The evolution of pesticide resistance in populations of redlegged earth mite presents an increasing challenge for growers of grains and pasture crops across Australia. This article outlines new research which could help counteract the evolution of pesticide resistance in this pest.
The rise of resistance
In Australia, the redlegged earth mite can be controlled chemically by one of five registered chemical groups, depending on time of year and crop type. However, growers rely heavily on only three of these – neonicotinoids as a seed dressing, and synthetic pyrethroids and organophosphates as foliar insecticides. The repeated application of these pesticide groups has placed a high selection pressure on redlegged earth mite to evolve resistance.
Resistance to synthetic pyrethroids, was first detected in Western Australia in 2006, followed by organophosphate resistance in 2014. Synthetic pyrethroid and organophosphate resistance is now widespread across much of the southern grain growing region of Australia.
Study of these resistant populations has shown that pyrethroid resistance is primarily attributed to a specific gene mutation. A similar gene mutation has been found in other arthropods which have become resistant, including mosquitoes (Aedes aegypti) and green peach aphids (Myzus persicae).
However, in these species the mutated gene comes at the cost of other survival factors. For example, resistant mosquito populations can have reduced female fertility and slower larval development which significantly reduces the rate at which resistant populations can reproduce. Resistant green peach aphid populations have been shown to have a reduced response to alarm pheromones which make them more likely to be predated by natural enemies. So, having gene mutations that enable mosquitoes and aphids to survive pyrethroid sprays can in fact reduce their fitness in the absence of pesticides.
What is fitness and why does it matter?
Fitness relates to the ability of an individual to survive and reproduce, and high fitness is essential for a population to persist in an environment. The possession of certain genes is what gives some individuals a fitness advantage over others.
The number of individuals which have these genes will change over time, as some environments and selective pressures (such as predation) favour some genes over others. For example, the gene for cryptic colouration will benefit an individual by hiding them from predators, so they will be more likely to survive and have offspring which also have the gene for this colour.
These changes in gene frequency in response to the environment form the basis of the emergence of pesticide resistance. The relative fitness of the mutant gene will be higher than the wild-type in situations where mites are exposed to pesticides. Repeated use of the same pesticide will select for those individuals with pesticide-resistant mutations and these will soon dominate the population. However, individuals carrying the resistant mutation can have relatively lower fitness than wild-type individuals in the absence of pesticides.
This led researchers to ask what the fitness cost to redlegged earth mites that acquire gene mutations associated with pesticide resistance are. And this is where it gets interesting.
Assessing fitness costs
To test this, researchers at The University of Melbourne and Cesar Australia collected pyrethroid resistant and susceptible redlegged earth mite populations from the field. They then housed the mites in a shade-house which simulated field conditions and tracked their offspring over several generations to assess if there were any differences in fitness between the susceptible and the resistant populations of mites. The researchers measured reproductive rates and tracked the presence of different genes.
The results of the research demonstrated significant fitness reductions associated with pyrethroid resistance. Populations that had a high proportion of resistant mites showed lower reproduction rates than those that had fewer, or no resistant mites. The researchers also found that the resistant genes became less common in the populations over time. So, when mite populations are no longer exposed to pyrethroids, they begin to lose their resistance after three generations.
What does this mean for the management of pesticide resistance?
Alongside the key strategies outlined in the redlegged earth mite insecticide resistance management strategy and the redlegged earth mite best management practice guide, which include monitoring, paddock risk assessments and strategic timing of pesticide applications, it is important to understand the fitness costs of pesticide resistance.
These findings provide evidence supporting the redlegged earth mite best management practice advice to carefully select and rotate pesticide groups in the field. The fitness cost imposed on resistant mites may help to maintain susceptible populations of redlegged earth mite (if managed correctly), as susceptible mites have a higher chance of surviving and passing on their genetic material in the absence of synthetic pyrethroid usage.
It also opens the door for other pesticide-resistance management strategies, such as the susceptible refuge strategy, to slow the evolution of synthetic pyrethroid resistance. This strategy involves maintaining refuges for susceptible mites to maintain a pool of susceptible genes in a population.
The research presented here raises a number of additional questions including: how large are the fitness costs? Is there also a fitness cost for genes associated with organophosphate resistance? Or for populations carrying resistance to multiple pesticide groups?
Answering these research questions could provide exciting avenues for developing alternative evidence based IPM strategies for managing and containing redlegged earth mite pesticide resistance into the future.
Want to know more?
This research was published in the Journal of Economic Entomology. You can find a link to the abstract here, or if you would like to read the full paper please get in touch.
If you suspect redlegged earth mite is showing signs of resistance in your area, you can send samples for free resistance testing. More details here.
If you would like more information on this topic please email our extension team (Lizzy Lowe or Leo McGrane) and we will answer your questions or put you in touch with the researchers from the project.
This research was completed as part of Xuan Cheng’s PhD, supported by the University of Melbourne, CSIRO, and the Taiwan Ministry of Education (GSSA). This research was an investment of the Grains Research and Development Corporation (Project #0903CR7_Mite Resistance). Thank you to Xuan Cheng, Lizzy Lowe, Leo McGrane and Paul Umina for contributions to this article.