Homozygous multiple-insecticide resistance in Australian invasive serpentine leafminer Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae)

In October 2020, Liriomyza huidobrensis (serpentine leafminer [SLM]) was first detected in Western Sydney, New South Wales (NSW), and subsequently Southern Queensland (QLD). Control failures were reported, and insecticide resistance was confirmed via bioassay, but mechanisms causing resistance remained unknown. We characterised the complete mRNA sequence of eight insecticide target genes of L. huidobrensis using RNA sequencing. We found that the Australian SLM carries three mutations (I129V, G227A and F331W) of the acetylcholinesterase (AChE) gene, causing resistance to mode of action (MOA) 1 chemicals; one mutation (A301S) of the gamma-aminobutyric acid receptor subunit beta (GABAR), causing resistance to MOA 2 chemicals; and two mutations (M918T and L1014L) of the voltage-gated sodium channel (VGSC) gene, causing resistance to MOA 3 chemicals. We consequently developed a multiamplicon-sequencing panel to screen 234 Australian field-collected SLM samples using next-generation sequencing. The multiplex panel includes mitochondrial cytochrome oxidase subunit 1 (COI) for species identification and AChE, VGSC, glutamate-gated chloride channel (GluCl) and chitin synthase 1 (CHS1) gene mutations. We confirm that all the individuals carry the multi-resistance alleles in a homozygous and fixed state. This is a rare phenomenon for a single individual to be homozygous for multiple resistance mutations, with only a few studies documenting such multi-resistance at the population level. Such resistance mechanism detection raises concern that there are limited chemical options for the control of invasive SLM in Australia.