Abstract
16 Accurate genomic knowledge can elucidate the global spread patterns of invasive pests. The high-17 profile invasive agricultural pest Spodoptera frugiperda (fall armyworm; FAW) is a case in point. Native to the 18 Americas, the FAW was first reported in West Africa in 2016 and has rapidly spread to over 64 countries across 19 the Old World, resulting in significant economic losses. The chronological order of reported detections has led 20 to the hypothesis that the FAW moved eastwards across Africa and then Asia, however genomic evidence 21 remains lacking to test this hypothesis and to identify the potential origin of invasive populations. Using a whole 22 genome sequencing approach, we explored the population genomic signatures of FAW populations from the 23 Americas and the Old World. Analyses of complete mitochondrial DNA genomes identified 12 maternal lineages 24 across the invasive range, while genomic signatures from 870 high-quality nuclear genome-wide single 25 nucleotide polymorphic (SNP) DNA markers identified five distinct New World populations that broadly reflected 26 their native geographical ranges and absence of host-plant preference status. Unique FAW populations in the 27 Old World were also identified that suggested multiple introductions underpinned their rapid global spread. We 28 identified in Asian FAW individuals, genomes lacking evidence of admixture; while analysis of identified complex 29 substructure revealed significant directional geneflow from Asia into East Africa, in contrast to a simple east-to-30 west spread. Our study highlights the need for population genomics approaches in analysing complex pest 31 invasions, and the importance of international partnership to address global biosecurity challenges presented 32 by emerging high priority insect pests. 33 34 35 36