Take home messages

• Due to evolving resistance in the redlegged earth mite, there is a need to reduce reliance on current insecticides and rethink management options for this pest.
• Detections of populations of RLEM resistant to SPs and OPs continue to increase, with the known range expanding, particularly in eastern Australia.
• Social benchmarking has identified attitudes towards insecticides and IPM, showing that agronomists can be more risk averse than growers.
• Updating the resistance management strategy ensures management options remain effective and useful to growers and advisers.

Background

The redlegged earth mite (Halotydeus destructor, RLEM) is a destructive and economically important pest in Australia’s grain and pasture crops. Redlegged earth mite control is heavily reliant on the applications of pesticides through foliar sprays and seed treatments. Of the five chemical groups available for RLEM control, three (synthetic pyrethroids, organophosphates, and neonicotinoids) are most frequently used. The repeated use of these chemicals creates substantial selection pressure for mites to evolve resistance. Consequently, insecticide resistance issues are now present across large areas of Western Australia and parts of south-eastern Australia. Many RLEM populations in these areas are resistant to synthetic pyrethroids (SPs), organophosphates (OPs), or both. This rise in resistance demonstrates a need to change the way insecticides are used to minimise the risk of further resistance in RLEM. Given that the dispersal ability of mites is limited, resistance tends to remain relatively localised. High selection pressure at individual farms drives resistance, therefore, the management actions of individual growers are vital.

In grain and pasture regions affected by this pest, resistance surveillance and the development of up-to-date management recommendations help to maintain the effectiveness of current chemical control options. Integrated pest management (IPM) strategies provide alternatives to insecticide use and support growers' efforts to manage insecticide resistance. Social research develops our understanding of current knowledge and attitudes regarding insecticide resistance management. These insights are used to ensure that management recommendations are applicable and achievable.

In this presentation, we will (1) present data on the current resistance status of RLEM in Australia; (2) present and discuss the project’s social benchmarking research; and (3) provide recommendations about insecticide resistance management, as specified by the RLEM resistance management strategy, to help growers and advisers stay on top of RLEM pest problems.

Field surveys and resistance testing

Since the first detection of SP resistance in RLEM in 2006, resistance surveillance has been undertaken every year, covering a wide geographical range throughout Western Australia and eastern Australia (including South Australia, Victoria, and New South Wales). RLEM populations collected for the resistance screening have been mostly collected from a mixture of paddocks with reported chemical control failures and paddocks with high insecticide and intensive cropping usage, with some indiscriminate collections also undertaken from paddocks and roadside vegetation.

Samples of mites from each population have been screened for resistance against SPs and OPs using phenotypic laboratory bioassays (Arthur et al. 2021). Molecular screening has been undertaken to assess SP resistance by using genetic markers targeting the known resistance mechanism (Edwards et al. 2018). It has been shown previously that resistance ratios in RLEM populations are much higher for SPs (~200,000 times) than for OPs (4-415 times). This is likely due to the underlying mechanisms conferring resistance. Furthermore, varying levels of resistance between populations to different OPs have been shown previously in RLEM (Arthur et al. 2021).

Following the screening, growers and advisers have been provided with a full report outlining the type of resistance present on their farms. The report is accompanied by management recommendations that are specific to the type of resistance present. Using this information, growers can minimise RLEM chemical control failures and the evolution and spread of insecticide resistance. More broadly, the surveillance and mapping of RLEM resistance provide important information that assists growers to implement successful management strategies to minimise the impact of RLEM. Resistance surveillance information is also used to update the RLEM resistance management strategy (RMS).

Social benchmarking

Using a national online survey, we investigated the current knowledge, practices, and attitudes of growers and advisers relating to RLEM insecticide resistance management. We aimed to understand how chemical-based management of RLEM is influenced by variables such as risk attitude, risk perception and knowledge. The findings provide information on current barriers to the adoption of sustainable management practices in different sub-populations and will inform the development of improved training and awareness outputs.

The survey was 24 questions long and was divided into the following categories: (1) demographics; (2) insecticide use patterns; (3) attitude towards risk in their management practice/advice; (4) knowledge of RLEM and insecticide resistance; and (5) connectivity in their community. Both growers and advisers were asked a range of questions about their chemical usage, attitude to risk about pest management, knowledge of the RLEM resistance status, ability to correctly identify the mite, and lifecycle, and estimate the number of non-chemical options available.

Updating the resistance management strategy

To help minimise the risk of developing resistance on their property, growers and advisers can refer to the RLEM-RMS. This document provides best practice management recommendations to manage RLEM resistance. This includes IPM strategies and chemical control strategies for situations where RLEM have resistance to certain chemical groups. The RLEM-RMS document is being updated so that it remains effective in helping growers manage RLEM resistance.

The revised document will be released in the autumn of 2024 and will incorporate updated information and findings from several sources. It will consider feedback from the surveys conducted under the social benchmarking activity. The information gained from the survey enables us to identify knowledge gaps and gain feedback on the current uptake and usefulness of the RLEM-RMS. We are planning to conduct a workshop, which will bring together resistance experts and engaged advisers to make decisions on improvements to be made to the RLEM-RMS. Finally, we will incorporate new scientific findings to update the scientific rationale of the RMS.

Results and discussion

Field surveys and resistance testing

Resistant RLEM populations have been found across Western Australia, South Australia, and Victoria since resistance surveillance began in 2006. Screening undertaken between 2006 and 2022 found SP resistance to be widespread across the southern regions of Western Australia and in some parts of South Australia (Arthur et al. 2021). Organophosphate resistance has been detected in the southern regions of Western Australia and parts of South Australia and Victoria.

Within Western Australia, the current distribution of SP and OP resistance is widespread, covering the southwest, great southern, south coastal and wheatbelt regions (Figure 1). Resistance in RLEM appears to be increasing in Western Australia, with new resistant populations being detected each year. For example, RLEM resistance to OPs has only just been detected for the first time in the southern coastal regions of Western Australia last year (Mata et al. 2022).

Over the last few years, we have seen a substantial increase in RLEM insecticide resistance in eastern Australia, particularly within South Australia. In South Australia, resistance was first discovered in 2016, and since then, resistance has been detected in several areas including Kangaroo Island, the Fleurieu Peninsula, and the south-east region (Figure 1). Over the last couple of years, there has been an increase in resistant populations to SPs and OPs in these areas, particularly in the south-east region. Approximately 60% of the RLEM that possessed resistance in eastern Australia were collected from pasture seed sites. More recently, resistant populations have been detected in the mid-north region.

Resistance in Victoria was first detected in 2018 to OPs, in Wanalta in north central Victoria (Arthur et al. 2021). Since then, several OP resistant populations have been detected in Victoria in the north central region and Minimay in the Wimmera region. There has been no SP resistance detected within Victoria to date.

In south-eastern Australia, the target areas for resistance screening in 2023 are New South Wales, the mid-north and south-east regions of South Australia and the Eyre Peninsula.

Figure 1. Current status of RLEM resistance in Australia as of 2022 to SPs (left) and OPs (right).

Social benchmarking

A total of 273 responses was received. When partial and invalid responses were removed (for example, had not encountered RLEM before or did not work as a grower or adviser in Australia), this resulted in 93 responses from growers and 97 responses from advisers. We found that, despite most growers and advisers reporting that they often or always employ an integrated pest management mindset, self-reported insecticide spray rates and advice to spray for RLEM were still high. When growers were asked how often they apply foliar insecticides specifically for control of RLEM, 5% answered several times a season, 35% every year, 25% once every 2–3 years, 9% once every 4–5 years, 14% rarely (once in 10 years), 11% never and 1% unsure. When making RLEM management decisions, 72% of growers consider environmental factors and 77% consider paddock history. For advisers giving management support, 81% consider environmental factors and 88% consider paddock history. We also found that knowledge about the resistance status in RLEM was low (Table 1), with only 31% of growers correctly identifying that resistance exists for both synthetic pyrethroids and organophosphates.

Table 1: A comparison of the percentage of growers and advisers who responded yes, no, or unsure when asked whether the RLEM has evolved resistance to the chemicals listed.

When respondents were asked about different risk scenarios relating to RLEM management, we found that growers are more worried about economic loss, but advisers are much more cautious about adopting new practices and wanting to see the results from farm trials before trying practices on their farms (Figure 2). These results will be used to provide locally relevant advice for insecticide resistance management strategies and to improve the extension and content delivery of the information in these strategies. We hope that the publication of our results will provide insight into programs that aim to reduce insecticide use and increase the adoption of insecticide resistance management strategies.

Figure 2. Growers’ and advisers’ responses to questions related to risk in on-farm pest management. The following abbreviations were used for each survey question: Economic loss – I am concerned about economic loss as a result of invertebrate pest damage; Take risk – I am willing to take risks when it comes to farming; New ideas – I am cautious about adopting new ideas and farm practices; New practice – A new farm practice must be proven on other farms or trials before I will use it. Agreement responses to a) demonstrate risk aversion, while the responses to b) are the inverse, that is, disagree responses show risk aversion.

Conclusions

The ongoing surveillance of RLEM provides growers with insight into the resistance status in their region, which can inform management decisions. In areas with high levels of resistance or in areas that require management of RLEM, growers and advisers can use the RLEM-RMS to inform their management practice. Some key recommendations for RLEM control include:

• assess RLEM populations over successive checks to determine if chemical control is warranted
• do not use the same chemical group across successive spray windows (on multiple generations of mites) as this will select for resistance to that chemical group
• reserve co-formulations or chemical mixtures 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, and where possible, use target-specific ‘soft’ chemicals, especially in paddocks with resistant RLEM
• contact DPIRD (Western Australia) or Cesar Australia (South Australia, Victoria and New South Wales) if you experience a chemical control failure involving RLEM and/or suspect insecticide resistance, as they can assist with advice and/or resistance testing.

The revised version of the RLEM-RMS will be available in the autumn of 2024 and will continue to support growers and advisers managing RLEM resistance. It will incorporate new science and advice from resistance experts and integrate feedback on the previous RMS obtained through the social benchmarking surveys.

Social research enables us to better understand motives and identify knowledge gaps, which will be used to produce regionally relevant management recommendations and extension materials which address the concerns of growers to help increase the adoption of IPM practice.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. The RLEM project (CES2010-001RXT) is being undertaken in collaboration with the University of Melbourne, Western Australia Department of Primary Industries and Regional Development (DPIRD) and James Ridsdill-Smith with funding from Grains Research and Development Corporation (GRDC). We would like to acknowledge Leo McGrane for his earlier work on this project.

The authors thank the many agronomists and growers who aided in mite collections, helped with field sites, and provided field history information. The authors thank Lisa Kirkland, Josh Douglas, Olivia Reynolds, Jenny Reidy-Crofts, Xuan Cheng, Matt Binns, Peter Mangano, Nick Bell, Clay Sutton, Thomas Heddle, Isobel Roberts, Tiana Rey and Paul Tyson for technical input and assisting with field collections. Thanks also to Amol Ghodke, Anthony van Rooyen, Moshe Jasper, Haylo Roberts, and Andrew Weeks for assisting with the molecular analysis of SP resistance.

References

Arthur A, Maino J, Hoffmann A, Jasper M, Lord A, Micic S, Edwards O, van Rooyen A, Umina P (2021) Learnings from over a decade of increasing pesticide resistance in the redlegged earth mite, Halotydeus destructor (Tucker). Pest Management Science 77, 3013-3024.

Edwards O, Walsh T, Metcalfe S, Tay W, Hoffmann A, Mangano P, Lord A, Micic S, Umina P (2018) A genomic approach to identify and monitor a novel pyrethroid resistance mutation in the redlegged earth mite, Halotydeus destructor. Pesticide Biochemistry and Physiology144, 83-90.

Mata L, Arturi A, Weeks A, Anthony V, Arthur A, Maino J, Lowe L, Umina P, Hoffmann A, MacMahon A, Kang H, Carew M, Yang Q, Pavri, Ridsdill-Smith J, Lord A, Micic S (2022) Future options for the control of RLEM in Australian grain crops – annual progress report 2022.

Miles M (1996) Control threshold and sampling recommendations for insect pests of field crops and pastures. Victorian Institute for Dryland Agriculture, Horsham VIC.

Resistance management strategy for the Redlegged Earth Mite in Australian grains and pastures

Redlegged earth mite best management practice guide – Southern

Free redlegged earth mite insecticide resistance testing 

Contact details

Luis Mata
03 9349 4723
lmata@cesaraustralia.com

Svetlana Micic
08 9892 8591
svetlana.micic@dpird.wa.gov.au