You’ve likely heard us announce over the years “We’re testing for resistance!”.
While this is our one battle cry to rally agronomists and growers to alert us to suspicious insecticide control failures – we have to tackle resistance from multiple angles, depending on both the invertebrate pest and the chemistry in question.
For example, to test the redlegged earth mite (RLEM) for synthetic pyrethroid resistance, we use molecular diagnostics and screen for a particular gene mutation. Whereas testing for organophosphate resistance in RLEM requires bioassays – lab tests that expose insects to varying concentrations of an insecticide and their response (essentially alive or dead) is recorded.
But although there is no one-size-fits-all model for testing insecticide resistance – there is some overlap.
Recently, cesar research scientists have developed a novel bioassay for testing not one, but TWO notorious crop pests, RLEM and lucerne flea, for sensitivity shifts to neonicotinoids (group 4A mode of action). The bioassay was developed through a GRDC-funded project and in partnership with CSIRO and the University of Melbourne.
After unsuccessfully trying insecticide residue exposure methods on leaves and glass vials, and even submersing them in the insecticide entirely, the research scientists found their answer!
The novel bioassay exposed RLEM and lucerne flea to different concentrations of insecticide (in this case imidacloprid) on filter paper and their mortality was assessed, allowing the scientists to capture data on how sensitive some populations are to neonicotinoids.
And how did they know that this novel bioassay works? Well it was validated first using green peach aphid (GPA), a pest already known to have developed resistance to neonicotinoids. Testing involving GPA showed that the bioassay method could detect small differences in imidacloprid resistance.
In broadacre farming, neonicotinoids are used as seed dressings to protect some crops and pastures from attack by several chewing and sucking pests during establishment, including RLEM and lucerne flea. Keeping a close eye on their resistance status and any sensitivity shifts is crucial. While they have not yet developed neonicotinoid resistance in Australia, they are ubiquitous across many crop types, and are frequently exposed to insecticides – the selection pressure is high!
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.
Cover image: Photo by Andrew Weeks, Cesar Australia
