Insecticide resistance status never stands still

Find out how an insecticide resistance monitoring project hunts for signs of trouble and guides the management of crop pest threats.

Around 10 years ago (April 2012), the latest phase of an insecticide resistance monitoring project began.

The Rothamsted Research-led work uses insects sent in from the field and exposes live samples to insecticide compounds at various screening doses.

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

Critically, in any sample, they can rule out resistance – helping to indicate that poor control is due to some other aspect of management.

The team has now accrued baseline bioassay data for many aphid pests and insecticides. This precious strategic resource can be used to rapidly assess potential efficacy shifts.

The screens do not require knowledge of the underlying genetic resistance mechanisms. However, where mechanisms are known, the team turns to 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, with significant resistance data stretching back to 1996.

Pyrethroids

The 2021 screens on peach-potato aphid continued to detect strong pyrethroid resistance, associated with the super-kdr target site resistance mechanism, in many samples.

DNA-based tests showed that the latest form – super-kdr (north European, Ne) – is common and widespread in the UK (found in 71% of the 2021 samples).

The presence of kdr (which confers moderate resistance to pyrethroids) was also found (in 24% of the 2021 samples).

The frequencies of these resistances are close to the levels observed in 2020.

Pirimicarb

The team also continues to detect resistance to pirimicarb (conferred by MACE) in UK peach-potato aphid populations.

The continuation of MACE in UK samples, despite the loss of pirimicarb as an authorised active ingredient in most UK crops, may be due to the resistance mutation ‘hitch hiking’ alongside super-kdr in the UK’s aphid ‘super clones’.

Organophosphates

For the first time during this project, the team did not detect high (R₂) or extreme (R₃) esterase-based resistance in peach-potato aphids collected from open-field crops. The result suggests that the esterase-based resistance mechanism is being selected against now organophosphate compounds are no longer used in the UK. This is referred to as a ‘fitness cost’.

Other compounds

For several other key compounds, the 2021 screens on peach-potato aphid samples found no evidence of resistance at levels that may compromise control. In fact, there was no evidence of any significant shifts in sensitivity from most of the diagnostic baselines.

Neonicotinoids

However, a worrying, albeit subtle, susceptibility shift to neonicotinoids in an aphid sample was detected (collected from oilseed rape in November 2021).

This was equivalent to metabolic-based resistance (Nic-R+), it is the first time this phenotype (moderate resistance) has been observed in the UK.

Additionally, strong (Nic-R++) neonicotinoid target site resistance in peach-potato aphid has been detected in southern Europe and North Africa. Recently, it has also been detected on sugar beet in Belgium, moving this resistance threat closer to our shores.

Resistance origins

In peach-potato aphids, rarer combinations of insecticide resistance mechanisms/genotypes are found significantly more often in samples collected from protected crop sites, compared to open-field sites, according to the researchers.

Some aphids in protected environments are thought to originate from more genetically diverse, sexually reproducing, populations on imported plant material. Consequently, it is essential to keep a close eye on this potential gateway for new forms of resistance.

Grain aphid: Levels of pyrethroid resistance are not greater than moderate. This means control failures are unlikely to occur if resistance management guidance is followed. This includes applying products at full recommended label rates with good aphid contact.

Bird cherry-oat aphid: No evidence of either resistance or reduced sensitivity to pyrethroids in the UK.

Cabbage stem flea beetle: The frequency of pyrethroid-resistant beetles (conferred primarily by a metabolic mechanism) has risen consistently over several years. There no longer appears to be a geographical resistance ‘hotspot’ in England. However, it is worth noting that a Scottish sample (taken in 2020) was fully susceptible to pyrethroids.

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

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

The project results 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 pest target 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 take insecticide resistance tests.

Image of peach-potato aphids on a leaf and (inset) Rothamsted Research entomologist Stephen (Steve) Foster, who leads the insecticide resistance monitoring project.