International effort needed to tackle insecticide resistance

A long-term project that monitors insecticide resistance in key pest species reveals how problems can potentially arrive in the UK from overseas. Jason Pole outlines the latest findings and how you can manage resistance threats.

The work, led by Rothamsted Research, receives insects from the field and exposes live samples to insecticide compounds at various screening doses.

When compared to expected mortality (from known insecticide-susceptible baselines), they can detect signs of resistance.

The screens do not require knowledge of the underlying genetic resistance mechanisms. However, if mechanisms are known, the researchers use DNA-based tools to identify which ones are present (and the frequency at which they occur).

The most-studied pest is the virus-transmitting peach-potato aphid.

Peach-potato aphids with high levels of resistance to pyrethroids (mode of action group 3) continue to dominate the UK population.

The good news is that there have been no detectible shifts in sensitivity to flonicamid* (mode of action group 29), which is the only alternative to pyrethroids available for use in UK oilseed rape.

Interestingly, the project discovered a peach-potato aphid sample that carried strong resistance to neonicotinoids (mode of action group 4).

It was collected from an oilseed rape crop in Suffolk in autumn 2023, with the resistance identified in aphid screening bioassays that tested two neonicotinoids (imidacloprid and acetamiprid).

In these assays, the researchers saw healthy adult aphids that could produce viable nymphs at doses that would normally kill susceptible individuals.

Although this phenotype has never been observed in the UK population before (screens have been conducted at Rothamsted Research since 1996), it has been seen in samples from mainland Europe.

Molecular assays also supported the finding, which detected a target-site mutation (R81T) associated with resistance to neonicotinoids. 

Concerningly, these aphids also carried the super-kdr (north European, Ne) mutation in a form that confers strong resistance to pyrethroids (homozygous, RR).

It is a genotype and resistance combination never seen anywhere before. In such aphids, neither neonicotinoids nor pyrethroids will work sufficiently.

Molecular testing at the James Hutton Institute suggests that the unique genotype most likely originates from overseas, where sexual reproduction (and mixing of genes) occurs.

These aphids could have flown/been blown into the UK. Alternatively, they may have reached our shores on imported plant material.

It is important to monitor the presence of such aphids in the UK, which impact other key UK crops, including sugar beet and potato.

*In 2023, there were also several reports of flonicamid control issues on peach-potato aphid on sweet peppers grown in glasshouses. Resistance to this active ingredient has been reported in mainland Europe. Once again, resistance may have been imported into the UK. The finding confirms how important collective action is when it comes to resistance management.

Note: In winter oilseed rape, only one flonicamid treatment is permitted, which must not be applied after GS18 (eight true leaves unfolded).

Grain aphid

For the first time, pyrethroid resistance greater than usually conferred by heterozygous (SR) kdr was found in UK grain aphid samples.

The issue was first identified by a key project collaborator (Dewar Crop Protection) in a Norfolk wheat crop that had received three pyrethroid spray applications. In spring 2024, the pest was at much higher levels than expected.

A sample was taken for genotyping and an insecticide screening bioassay. Although the former did not detect the presence of homozygous (RR) kdr, the latter detected a far higher resistance response than the standard heterozygous (SR) genotypes present in the UK.

The researchers will continue to monitor the situation.

Cabbage stem flea beetle

The frequency of pyrethroid-resistant beetles (conferred primarily by a metabolic mechanism) has risen over several years.

Based on screens of 14 samples collected following harvest 2023, all but one contained pyrethroid-resistant adults (ranging in frequency from 7% to 95%). For the first time, resistance was detected in Scotland – one sample contained 27% resistant beetles.

Pollen beetle

Pollen beetle samples from Hertfordshire (3) contained pyrethroid-resistant beetles at the equivalent of full label rate.

Bird cherry-oat aphid and rose-grain aphid

There is no recent evidence of resistance or reduced sensitivity to pyrethroids in the UK. However, no tests on live aphids have been conducted since 2020.

The latest annual report includes results on a wider range of pest species.

The Insecticide Resistance Action Group (IRAG) considers the results from the resistance screening work and updates its management guidance annually.

Integrated pest management (IPM) approaches, which balance non-chemical and chemical control, are at the heart of the guidelines.

The results from our monitoring project also support the regulatory decision-making process, via Defra/CRD.

If an insecticide has been applied optimally (in line with the label recommendations) and fails to control the target pest as expected, do not make repeat applications of any insecticide from the same mode of action.

Report any new resistance concerns to a BASIS-qualified adviser and contact stephen.foster@rothamsted.ac.uk to assess the potential to conduct insecticide resistance tests.