Resistance bioassays and allele characterisation inform Spodoptera frugiperda (Lepidoptera: Noctuidae) management and introduction pathways in Asia and Australia

The fall armyworm Spodoptera frugiperda is present in over 70 countries in Africa, Asia, and Oceania. Its rapid dispersal since 2016 since first reported in western Africa, and associated devastation to agricultural productivity, highlight the challenges posed by this pest. Currently, its management largely relies on insecticide sprays and transgenic Bacillus thuringiensis toxins, therefore understanding their responses to these agents and characteristics of any resistance genes enables adaptive strategies. In Australia, S. frugiperda was reported at the end of January 2020 in northern Queensland and by March 2020, also in northern Western Australia. We undertook bioassays on two Australian populations, one each from these initial points of establishment. To assist with resistance response interpretation, two related endemic noctuid pest species, Helicoverpa armigera and Spodoptera litura , were concurrently screened to obtained LC50 estimates for larvae against a range of insecticides. We characterised known resistance alleles from the VGSC, ACE-1, RyR, and ABCC2 genes for five populations to compared with published allele frequencies and bioassay responses from native and invasive S. frugiperda populations. An approximately 10x LC50 difference for indoxacarb was detected between Australian populations, which was approximately 28x higher than an Indian population. Characterisation of ACE-1 and VGSC alleles provided further evidence of multiple introductions in Asia, and multiple pathways involving genetically distinct individuals into Australia. Bioassay results and resistance allele patterns from invasive S. frugiperda populations therefore suggest multiple introduction events have contributed to the pest′s spread in the Old World, and challenge the axiom of its rapid ′west-to-east′ spread. ### Competing Interest Statement The authors have declared no competing interest.