Serpentine leaf miner (SLM), Liriomyza huidobrensis, invaded New South Wales and Queensland late 2020. Overseas, SLM has a history of developing resistance to several chemical insecticides. Since its Australian discovery, SLM has caused significant control issues in Australian horticultural industries. The project aimed to develop and deliver cost-effective and accurate insecticide resistance surveillance tools and management strategies for SLM in Australia.

First, we developed efficient bioassay methods against both SLM larval and adult stages to test responses to insecticides. We found that dimethoate (Group 1) and imidacloprid (Group 4) were only able to control SLM at application rates of 33 and 200 times the field-recommended rates, respectively. Furthermore, we found that 64-fold the field-recommended rate of chlorantraniliprole (Group 28) controlled only 94% of one tested population. Similarly, spirotetramat (Group 23), tested at 64-fold the field-recommended rate, only achieved 84-94% mortality. As none of the above-tested insecticides will likely completely control Australian invasive SLM, the project made additional effort to find an insecticide that may work against SLM. Encouragingly, we found spinetoram (Group 5) fully controlled tested populations at a rate 160-640 times lower than the field-recommended rate.

The whole genome of Australian SLM was sequenced for the first time in partnership with a NSW DPI AUSGEM research grant. The SLM genome sequence facilitated designing a primer panel and the development of a multiplex amplicon sequencing platform for DNA-based target-site resistance causing detection. The DNA-based assay further supported the bioassay data by elucidating fixed resistance genes against several chemical insecticides, including Group 1 – Carbamates & Organophosphates; Group 2 – Cyclodienes and Fiproles; Group 3 – Pyrethroids, and Group 28 – Diamides. Notably, the current DNA assay did not detect Group 4 target-site resistance in Australian SLM, suggesting that other mechanism(s) are involved in imidacloprid’s ineffectiveness.

The project has made a significant contribution to sustainable SLM management by providing a comprehensive understanding of the current status of insecticide resistance in Australian SLM populations. The successful sequencing of the SLM genome not only facilitated a DNA-based target-site resistance detection capability for Australian SLM but also provided a valuable DNA-based resource for researchers in the fields of insecticide resistance, population genetics and species diagnostics.

Successful resistance management of any arthropod species can only be achieved via rotation of compounds from different modes of action groups. For that reason, we strongly recommend expanded bioassay testing to include other permitted insecticide groups and biological products. This will significantly enhance a sustainable SLM management because the control strategy currently relies solely on the effectiveness of Spinetoram. Further, understanding the mechanism of spirotetramat resistance is a crucial step towards implementing a sustainable management strategy assuming reversion to susceptibility. Routine resistance surveillance using the DNA-based and bioassay testing developed by this study provides the industry with early signs of resistance, empowering them to take proactive management measures before consequences occur at the field level. With the above recommended tools and strategies, we are confident that Australian horticultural industries can effectively manage insecticide resistance in SLM thereby ensuring sustainability and productivity.