Test to take the guess work out of RLEM resistance

Author: Melissa Williams | Date: 20 Jun 2013

Extreme close-up photo of three mites: Bdellidae (Snout mite), Penthaleidae (RLEM) and Erythraeidae (Balaustium mite). (C) DAFWA Entomology, Pia Scanlon.

Redlegged earth mites (RLEM) that survive registered rates of insecticide treatments and/or are suspected of being chemically resistant should be tested for resistance levels.

Twenty nine WA properties have been confirmed with RLEM populations that are resistant to all synthetic pyrethroids (SPs – Group 3A) out of 148 properties tested between Boyup Brook and Esperance from 2006 to 2012.

Researchers say the incidence of RLEM resistance to SP insecticides in this State could be more widespread than these figures suggest and is rising, especially to frequently used alpha-cypermethrin and bifenthrin.

Research by the University of Melbourne has shown that resistant RLEM are up to 240,000 times more tolerant to SP insecticides than susceptible RLEM and this resistance is genetic – surviving in several generations.

Fortunately, tests have shown WA’s SP resistant RLEM populations do not have cross resistance to other insecticide groups, such as organophosphates (OPs – Group 1B), and can continue to be controlled with these products.

As part of a GRDC-funded project, Department of Agriculture and Food WA (DAFWA) entomologists will this winter continue to test properties across the State where RLEM are found to survive insecticide treatments.

Growers and their advisers are advised to contact DAFWA entomologists to arrange sampling and laboratory tests if they suspect RLEM resistance.

As part of this project, Albany-based DAFWA entomologist Svetlana Micic is researching the extent and geographical spread of RLEM resistance to help develop integrated management strategies to deal with this pest.

Inspect for damage

She says RLEM hatched across much of the WA grain belt during May - in areas that received late autumn-early winter rainfall accompanied by several days of temperatures below 20 degrees Celsius.

Numbers and consistency of hatching are dependent on paddock history, including how RLEM were managed last spring and the crop type. Canola crops are more at risk than cereals and late sown crops are at higher risk of damage from these pests than earlier sown crops.

Threshold numbers for economic control of RLEM are variable, but if populations are high there can be total decimation of canola seedlings.

Emerging cotyledons and young seedlings can often tolerate low to moderate numbers of mites without sustaining plant losses, especially under good growing conditions.

But if pests are considered to be in damaging numbers, correct identification of the specific species at work is vital to ensure the right choice and rate of insecticide.

The commonly occurring bryobia mite, brown wheat mite and/or Balaustium mite can be easily mistaken as RLEM unless a magnifying glass is used to examine the pests.

Some mites are also beneficial, such as the pasture snout mite that is a predator of lucerne flea and pest mites.

Extreme close-up photo of three mites: Bdellidae (Snout mite), Penthaleidae (RLEM) and Erythraeidae (Balaustium mite). (C) DAFWA Entomology, Pia Scanlon.

Click the image above to view a larger version.

If there is a spray failure

Svetlana says the failure of an insecticide spray to control RLEM does not necessarily mean mites are resistant to that product. Resistance testing will confirm this.

If resistance to SPs is found, these products should not be used for RLEM control.

Alternative registered insecticide groups for the control of RLEM are limited to OPs such as omethoate, dimethoate (under APVMA permit 13155) and chlorpyrifos in crops such as canola.

To prolong the efficacy of all insecticide groups, it is vital to rotate products within and between seasons to minimise the risk of resistance developing and to limit ‘insurance’ - or prophylactic - spraying unless there is a genuine risk of pest problems.

Every time an SP is used to control pests such as weevils, caterpillars and aphids, RLEM also receive a dose of this insecticide – despite not necessarily being the primary target.

Svetlana advised growers to get the most out of the effective and cheap SP insecticides they are currently using by taking advantage of the free resistance testing offered by DAFWA this year.

Understanding the levels of resistance of RLEM will allow better management of properties to keep pest levels low and use of insecticides when they are really needed.

Preventative action in spring

In-crop weeds and weeds along fencelines have been found to harbor residual populations of SP resistant RLEM that can re-infest surrounding paddocks.

Controlling these weeds will reduce mite numbers and levels of over-summer eggs, lowering the burden of RLEM carried over into next autumn.

Unsprayed and under-grazed pastures in spring are known to be favourable to RLEM, which can produce populations of more than 50,000 mites per square metre – compared to less than 200 mites per square metre in crops where weeds have been controlled.

Grazing pasture paddocks in spring to levels below two tonnes per hectare of feed on offer can reduce RLEM populations in subsequent canola crops to levels that are almost as low as those achieved by insecticides.

Another tool for RLEM control is Timerite®, the web-based package that provides growers with a spring date to spray RLEM on pastures to stop females producing over-summer eggs.


More information

To find DAFWA entomologists:

Svetlana Micic, 08 9892 8591, svetlana.micic@agric.wa.gov.au
Peter Mangano, 08 9368 3753, peter.mangano@agric.wa.gov.au

For information about in-crop pest control:


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Region West