The rise of insecticide resistance in the redlegged earth mite

Understanding the impact of insecticide reliance on RLEM populations

The RLEM poses a significant threat to establishing crops and pastures across the southern growing regions of Australia. They are a major pest affecting the pasture seeds industry. Currently, three main chemicals – synthetic pyrethroids (SPs), organophosphates (OPs), and neonicotinoids are heavily relied upon for control. Diafenthiuron and fiproles are two other chemical groups registered for the control of RLEM.

However, extensive use of these chemicals has created significant selection pressures on RLEM populations to develop resistance to two of the commonly used chemical groups, SPs and OPs. There has been no neonicotinoid resistance detected in RLEM populations so far, but it could evolve in the future, leaving us with very few defences against this pest.

Controlling the spread of insecticide resistance involves recognising the risk associated with the repeated use of the same chemical group within a season or between seasons. This is especially important because these chemicals are also used for controlling other pests like blue oat mites or lucerne fleas, inadvertently exposing RLEM to the chemicals and increasing resistance risk.

The expanding geographical footprint of insecticide resistance

Resistance can arise through both independent evolutionary events and the introduction of resistant mites, which most often occurs at a local scale. Factors like wind and farm machinery contribute to the spread of mites and mite eggs. However, the geographically distinct locations of existing resistance imply isolated development of resistance in individual RLEM populations within properties. Thus, the actions of individual growers are crucial in preventing the spread of insecticide resistance.

Ongoing surveillance has revealed a widespread emergence of resistance across the southern grain and pasture growing regions of Australia. Figures 1 and 2 respectively show the current distribution of SP and OP resistance in the RLEM across Australia.

SA
In SA, resistance to SPs and OPs is present in several areas including Kangaroo Island, the Fleurieu Peninsula, the mid-north region, and the south-east region. Over the last few years, the southeast region has experienced a significant increase in resistance, with a large proportion of resistant populations coming from pasture seed paddocks – particularly lucerne.

VIC
In VIC, resistance to OPs was first identified in the north-central area in 2018, and subsequent populations with OP resistance were found near Minimay in the Wimmera region. No resistance to SPs has been discovered in Victoria to date.

WA
Both SPs and OP resistance are now prevalent in WA, covering various regions such as the southwest, great southern, south coastal, and wheatbelt regions. Notably, resistance in RLEM appears to be increasing in WA with new resistant populations being detected annually. For example, resistance to OPs was detected for the first time in the south coastal regions of WA in 2022.

Figure 1. Current resistance of RLEM to SPs as of 2023

Figure 2. Current resistance of RLEM to OPs as of 2023

What does this mean for redlegged earth mite management?

Despite the continued role of insecticides in RLEM control, the increasing emergence of resistance across Australia raises concerns about the long-term effectiveness of chemical control.

In areas with high resistance levels or the risk of resistance, growers and advisors are encouraged to 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 RLEM 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. Use target-specific ‘soft’ chemicals where possible, especially in paddocks with resistant RLEM.

Other management strategies:

• Implement pre and post-sowing weed management practices, acknowledging the significance of capeweed as an important RLEM host. Control all weeds within paddocks and along fence lines at least two weeks in advance.
• Utilise spring spraying with Timerite, a carefully timed chemical application in spring to minimise over-summering egg production and subsequently reduce the RLEM population the following autumn.
• Enhance management by knowing RLEM hatch dates in autumn. This indicates when to increase crop monitoring and recognise the optimal spray timing just after the hatching of oversummering eggs which occurs in autumn. The online hatch timing tool developed by Cesar Australia uses local climatic data to predict hatch dates. Access the tool here.

If you experience a chemical control failure involving RLEM and or suspect insecticide resistance, contact DPIRD (WA) or Cesar Australia (SA, VIC, NSW) who can assist with advice and/or resistance testing.

Contact information
Cesar Australia: Adriana Arturi aarturi@cesaraustralia.com
DPIRD: Svetlana Micic svetlana.micic@dpird.wa.gov.au

For more information about the project go to https://www.agrifutures.com.au/related-projects/future-options-for-the-control-of-the-rlem-earth-mite-in-australian-pasture-seed/

Learn more about the AgriFutures Pasture Seeds Program

Acknowledgements

This project 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 Australia. 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.