Adriana Arturi 
We’ve been keeping a close eye on insecticide resistance in the redlegged earth mite.
Since the first discovery of resistance in the redlegged earth mite in 2006 in Western Australia, the incidence of resistance has increased and is now present in populations across Western Australia, South Australia, and Victoria.
This article gives an update on the current resistance status of redlegged earth mite across these regions.
A brief history of insecticide resistance in the redlegged earth mite
The redlegged earth mite is a significant pest in establishing crops and pastures across the southern growing regions of Australia.
Currently, five chemicals are registered for redlegged earth mite control. Of these, growers rely heavily on three; synthetic pyrethroids and organophosphates as foliar sprays, and neonicotinoids as seed treatments.
Repeated applications of these chemicals have created significant selection pressures on redlegged earth mite populations to evolve resistance. This, along with high densities of mite populations, has resulted in resistance issues to two of the commonly used chemical groups, synthetic pyrethroids and organophosphates.
Insecticide resistance in the redlegged earth mite was first detected in WA in 2006 to synthetic pyrethroids.
In 2014, resistance to a second chemical group, the organophosphates, was discovered again in WA, with several populations exhibiting dual resistance to both synthetic pyrethroids and organophosphates.
In 2016, resistance to both synthetic pyrethroids and organophosphates was discovered for the first time outside of WA in a population in the Upper South East district in South Australia.
Current status of insecticide resistance in the redlegged earth mite
Since the discovery of these cases of resistance, ongoing surveillance shows that resistance in the redlegged earth mite has emerged over a wide area of the southern grain-growing regions of Australia.
Resistance to both pyrethroids and organophosphates is now common across large areas of WA and present in several areas within SA and Victoria. Figures 1 and 2 show the current distribution of synthetic pyrethroid and organophosphate resistance in the redlegged earth mite across Australia, respectively.
Western Australia
The current distribution of synthetic pyrethroid and organophosphate resistance is widespread across WA, covering the southwest, great southern, south coastal, and wheatbelt regions. Resistance in redlegged earth mite appears to be increasing in WA with new resistant populations being detected each year. For example, resistance to OPs was detected for the first time in the south coastal regions of WA in 2022.
South Australia
Within SA, redlegged earth mite resistance to synthetic pyrethroids and organophosphates is present in several areas including Kangaroo Island, the Fleurieu Peninsula, and the south-east region.
Since the first cases of resistance in SA, there has been a significant increase in insecticide resistance in these areas, particularly within the southeast region. More recently, resistant populations have been detected in the mid-north region.
Victoria
Resistance to organophosphates in the north-central area was first discovered in Victoria in 2018.
Since then, several more populations have been detected with organophosphate resistance in the north-central area and near Minimay in the Wimmera region.
To date, no resistance to synthetic pyrethroids has been discovered in Victoria.
What does this mean for redlegged earth mite management?
Insecticides will continue to play a role in the control of the redlegged earth mite. However, the increasing emergence of resistance across Australia raises concerns about the long-term viability of chemical control.
In areas with high levels of resistance or risk of resistance growers and advisors can use the “Resistance management strategy for the redlegged earth mite in Australian grains and pastures” to inform their management practice.
Key recommendations for control include:
- Assess mite populations over successive checks to determine if chemical control is warranted.
- Avoid using the same chemical group across successive spray windows (on multiple generations of mites) as this will select for resistance to that chemical group.
- Co-formulations or chemical mixtures are best reserved for situations where damaging levels of redlegged earth mite and other pest species are present, and a single active ingredient is unlikely to provide adequate control.
- Consider the impact on target and non-target pests and beneficial invertebrates when applying insecticide sprays. Where possible, use target-specific ‘soft’ chemicals, especially in paddocks with resistant redlegged earth mite.
If you experience a chemical control failure involving redelgged earth mite and or suspect insecticide resistance, contact DPIRD (WA) or Cesar Australia (SA, Vic, NSW) who can assist with advice and/or resistance testing.
Acknowledgements
This project (CES2010-001RXT) is being undertaken in collaboration with the University of Melbourne, WA Department of Primary Industries and Regional Development (DPIRD) with funding from Grains Research and Development Corporation (GRDC) and co-investments from Meat & Livestock Australia (MLA), and Agrifutures
The research undertaken as part of this project is made possible by the significant contributions of the agronomists and farmers who aided in mite collections. Thanks to Svetlana Micic and Alan Lord for mite collections in WA.
Cover image: Photo by Cesar Australia
