Mites demand measured response

Key points

  • WA has RLEM resistance to synthetic pyrethroids (SP) confirmed on 49 properties
  • RLEM from a WA property were found in 2014 with both SP resistance and tolerance to the organophosphate (OP) omethoate
  • SP resistance and OP tolerance in RLEM appear to be heritable and persistent, requiring integrated control tactics 
Image of red legged earth mites

Insecticide resistance in redlegged earth mites in WA may be heritable and persistent, reinforcing the need for a spring spray only where necessary.

PHOTO: DAFWA

Increasing, and heritable, resistance to some insecticide groups in WA populations of redlegged earth mites (RLEM) has prompted a warning to only spray if necessary.

RLEM (Halotydeus destructor) is a major pest of grain cropping and livestock enterprises and the GRDC and CSIRO are investing in new mite prediction models, control methods and resistance testing techniques.

Department of Agriculture and Food, WA (DAFWA), entomologist Svetlana Micic says there are increasing levels of RLEM resistance to synthetic pyrethroids (SPs) and tolerance to the organophosphate (OP) omethoate.

Coupled with evidence that SP resistance is heritable and omethoate tolerance may be heritable, she is urging growers to adopt a cautious approach to insecticide use in coming months.

“The common management option used against pests in broadacre crops is the application of SP insecticides,” she says. “Repeated use of SP insecticides within and between seasons suggests there is strong selection pressure for RLEM to develop resistance to this chemical group.

“Every time a grower sprays an SP against pests, such as aphids, RLEM also receive a dose of the chemical – even though they are not the direct target.

“It is likely that the repeated cumulative exposure of RLEM to SPs is one of the factors behind the development of resistance.”

Ms Micic says if RLEM are found in high numbers, such as if clovers are silvered with RLEM feeding damage, then spraying pasture paddocks (especially those to be cropped next year) on the optimum TIMERITE® day for that paddock (taking into account climatic variables) will minimise damage to crops and pastures in 2016.

The web-based TIMERITE® package, developed by CSIRO, targets the mites after they have finished laying normal winter eggs on crops and pasture, but before they produce over-summering eggs.

This means a big proportion of the population can be controlled and numbers of RLEM hatching the following autumn will be significantly reduced. CSIRO research has shown TIMERITE® is highly effective at controlling RLEM.

Ms Micic says if RLEM or RLEM feeding damage on clovers cannot be seen, it is probably not worth spraying in spring.

She says routine, preventive-type spraying of pasture paddocks in spring using TIMERITE® is not likely to be sustainable and should not be used in the same paddocks in consecutive years.

“It is best to use TIMERITE® as part of an integrated pest management plan that includes crop rotations and grazing management,” she says.

Ms Micic says if a TIMERITE® spray is used in spring to reduce mite numbers in a paddock that will be cropped to canola, it is vital to continue monitoring that paddock in autumn for RLEM.

She says other preventive action to manage RLEM numbers in spring includes control of in-crop weeds and weeds along fence lines. “These weeds can harbour residual populations of SP-resistant RLEM that can then re-infest surrounding paddocks,” she says.

“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. This compares to less than 200 mites/m2 in crops where weeds have been controlled.”

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

To boost grower tools for RLEM and resistance control, new models to predict the risk of an RLEM outbreak are being developed at the University of Melbourne.

Complementing these better forecasting systems for RLEM, researchers from DAFWA, CSIRO, cesar and the University of Melbourne are continuing to test and monitor mite resistance to insecticides.

The project team is also working with the agrichemical industry to identify new chemical modes of action against RLEM.

More information:

Svetlana Micic, DAFWA,
08 9892 8591,
svetlana.micic@agric.wa.gov.au

Useful resources:

GRDC RLEM Hot Topic – 'Prevent RLEM resistance'
DAFWA Farmnote: ‘Prevent redlegged earth mite resistance
TIMERITE®

Australian Wool Innovation Helpline – optimal spray dates

1800 070 099

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