Current efforts to stop the spread of resistance through the use of pesticide mixtures might sometimes “be doing as much harm as good”, says ENDURE partner Rothamsted Research.
Reporting on research published in Nature Communications , it notes that farmers widely use mixtures of multiple pesticides to stop resistance arising in weeds, insect pests and diseases, with antibiotic mixtures also used to stop resistant microbes evolving.
The study uses the herbicide strategy often adopted by farmers to control highly problematic black-grass (pictured right) to demonstrate that mixtures can have the unintentional consequence of promoting a different ‘type’ of resistance.
It reports: “This large-scale case study into the effectiveness of herbicide mixtures, found that whilst mixture strategies are very effective at slowing resistance towards specific treatments, they may also lead to the development of broader, more generalist types of defence.
“According to lead author Dr David Comont, this unintentional side-effect can provide harmful weeds protection against an even greater array of chemical control compounds - potentially even ones they have never encountered.”
David told the Rothamsted Research website: “Mixtures are almost universally advocated to mitigate resistance in pests, weeds and diseases. Whilst they are undeniably effective, our work highlights that where generalist mechanisms can evolve, even mixture strategies might not be resistance-proof.”
The website explains how using pesticides with different modes of action alters the type of resistance which evolves. Black-grass populations with long-term exposure to a single herbicide develop a highly specific resistance to that type of compound. However, when populations are exposed to the greater use of mixtures with multiple modes of action, they develop a more general resistance mechanism.
Rothamsted Research says these results could have implications for other pests, weeds, or disease species where similar generalist resistance has been shown to evolve.
“We have good evidence that such mixture strategies can be very effective at delaying resistance,” David told the website. “Unfortunately, we now know that some species can also evolve broader, more wide-reaching resistance mechanisms, involving other genetic and biochemical pathways.”
He added: “Mixture strategies will remain a key approach to mitigate resistance, but it’s important to know that even successful strategies like this are not always going to be resistance-proof.
“Future control of pests, weeds and diseases will become increasingly reliant on rapid and accurate resistance diagnostics in order to select the best combinations of chemicals to use, along with the integration of other, non-chemical strategies to slow the evolution of resistance.”
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