Genetic variation associated with increased lambda-cyhalothrin resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) in West Africa

The fall armyworm Spodoptera frugiperda (J.E. Smith) is a new serious destructive and widespread pest of corn which recently invaded subtropical regions worldwide. As this invasive species is spreading across the continent, it is vital to assess its susceptibility to currently used insecticides and establish the potential underlying resistance mechanism to better inform control programmes. In this study, we characterized the strains from eighteen fall armyworm populations from different countries in West Africa, established their susceptibility profiles to the main insecticides and genotyped the target site resistance alleles. The RFLP-PCR method showed that the majority of the population tested was a corn strain (72.5 – 100%) compared to the probable rice strain (0 – 27.5%). Tpi sequencing of the suspected rice strains revealed that almost all the samples analysed were from corn (>97%). Additionally, among the three insecticides tested, λ-cyhalothrin and chlorpyrifos induced the lowest toxicity against this pest, while emamectin benzoate exhibited moderate toxicity. Synergism tests performed to investigate the biochemical mechanism used by fall armyworm to breakdown λ-cyhalothrin indicated that metabolic enzymes (oxydases, esterase and the glutathion-S-transferase) play major roles in the resistance of λ-cyhalothrin observed in western Africa. Target mutation tests (qPCR) combined with previous synergetic tests showed that resistance to organophosphates and pyrethroids could be due to a biochemical mechanism + amino acid mutations (presence of the F290V mutation) or a biochemical mechanism (absence of the T929I mutation), respectively. The results of this study provide valuable information for supporting decisions related to sustainable fall armyworm control and applied resistance management.