Francesca Noakes The redlegged earth mite (Halotydeus destructor) might be a lover of a variety of plant crops, but they are certainly not beloved by farmers.
To help growers better manage redlegged earth mites in the future, a new research project is investigating control options for this pest and improved understanding of insecticide resistance in Australian mite populations.
The redlegged earth mite is one of the most common and destructive of pests for southern Australian grain crops and pastures. In crops like wheat and canola, redlegged earth mite are of greatest concern during seedling establishment when the crop is most vulnerable, and damage during this period can result in substantial economic costs for growers.
As a result, ensuring that grain growers have effective control options available for managing this pest is important for protecting growers’ bottom-lines.
Insecticide resistance in redlegged earth mite
Unfortunately, control of redlegged earth mite is complicated by increasing resistance issues to key chemicals in Australian populations of the mite.
There are five chemical groups registered for use against redlegged earth mite in Australian grain crops: organophosphates (Group 1B), fiproles (Group 2B), synthetic pyrethroids (Group 3A), neonicotinoids (Group 4A) and diafenthiuron (Group 12A). Of these, growers rely heavily on organophosphates, synthetic pyrethroids and neonicotinoids. These chemicals are often applied prophylactically to safeguard against damaging infestations, creating strong selection pressures and driving the evolution of resistance.
Resistance to synthetic pyrethroids in redlegged earth mites was first detected in WA in 2006, followed by the detection of organophosphate resistance in the state in 2014. Since these discoveries, resistant populations have been found across large areas of WA as well as being detected in new regions, including in South Australia and in 2019 in Victoria to organophosphates.
Further, there is significant concern regarding the potential for neonicotinoid resistance to evolve in this pest given the high selection pressures from widespread use of seed dressings in grain crops.
While luckily there has been no recorded cases of neonicotinoid resistance for redlegged earth mite so far, the importance of this insecticide in control of mites (and other key grain pests) should spur caution towards practices that could drive neonicotinoid resistance evolving in the future.
Previous resistance research for the redlegged earth mite
Since the first detection of pyrethroid resistance in redlegged earth mites in 2006, resistance surveillance has been undertaken on a yearly basis, with 1029 populations being tested over the last 13 years under a GRDC investment. One hundred and ninety-five redlegged earth mite populations have now been detected with pyrethroid resistance, 59 populations have been detected with organophosphate resistance and 24 populations with resistance to both chemical groups. Surveillance has covered a wide geographical range throughout Western and eastern Australia, covering a large portion of the entire known Australian distribution of redlegged earth mites.
Given the wide distribution of resistance for the pest, genomic testing was also undertaken to identify the genetic variation between resistant populations. It indicated that resistance evolved independently at multiple locations rather than simply spreading from a single mutation.
These results are significant as they suggest that the pest management practices on individual farms are important when it comes to controlling resistance evolution events. It should still be noted however that proximity to an existing resistant population is a strong indicator of resistance emerging in a location with spread across larger distances assisted by the dispersal of over-summering eggs on wind or contaminated farm machinery.
Research has also shown that in instances of regular pyrethroid exposure, the frequency of pyrethroid resistance in the redlegged earth mite can increase rapidly, but there were no such increases seen when the mites were subject to the same conditions with organophosphates.
Pyrethroid resistance in redlegged earth mite populations in Australia has found to be a recessive trait, meaning that both parents must carry the genetic mutation that confers resistance for offspring to be resistant.
Additionally, pyrethroid resistance has been shown to confer a fitness cost to redlegged earth mites. This means that susceptible individuals likely have a higher chance of surviving and will outcompete resistant mites in the absence of pyrethroid applications.
Therefore, despite the speed at which pyrethroid resistance can increase through a population, there is also an opportunity to revert largely resistant populations to a susceptible state through management practices that promote the presence of susceptible mites.
Pre-emptive research was also undertaken to understand current sensitivities in the redlegged earth mite to neonicotinoids and diafenthiuron that will support early detection of insecticide resistance to these chemicals.
New research project investigating insecticide resistance
The new GRDC investment (CES2010-001RXT) led by Cesar Australia, in collaboration with the University of Melbourne and the WA Department of Primary Industries and Regional Development (DPIRD), will continue resistance surveillance of redlegged earth mite across Australia utilising improved resistant monitoring tools such as the molecular pyrethroid resistance screening test and modelling that predicts ‘at risk’ areas.
This project will also investigate new chemical and biological control options, the role that natural enemies play in the pest’s population dynamics, as well as tools to increase confidence in seasonal redlegged earth mite risks and management options.
A review will be undertaken of the resistance management strategy for redlegged earth mite and updated according to the latest research. The updated strategy will support grain growers in using a range of cultural and chemical control options for managing this pest and reducing selection pressures that could drive further resistances evolving.
Acknowledgements
This new project (CES2010-001RXT) is being undertaken in collaboration with the University of Melbourne, WA Department of Primary Industries and Regional Development (DPIRD) and James Ridsdill-Smith with funding from Grains Research and Development Corporation (GRDC).
Cover image: Photo by Andrew Weeks, Cesar Australia
