REDLEGGED earth mites (RLEM) resistant to insecticides have been found in South Australia and authorities believe their spread eastward to Victoria and southern NSW is inevitable.
Paul Umina, entomologist with Cesar and The University of Melbourne, was not surprised the resistance, first observed in Western Australia over a decade ago, has now been found in other parts of Australia.
“You could say it was just a matter of time,” he said.
Tests have found multiple RLEM populations resistant to insecticides based on synthetic pyrethroids (SPs, such as bifenthrin and alpha-cypermethrin, and organophosphates (OPs), including omethoate and chlorpyrifos.
Dr Umina said in most cases SP resistance was more pronounced than OP resistance.
Worryingly for growers, some OP products, such as omethoate, are on the Australian Pesticides and Veterinary Medicines Authority (APVMA) list to phase out due to safety concerns.
This means they will have to make judicious use of remaining insecticides to avoid compounding the resistance problem.
He said avoiding the prophylactic application of insecticides was paramount to reducing the risk of resistance in RLEM.
“As an industry, we must do everything we can to prevent further resistance development,” he said.
Dr Umina said with RLEM activity starting to ramp up across most cropping regions following ideal conditions for mites, growers should monitor paddocks.
“Routine monitoring of emerging crops and pastures needs to take place so growers can tell if they have a problem.”
Dr Umina urged growers in any region to report chemical spray failures involving RLEM and to utilise a free resistance testing service which is being provided through a Grains Research and Development Corporation (GRDC) funded project.
Dr Umina said WA growers with resistant RLEM populations often relied heavily on seed treatments to keep crop damage to a minimum.
“Growers with resistant mites will struggle to achieve adequate control with the SPs, however if a chemical spray is unavoidable, the careful selection and use of an OP may provide some level of control despite the efficacy being somewhat diminished.”
RLEM is a threat to pastures and all major broadacre crops including oilseeds, cereal and pulses. It is also a pest in the vegetable industry.