Farm Management

Redlegged earth mite insecticide resistance

rlem-photo-a-weeks
Redlegged earth mite is a threat to a variety of Australian crops and pastures, with canola, lupins and legume seedlings the most susceptible to attack. Photo: Andrew Weeks

Scientists have for the first time discovered insecticide resistance in redlegged earth mites (RLEM) in the Grains Research and Development Corporation (GRDC) southern cropping region.

Recent laboratory studies on several RLEM populations in South Australia have confirmed resistance to both synthetic pyrethroids, including bifenthrin and alpha-cypermethrin, and organophosphates, including omethoate and chlorpyrifos.

This is the first time resistance has been detected in Australian RLEM populations outside of Western Australia, where resistance can be found throughout Western Australia’s grain belt.

Research, in which the GRDC invests, suggests that whilst it is too early to determine just how widespread the distribution of resistance is in the south, resistant samples have come from more than one paddock.

Entomologist Dr Paul Umina, from cesar and the University of Melbourne, says further sampling will be undertaken in 2017 to enable more detailed mapping of the extent of resistance in the southern and northern regions.

RLEM is a threat to the profitability of a range of Australian crops and pastures, with canola, lupins, and legume seedlings the most susceptible to attack.

Mite feeding can lead to distortion or shrivelling of leaves, and affected seedlings may die at emergence when mite populations are high.

RLEM are most active from autumn to late spring.

Infestations are commonly controlled using seed treatments, or foliar applications of synthetic pyrethroid and organophosphate insecticides.

Dr Umina says the discovery of resistance in samples collected from SA in 2016 means growers and advisers will need to reconsider their RLEM strategies, requiring a focus on ensuring insecticide applications are used judiciously against this pest as part of an integrated approach to control.

“As an industry, we must do everything we can to prevent further resistance development,” Dr Umina says.

To guide growers and their advisers in their efforts to control RLEM and reduce the risk of resistance occurring, a Resistance Management Strategy (RMS) for South Australia, Tasmania, Victoria, and southern New Wales has been developed (in addition to a separate strategy for WA).

The RMS for the Redlegged Earth Mites in Australian Grains and Pastures, developed through the National Insecticide Resistance Management (NIRM) working group and endorsed by CropLife Australia, has been published by the GRDC and is available for viewing and downloading at https://grdc.com.au/FS-RLEM-Resistance-strategy-South or at http://ipmguidelinesforgrains.com.au.

The strategy’s key recommendations include not using the same chemical groups across successive spray windows (on multiple generations of mites), reserving co-formulations (or chemical mixtures) for situations where damaging levels of pests are present, and a single active ingredient is unlikely to provide adequate control, according to Dr Umina.

“The most important message is this: only use chemicals when needed, and if they are required, rotate between different groups of chemicals,” he says.

Other recommendations in the RMS, as part of an overall integrated pest management (IPM) approach include:

  • Consider the impact on target and non-target pests and beneficial invertebrates when applying insecticide sprays. Where possible, use target-specific ‘soft’ insecticides, especially in paddocks with resistant RLEM;
  • Bee-aware;
  • Correctly identify the mite species to ensure the most effective insecticide and recommended label rate is used. Misidentification and incorrect insecticide selection may result in poor control and contribute to selection for resistance;
  • Assess mite and beneficial populations over successive checks to determine if chemical control is warranted. Use economic spray thresholds where available and do not spray if pest pressure is low;
  • Monitor RLEM numbers before and after insecticide application to determine control levels achieved. Where poor control is observed re-evaluate future control tactics and seek expert advice.

Growers are again in 2017 being encouraged to use a RLEM insecticide resistance testing service, available at no extra cost to growers through a national GRDC-funded project led by the University of Melbourne, in collaboration with cesar, the Department of Agriculture and Food Western Australia, and CSIRO.

Dr James Maino, a cesar researcher, is urging growers to pay close attention to RLEM populations sprayed with insecticide so that any potential resistance can be detected early before developing into a more serious issue.

“If growers notice poor efficacy from spraying or experience chemical control failures over autumn and winter, we want to know about it,” Dr Maino says.

Growers and advisers suspecting chemical resistance in RLEM should contact Dr Maino on 03 9349 4723. Samples will then be collected from the property and follow-up advice as to the status of resistance will be provided.

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