Grain growers and advisers in western and northern regions are encouraged to have best-practice management strategies in place for redlegged earth mite (RLEM, or Halotydeus destructor) this season - with numbers likely to increase in areas experiencing cold and wet conditions.

What's happening in WA

Western Australian Department of Primary Industries and Regional Development (DPIRD) entomologist, Svetlana Micic, says with crop sowing almost complete and seasonal conditions right for RLEM numbers to build-up, it is critical to have clear management strategies in place.

She says this is particularly important due to the incidence of resistance to insecticides.

"RLEM adults and nymphs have been reported in some areas of the WA grainbelt in recent weeks and seasonal conditions favour an increase in mite populations," she says.

"It is important to be on the look-out for RLEM in crops and pastures, as the plants on which this pest feeds are most vulnerable during the establishment stage."

What's happening in NSW

GRDC Crop Protection Manager - North, Vicki Green, says winter crop sowing was underway and seasonal conditions were right for RLEM build-up in parts of NSW - so growers there also needed clear control plans for this pest.

"Seasonal conditions are conducive to increases in RLEM populations and we are already hearing reports of growers with early infestations," she says.

"RLEM feed on a range of plants, including canola, clover, faba beans and lupins, and crops are most vulnerable during the establishment stage.

"The downside of this season getting off to a good start for growers is that conditions are also good for RLEM population build-up.

"So growers who are now implementing control tactics need to be aware of best-practice guidelines, because it is the only way we will prevent resistance and ensure we continue to have the chemistries available to control this pest."

Insecticide resistance status

The call for a best practice approach comes in the wake of insecticide resistance being detected in RLEM populations in Victoria last month (April).

While RLEM has had high levels of insecticide resistance in WA for decades, this is the most eastern point resistance has been detected in Australia.

Since 2016, resistance to both synthetic pyrethroids, including bifenthrin and alpha-cypermethrin, and organophosphates, including omethoate and chlorpyrifos, has been detected in RLEM populations.

Entomologist Paul Umina, from the scientific research organisation cesar, says the recent detection of resistance in Victoria was 'low-level', but served as a timely reminder for industry.

"If higher levels of insecticide resistance were to evolve to multiple chemistries and across larger areas, RLEM would be far more difficult to control on vulnerable establishing crops than what it is today in south-eastern Australia," Dr Umina says.

Through a GRDC investment which supports scientific surveys and testing of suspect RLEM populations, cesar research scientist Aston Arthur collected RLEM populations in Victoria's north in 2019 - after an agronomist reported a spray failure with omethoate in 2018.

The mites were collected from three paddocks in close proximity.

Back in the laboratory, Dr Arthur tested two organophosphate insecticides, omethoate and malathion, against these RLEM.

To determine if the test populations were expressing resistance, Dr Arthur ran a bioassay to compare their LC50 (the lethal concentration required to kill 50 per cent of the population) values to that of a known insecticide-susceptible population of RLEM.

Dr Arthur found at most a 7-fold increase when mites were tested against omethoate, and a 70-fold increase with malathion.

The test populations also underwent molecular testing to screen for resistance to synthetic pyrethroids.

No synthetic pyrethroid resistance was found in any of the test populations.

Dr Umina says the ubiquity of the RLEM across many broadacre crops and pastures means that the pest is frequently exposed to insecticides and faces a high selection pressure to evolve resistance.

Ms Micic says, in WA, RLEM with high levels of insecticide resistance have been detected since 2006.

"Resistance was first detected against synthetic pyrethroids, including bifenthrin and alpha-cypermethrin, in 2006," she says.

"Since then, with GRDC co-investment, DPIRD has undertaken testing throughout WA and found there are populations of RLEM with resistance to not only SPs but also organophosphates (OPs), including omethoate and chlorpyrifos.

"This means SPs and OPs cannot be relied on to control RLEM effectively."

Latest information about the extent of RLEM insecticide resistance in the WA grainbelt was presented by Ms Micic at the 2020 GRDC Grains Research Update, Perth, and the Updates paper can be found by searching 'RLEM updates' on the GRDC website.

Key mite management information and tools

Ms Micic says it is important that growers and advisers right across Australia who are implementing control tactics for RLEM are aware of best-practice guidelines to minimise the development of insecticide resistance - and to ensure chemistries continue to be available to control this pest.

Information about how to prevent resistance and how to predict hatchings of RLEM to target control strategies is available on the DPIRD website by searching 'RLEM resistance'.

Best-practice help on minimising the incidence of RLEM, as well as minimising the need for insecticide application, is also available in the GRDC's Resistance management strategy for the redlegged earth mite in Australian grains and pastures.

Watch the crops

Ms Micic says knowing roughly when the first autumn hatchings of RLEM occur on a grower's property would help to determine if they will coincide with crops being at the seedling stage.

"RLEM hatch in autumn from their over-summering egg stage, after adequate rainfall and at least seven days of average temperatures below 20°C," she says.

"Crops sown in seasons with 'early breaks', with maximum temperatures well above 20°C (for example, canola germinating in April), will not be damaged by RLEM.

.RLEM may be mistaken for other crop-feeding mites.

For help with mite identification, see the GRDC Crop Mites Back Pocket Guide and the Integrated Pest Management (IPM) guidelines.

Help with mite identification is also available in cesar's PestBites episode.

GRDC has also produced a podcast featuring Professor Ary Hoffman from the University of Melbourne discussing the evolution of pesticide resistance in RLEM.

NOTE: Project partners in this GRDC investment are cesar, CSIRO, the University of Melbourne, and DPIRD.

GRDC Research Code UM00057

More Information: Svetlana Micic, DPIRD entomologist, 0427 772 051, svetlana.micic@dpird.wa.gov.au