Global FAW population genomic signature supports complex introduction events across the Old World

Accurate genomic knowledge can elucidate the global spread patterns of invasive pests. The high-profile invasive agricultural pest Spodoptera frugiperda (fall armyworm; FAW) is a case in point. Native to the Americas, the FAW was first reported in West Africa in 2016 and has rapidly spread to over 64 countries across the Old World, resulting in significant economic losses. The chronological order of reported detections has led to the hypothesis that the FAW moved eastwards across Africa and then Asia, however genomic evidence remains lacking to test this hypothesis and to identify the potential origin of invasive populations. Using a whole genome sequencing approach, we explored the population genomic signatures of FAW populations from the Americas and the Old World. Analyses of complete mitochondrial DNA genomes identified 12 maternal lineages across the invasive range, while genomic signatures from 870 high-quality nuclear genome-wide single nucleotide polymorphic (SNP) DNA markers identified five distinct New World populations that broadly reflected their native geographical ranges and absence of host-plant preference status. Unique FAW populations in the Old World were also identified that suggested multiple introductions underpinned their rapid global spread. We identified in Asian FAW individuals, genomes lacking evidence of admixture; while analysis of identified complex substructure revealed significant directional geneflow from Asia into East Africa, in contrast to a simple east-to-west spread. Our study highlights the need for population genomics approaches in analysing complex pest invasions, and the importance of international partnership to address global biosecurity challenges presented by emerging high priority insect pests. ### Competing Interest Statement The authors have declared no competing interest.