genotyping of field-collected samples employing a TaqMan-based allelic discrimination assay, revealed a low frequency of RyR I4790M/K mutations significantly correlated with phenotypic diamide resistance. Our findings will help to sustainably employ diamides in FAW resistance management strategies across crops.