Don’t guess – test: How to monitor for pesticide resistance

Ellie Dearlove explains how to monitor for resistance to plant protection products in crop pests (insects, weeds and disease pathogens).

All pest populations have natural genetic variation in pesticide sensitivity.

With frequent use of the chemistry, particularly those with the same mode of action (MoA), less-sensitive individuals survive and reproduce.

This process can go unnoticed, until field control is compromised.

Today, resistance to insecticides, herbicides and fungicides is seen in many cropping systems, which reduces control, limits options and increases costs.

Resistance tests can reveal threats early and help you avoid ineffective products (saving time, money and inputs) and implement better management.

Consider testing if: 

• Control performance declines/survivors remain after treatment*
• The same MoA has been used repeatedly
• Resistance is in your area
• You're considering switching chemistry 

*Application issues also cause sub-optimal control, so it's important to rule this out.

Resistance is dynamic and periodic testing aids swift detection.

There’s no set testing interval, as it depends on the pest population and system risks, which are driven by:

• The crop grown
• Chemistry use (type and intensity)
• Weather
• Cultural practices

Certainly, testing is not a one-off tick box.

Pyrethroid resistance is the most common concern.

As insects can move hundreds of miles in the wind (especially aphids), previous results may not reflect this year’s risk.

Additionally, many insect species have multiple generations every year, so resistance can spread fast.

It's important to monitor crops closely and test when control failures are suspected.

As part of a Defra-funded monitoring project, ADAS will screen peach-potato aphid, bird-cherry oat aphid, grain aphid and cabbage stem flea beetle for insecticide resistance (sent in dead or alive).

Many of the most active herbicides pose a very high resistance risk.

In contrast to insecticide and fungicide resistance, herbicide resistance often develops independently in fields, with each having its own weed-resistance profile.

As a foundation for monitoring:

• Keep field records of cropping, cultivations, herbicide use, weed burdens and control levels
• Monitor problematic patches closely
• Increase monitoring/testing frequency where resistance pressure is high or in areas of persistent weeds

Where resistance is not suspected or hasn’t been confirmed, try to test at least once per rotation and at least every three years for major weeds.

Screens often use seed or leaf samples, but sometimes live plants are needed, such as for the rapid indication of glyphosate resistance risks in Italian rye-grass.

Fungicide resistance monitoring is coordinated across the UK on behalf of farmers.

Fungicide resistance, which can develop gradually or rapidly, impacts commercially important MoAs.

Septoria tritici in wheat is a high-profile example, with rapid control losses for strobilurins (quinone outside inhibitors, Qols) seen in the early 2000s and more recent gradual changes for demethylation inhibitors (DMIs/azoles) and succinate-dehydrogenase inhibitors (SDHIs).

Our fungicide performance programme (for wheat, barley and oilseed rape) tracks product efficacy changes and supports the design of tailored fungicide programmes that balance disease control and resistance management, through product/MoA selection and appropriate doses.

Our laboratory sensitivity analyses also quantify resistance shifts long before they are detected in the field and help reveal cross-resistance risks, which is where a pathogen becomes less sensitive to a fungicide that was not applied (with the same MoA).

For example, we’ve seen this with the newer SDHIs, based on tests of pydiflumetofen (adepidyn, Miravis Plus) and isoflucypram (iblon, Vimoy).

Using these SDHIs in the same programme is unlikely to benefit resistance management.

As there's no known field resistance to multi-site fungicides, such as folpet, these are a vital resistance management tool. 

Sample survivors of recent treatments or suspect individuals/populations and consider how representative the samples are to wider field populations.

Timing, handling and sampling technique are essential for reliable results. 

Work with trusted companies that test samples under controlled conditions (in laboratories or glasshouses).

Tests may include:

• Dose-response assays to measure sensitivity to active ingredients
• Discriminating dose tests to identify resistant individuals
• Molecular diagnostics to detect known resistance mutations

Always ask how best to sample and how quickly you can expect results.

Ask for guidance to help you make sense of the results.

It may not be as clearcut as ‘resistant’ or ‘susceptible’, with various levels of reduced sensitivity or partial resistance often described.

For example, the results can help show:

• Which chemistry is likely to give reliable control
• The highest risk MoA
• Where more non-chemical controls may be needed
• How urgently strategies need to change 

If you would like to learn more about the management of pesticide resistance, then sign up to one of our remaining winter Resistance Roadshow events (funded by Defra).