Test and target mites to suppress resistance

Author: Melissa Williams | Date: 15 May 2014

Image of redlegged earth mites (RLEM)

WA growers are this year being urged to test redlegged earth mites (RLEM) that survive insecticide treatments.

This is the only Australian state where this costly pest has been confirmed to have developed resistance to synthetic pyrethroids (SPs – Group 3A), including bifenthrin and alpha-cypermethrin, and there are now more than 30 properties affected.

But researchers say there are likely to be resistant populations spread right across the WA grainbelt and possibly into other areas of southern Australia.

University of Melbourne research indicates SP-resistant RLEM are up to 240,000 times more resistant to some SP insecticides than susceptible RLEM and this resistance is genetic – surviving through several generations.

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

As part of a GRDC-funded national project, CSIRO and Department of Agriculture and Food WA (DAFWA) researchers will this year continue to test properties across the State where RLEM are found to survive insecticide treatments. Testing will also take place in 2014 in Victoria, NSW and SA through this project.

This collaborative research aims to monitor the extent, geographical spread and dynamics of RLEM resistance, finetune recommendations for integrated management strategies and determine more targeted spraying tactics for this pest.

Testing for resistant RLEM

If RLEM survive registered rates of insecticide treatments, or there is suspicion that mites resistant to chemicals are present on a property, contact DAFWA’s broadacre entomologists to have mites sampled and tested.

For more information contact:
Svetlana Micic
08 9892 8591

Updated testing methods developed by CSIRO in collaboration with the University of Melbourne and DAFWA - with GRDC funding - mean much smaller populations of mites can be tested for resistance this year and tests can occur at any time of the year.

Tactics to prevent RLEM resistance this season

Spray only if necessary

DAFWA research has found that growers with populations of resistant RLEM have mostly used repeated applications of SP insecticides.

To reduce the likelihood of resistance developing, it is best to apply insecticides only on paddocks that have damaging numbers of pests.

Resistant RLEM populations can currently be controlled using insecticides from the OP group, such as dimethoate or omethoate.

In situations where spray failures have occurred, it is important to correctly identify the mite. Blue oat mites (BOM) are controlled by all chemicals registered for RLEM control, whereas chemical controls for Bryobia mite and Balaustium mites differ.

Where spraying is needed, rotate chemical groups

Rotation of chemical groups, such as SPs and OPs, within and between seasons will help to reduce RLEM resistance developing.

If spraying other pests, such as aphids, try not to use SP’s consecutively. Consider other insecticide options.

Predict hatchings of RLEM

Knowing when the first autumn hatchings of RLEM are occurring will help to determine if these mites will coincide with seedling crops.

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

Crops sown in seasons with early breaks and maximum temperatures well above 20°C (e.g. canola sown in April) are unlikely to be damaged by RLEM.

Use insecticide seed treatments

Use insecticide seed treatments for vulnerable crops and sown pastures with moderate pest pressure, rather than spraying whole paddocks.

Seed treatments allow smaller quantities of pesticide to be used that directly target plant feeding pests, allowing any predatory insects to continue their beneficial role.

Plan ahead

Paddocks prepared in the preceding season will have lower numbers of RLEM on crops.

This can be achieved by controlling in-crop and fenceline weeds that provide a habitat for mites and using controlled grazing of break-crops and pastures to reduce mite numbers in spring.

DAFWA research has found sustained grazing of pastures during spring to maintain Feed On Offer (FOO) levels below two tonnes per hectare dry weight will restrict mite numbers to low levels.

Applying insecticides to some paddocks during spring to prevent RLEM populations producing over-summering eggs will also decrease the pest population the next autumn.

Only specific paddocks should be selected for spring spraying based: on FOO levels; future grazing management options; seed production requirements; and intended paddock use next season.

Timerite® is a free package that provides a date in spring for a spraying to stop RLEM from producing over-summering eggs. Although, growers should be aware that the routine spraying of all pasture paddocks in spring using ‘Timerite®’ dates is unlikely to be sustainable.

Crop rotations can decrease reliance on pesticides.  Some paddocks will have a higher risk of RLEM damage if they have been in long term pasture (with high levels of broad-leafed plants) where mite populations have been uncontrolled.

Lower risk paddocks that generally do not require mite control are often those following a cereal or canola weed-free crop, where conditions are less favourable for mite increase.

More information:

Svetlana Micic, entomologist DAFWA
08 9892 8591

GRDC Project Code UM00049

Region West