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Resistance detected in redlegged earth mites

Populations of redlegged earth mites (Halotydeus destructor) have been found to have high levels of resistance to widely used synthetic pyrethroid insecticides, bifenthrin and alpha-cypermethrin. This discovery has lead many entomologists to question the effectiveness of current control methods for this pest.

Resistance was discovered after a farmer found that he was unable to control an infestation of redlegged earth mites in his seedling canola crop in 2006. Four applications of registered rates of synthetic pyrethroids failed to control the mites in this canola paddock, which subsequently suffered extensive feeding damage and considerable yield loss. Importantly, this resistance has been demonstrated to have a genetic basis, meaning it is passed on to future generations of mites and could persist in the field indefinitely. For one synthetic pyrethroid, the resistant mites exhibited greater than 240,000 times the resistance of control mites.

At this stage, resistance has only been confirmed in the Esperance region of Western Australia, although it’s quite probable that chemical-tolerant populations are present elsewhere across the continent and Esperance farmers are simply the first to report problems.

Unfortunately, insecticides are often applied in an ad hoc fashion with little concern for the long-term implications. This is worrying because the continued application of insecticides places enormous selection pressure on a species to develop resistance. We need to monitor this development closely and reconsider standard industry practices for controlling redlegged earth mites. Chemicals have been used as the main method of controlling this pest for more than 50 years in Australia, so it’s perhaps surprising that we haven’t encountered resistant redlegged earth mites before now.

In recent years, the increased usage and reliance on low cost insecticides has accelerated the selection pressure placed on pest populations. To reduce the risk of problems developing in the future, the use of ‘insurance sprays’ should be avoided. Only apply insecticides after careful monitoring and correct identification of pest species.

Alternative treatment options are available, so we need to consider these with a view to controlling redlegged earth mites in a sustainable manner. There is a real need for integrated pest management (IPM) strategies, but this requires careful monitoring and further research to better understand non-chemical control, including the role of natural biological control agents.

If you suspect any resistance problems, contact your local agronomist or Paul Umina on (03) 83442522.

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