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

Abstract

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.

Complex multiple introductions drive fall armyworm invasions into Asia and Australia

“…In China, Lv et al (2021) reported diamide resistance alleles in individuals from Guangxi and Guizhou Provinces. While this study failed to source Indonesian and Cambodian populations (due to the difficulty of sharing and sending biological material from Indonesia and Cambodia to Australia), significant differences in resistance to indoxacarb was nevertheless detected between WA (Kununurra) and Qld (Strathmore) populations (Tay et al 2021a, Tay et al 2022c, and could be due to the different source populations for the WA and the Qld populations. On-going studies to monitor for potential arrivals of novel resistance alleles and those already reported in Indonesia and China to Australia will therefore be needed to inform and impact future FAW management strategies.…”

“…While multiple introductions especially in the Asian continent (e.g., China, Vietnam, Malaysia; Vu 2008, Nguyen and Vu 2009, Tay and Gordon 2019, Gui et al 2020, Tay et al 2021b, Jiang et al 2022, Tay et al 2022d; this study) have played an important role leading to the perception of rapid spread, introductions from elsewhere have also contributed to the African establishment and expansion. Further to genomics evidence, resistance allele characterisation and bioassay experiments have also identified invasive populations with non-overlapping unique insecticide resistance profiles in China (e.g., Lv et al 2021), Indonesia (Boaventura et al 2020), Australia (Tay et al 2021a, Tay et al 2022c), India (Deshmukh et al 2020), and Africa (Eriksson 2019, Worku and Ebabuye 2019).…”

“…The copyright holder for this preprint (which this version posted July 16, 2022. ; https://doi.org/10.1101/2022.06.11.495773 doi: bioRxiv preprint profiles in China (e.g., Lv et al 2021), Indonesia (Boaventura et al 2020), Australia (Tay et al 2021a, Tay et al 2022c, India (Deshmukh et al 2020), and Africa (Eriksson 2019, Worku andEbabuye 2019).…”

“…Understanding the spread patterns (e.g., single invasive bridgehead effect (Guillemaud et al 2011) vs. mass dispersion (Wilson et al 2009)) and frequencies of introduction (e.g., single introduction (Cock et al 2017, Nagoshi et al 2018, Nagoshi et al 2019b, Gui et al 2020 vs. multiple introductions (Schlum et al 2021, Tay et al 2021a, Tay et al 2021b, Jiang et al 2022, Tay et al 2022c)) of the current invasive FAW . CC-BY-NC-ND 4.0 International license available under a was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.…”

“…Methods of specimen preservation involved collection of larvae/adults from fields and placing directly into highly concentrated ethanol (95-99.9%) and storing at -20˚C until being used in DNA extraction. Samples sent to CSIRO from Queensland (CSIRO code: Sf20-1) and Western Australia (CSIRO code: SF20-4) were first used for bioassays purpose(Tay et al 2021a, Tay et al 2022c prior to being stored at -20˚C (once laboratory colonies were established) for DNA extraction purpose. Individuals from these two locations used therefore represent the original field-collected populations from northern Australia.DNA extraction and genome library preparation DNA of individual FAW samples (TableS1) was extracted using the Qiagen DNA extraction kit and eluted in 200µL elution buffer following the manufacturer's protocol.…”