Leo McGrane
With much of south-eastern Australia receiving a welcome drink of rain and cooler autumn temperatures arriving, the redlegged earth mite, Halotydeus destructor, will have started popping up in crops and pastures during April.
This autumn we are asking growers and agronomists to revisit their redlegged earth mite management strategies on the back of an important finding.
We have recently detected insecticide resistance in redlegged earth mite populations in Victoria for the first time.
How was the detection made?
In 2019, cesar research scientist Dr Aston Arthur collected redlegged earth mite populations in Victoria’s Northern Country, after an agronomist reported a spray failure with omethoate in 2018.
The mites were collected from three paddocks in close proximity.
Back in the laboratory, Dr. Arthur tested two organophosphate insecticides, omethoate and malathion, against these redlegged earth mite populations – we will call these the ‘test populations’.
To determine if the test populations were expressing resistance, Dr Arthur ran a bioassay to compare their LC50 values to that of a known insecticide-susceptible population – the ‘control population’.
The LC50 is the concentration of insecticide needed to kill at least 50% the tested population.
Dr Arthur found at most a 7-fold increase in the LC50 of one of the test populations compared to the control population with omethoate, and up to a 70-fold increase with malathion.
The test populations also underwent molecular testing to screen for resistance to synthetic pyrethroids.
No synthetic pyrethroid resistance was found in any of the test populations.
This detection was made possible through a GRDC investment, which supports our scientists to survey eastern Australia and test suspect redlegged earth mite populations.
Why is this detection important?
The redlegged earth mite has been showing high levels of resistance to insecticides for more than a decade in Western Australia.
However, this is not just a concern for the west; cases of insecticide resistance are now popping up in south/south-eastern growing regions.
Since 2016, resistance to both synthetic pyrethroids, including bifenthrin and alpha-cypermethrin, and organophosphates, including omethoate and chlorpyrifos, has been detected in redlegged earth mite populations in numerous locations within South Australia.
This recent detection in Victoria is the furthest eastern point in Australia that insecticide resistance has been detected in the redlegged earth mite.
While the level of resistance detected in Victoria is considered ‘low-level’, we can’t stress enough, the importance of having a resistance management strategy at the forefront of redlegged earth mite control in the south-east.
The redlegged earth mite has a wide appetite, feeding on a broad range of plants including but not limited to canola, clover, faba beans and lupins.
If higher levels of insecticide resistance were to evolve to multiple chemistries and across larger areas, it would make protecting establishing crops more complex and challenging.
How can we stop insecticide resistance in its tracks?
The ubiquity of the redlegged earth mite across many broadacre crops and pastures means that it is frequently exposed to insecticides and faces a high selection pressure to evolve resistance.
With sowing of winter crops and pastures already underway or nearing for others, there are two resources from which growers and advisers can source the best available information on the redlegged earth mite and resistance management in broadacre crops and pastures.
The first resource is the Redlegged earth mite best management practice guide – Southern.
This best management practice guide is a useful tool to not only help minimise the incidence of redlegged earth mite but also minimises the need to apply insecticides to control them.
This guide is not based on any one crop, but rather lists multiple cropping scenarios based on whether the previous year a paddock was cropped, or it was in pasture or fallow. These scenarios generate a risk score – low, moderate or high, and the guide provides management strategies based around 4 different times of the growing season (late spring, pre-sowing, seedling establishment and winter-spring).
The second resource is the Resistance management strategy for the redlegged earth mite in Australian grains and pastures.
The resistance management strategy outlines various chemical control approaches based on scenarios where there is no resistance, resistance to synthetic pyrethroids only, resistance to organophosphates only, or resistance to both synthetic pyrethroids and organophosphates.
The redlegged earth mite may be mistaken with other crop-feeding mites. For help with mite identification, watch our PestBites episode on Crop and pasture mite identification below or contact our team at PestFacts south-eastern.
Acknowledgements
This work was made possible with financial support from the Grains Research & Development Corporation (UM00057). Our project partners are CSIRO, the University of Melbourne, and the WA Department of Primary Industries and Regional Development.
Thanks to Dr. Aston Arthur for reviewing this article.
Cover image: Photo by Andrew Weeks, Cesar Australia
