DAFWA entomologist Svetlana Micic is studying the incidence and control of insecticide-resistant redlegged earth mites in WA as part of a national GRDC-funded research project into this costly pest.
PHOTO: David Cappaert, Michigan State University
Growers who find redlegged earth mites (RLEM) that have survived insecticide treatments are urged to test for resistance and start planning spring control strategies.
Department of Agriculture and Food, Western Australia (DAFWA) entomologist Svetlana Micic says WA growers should contact their local DAFWA office if they have had difficulty controlling RLEM this year, as this could indicate resistance to synthetic pyrethroids (SPs – Group 3A) – including bifenthrin and alpha-cypermethrin.
WA is the only state with confirmed resistant RLEM populations, with 30 properties known to be affected.
However, researchers say resistant populations are likely to be widespread across the WA grainbelt and possibly in parts of eastern 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 national, GRDC-funded project, led by Dr Paul Umina from the University of Melbourne’s Centre for Environmental Stress and Adaptation Research, CSIRO and DAFWA researchers are this year continuing to test properties across WA where RLEM are found to survive insecticide treatments.
Testing will also take place in 2014 in Victoria, New South Wales and South Australia through this project.
Redlegged earth mites
- Redlegged earth mites (RLEM) grow to about one millimetre in length.
- Adults have a velvety black body and eight red legs.
- Newly hatched mites are pinkish-orange with six legs and are 0.2mm long.
- Nymphs develop into mature adults in four to six weeks.
- In autumn, over-summering eggs hatch when there is significant rainfall and the mean daily temperatures fall below 21°C.
- Three generations of RLEM can be produced per season.
- Canola, pulses and other legume seedlings are the most susceptible.
- RLEM feed on broadleaf weeds, particularly capeweed.They also attack cereals and grasses, especially when selective herbicides eliminate preferred hosts.
- RLEM are often found in feeding aggregations of up to 30 individuals.
- Feeding causes silvering or white discoloration of leaves and distortion or shrivelling in severe infestations.
This collaborative research aims to monitor the extent, geographical spread and dynamics of RLEM resistance, fine-tune recommendations for integrated management strategies and determine more targeted spraying tactics for this pest.
Ms Micic says to prolong the efficacy of all insecticide groups and minimise resistance risks, it is vital to rotate chemical products within and between seasons and limit ‘insurance’, or prophylactic, spraying.
“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,” she says.
Ms Micic says research has shown that weedy habitats, including in-crop weeds and weeds along fencelines, can host residual populations of RLEM, which have the potential to re-infest surrounding paddocks.
“Unsprayed and under-grazed pastures in spring are also known to be favourable to RLEM,” she says.
DAFWA research has found sustained grazing of pastures during spring to maintain feed on offer (FOO) levels below two tonnes per hectare dry weight – ideally about 1.4t/ha – will restrict mite numbers to low levels. These paddocks will often not require spraying for RLEM.
Applying insecticides to some paddocks – including pastures with FOO more than 3t/ha or legume break crops – during spring to prevent RLEM populations producing diapause (over-summering) eggs will also reduce the pest population next autumn.
Ms Micic says 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, specific to a locality, for a spraying to stop RLEM from producing over-summering eggs.
CSIRO studies have shown spraying on the optimum date, or two weeks earlier, provides effective RLEM control. Waiting for two weeks after that date can significantly increase the carry-over RLEM population.
Growers should be aware that the routine spraying of all pasture paddocks in spring using Timerite® is not likely to be sustainable.
08 9892 8591,
For related GRDC resources go to Ground Cover Direct
DAFWA RLEM information hub: www.agric.wa.gov.au/prevent-redlegged-earth-mite-resistance
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