How to combat resistance
Pesticide resistance is when target weeds, pests or a disease pathogen develops the capacity to survive the active ingredient approved for its control.
It develops when targeted individuals in a weed, pest or disease population have genetic traits that enable it to survive the impact of the active ingredient, reproduce and then pass on those traits.
Progressively the target population will alter to include more individuals with the genetic traits that enable it to resist the active ingredient.
Causes of resistance are widely understood and often linked to patterns of product use. Commonly cited causes of resistance include repeated use of the same active ingredient and/or active ingredients with the same mode of action, lack of suitable active ingredient rotation, poor practices and incorrect application rates (litres/ha).
The progressive reduction in the availability of key active ingredients, through withdrawal of approval, is now limiting the options. And this is likely to be increasing the challenge.
Although product resistance is a significant threat that could undermine crop production and increase the cost of crop protection, appropriate active ingredient selection and application can substantially reduce the risk of resistance developing.
Many product labels include details and guidance about pesticide resistance in reference to the active ingredient in the product. Some information may be found in the ‘Other Specific Restrictions’ section of the label and these must be strictly followed as a mandatory part of the label.
Other information about combatting resistance may also be found on directions for use or in a designated section in the guidance component in the label.
It is a legal responsibility for operators to always read the label, and they adhere to the instructions and guidance, including any specific label restrictions or guidance that relates to concerns regarding pesticide resistance.
Applying products at application rates above or below recommended rates can increase the risk of targets developing resistance to products.
Treatments at application rates below those recommended on product labels can result in sub-lethal doses of the product hitting the target. Pests, weeds and pathogens that survive this exposure can pass on their genes, including any that confer resistance, to future generations.
Conversely, applying at above recommended application rates results in overuse of active ingredients and could impact negatively on populations of beneficial insects – favouring pests.
Operators need to regularly check and calibrate sprayers and applicators to ensure they are accurately applying the correct dose rate. It is also important to ensure treatments are precisely applied, taking care to avoid any misses or overlaps.
Correct nozzle selection also plays an important role, ensuring the product is applied using the recommended droplet spectrum to guarantee correct coverage and deposition.
Products must be applied only to target areas, which helps reduce the risk of resistance developing. It is also important to protect beneficial insects in habitat areas around crops, such wild flower areas or hedgerows, because useful crop pest predators can help to reduce the population of resistant pests.
Non-cropped areas may also host weeds, but remain undisturbed by cultural control or other treatments in the cropped field, which can result in creating a reservoir of resistant weeds. Drift onto these areas will apply a sub-lethal dose of herbicide, which could worsen the problem – further encouraging resistance. These resistant plants can also migrate into crops.
Operators must complete an environmental risk assessment to ensure all the necessary measures required to minimise the risk to non-target areas, especially from spray drift, are identified and applied.
A key part of any strategy to combat pesticide resistance is to follow up the success of any applications on the target weed, pest or disease. Always assess the results by, for example, checking for surviving weeds after an herbicide application. Investigate any unexpected results and report these to the product’s manufacturer and appropriate resistance action groups (see below).
As well as being a legal requirement, application records are also a vital tool for keeping track of treatments against which the response of the target can be judged. Weather conditions (temperature, humidity), ground conditions at the time of application, crop growth stage at treatment, weed, pest or disease incidence and any tank mix information are the types of useful information that can be added to records. These details can help provide clues to the success, or otherwise, of any treatment.
In addition to reading the guidance on product labels, operators should follow any updated guidance in any specific product stewardship programmes – a potential CPD activity. AHDB-supported Resistance Action Groups (RAGS) publish further information regarding the status and actions in relation to weed, pest and disease resistance.
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