The hidden cost of resistance

Teagasc research is looking at how grain aphids’ resistance to insecticide – or “knockdown resistance” – can be a double-edged sword for growers.

The English grain aphid is a major pest of cereals in Ireland and Europe, and its populations are showing increasing resistance to insecticide. Photo credit: Teagasc

Two recent publications from Teagasc researchers have shown that resistance to insecticide in grain aphids – colloquially termed as knockdown resistance – does not increase the transmission of virus by the grain aphid in barley. In fact, the studies highlight that the resistance trait comes with significant biological costs for the aphid. Together, these findings provide new insights into the trade-offs shaping aphid populations and their impact on crop protection.

The English grain aphid (Sitobion avenae) is a major pest of cereals in Ireland and across Europe and an important vector of barley yellow dwarf viruses (BYDVs). The primary control option available to farmers is the pyrethroid insecticide – however, resistance to pyrethroid insecticide has been detected in grain aphid field populations.

Experiments at Teagasc have shown that resistant aphids are no more effective than susceptible aphids at spreading BYDVs. It was also found that resistance comes with a biological cost: slower growth, shorter lifespan, and fewer offspring. As such, researchers note that Integrated Pest Management (IPM) remains essential for sustainable cereal production.

Rise in resistance

Cereal growers are well aware of the risks posed by aphids. These tiny insects not only weaken crops by feeding on plant sap, but they also act as efficient carriers of viruses that can devastate yields. Among these, BYDVs are the most important, with yield losses of up to 80% reported in severe outbreaks.

Since the ban on neonicotinoid seed treatments, pyrethroid insecticides have been the main weapon against cereal aphids such as the English grain aphid, S. avenae. They are cost-effective; however, their repeated use has applied strong selection pressure on aphid populations. As a result, resistant aphid clones have been detected in Ireland, the UK, and further across Europe.

The rise of resistance in the English grain aphid has naturally raised questions, notes Louise McNamara, a Research Officer in the Crop Science Department at Teagasc Oak Park.

“Would resistant aphids pose an even greater risk to farmers by becoming more efficient virus spreaders? Or would carrying resistance genes make them weaker competitors?”

Recent research by Teagasc and collaborators offers some clear answers.

Reassuring results

In one study, researchers compared resistant and susceptible clones of S. avenae for their ability to spread two strains of BYDV: BYDV-PAS and BYDV-MAV, explains Louise.

“We carried out this research in response to some anecdotal evidence that resistant clones might be better spreaders of BYDV. However, the results were reassuring.”

Transmission of BYDV-PAS was consistently low, with only 0–12% of plants infected, while transmission of BYDV-MAV was higher, ranging from 26–40%. Crucially, Louise points out, resistance status had no effect on these rates.

“This means that resistant aphids are no more dangerous as virus vectors than their susceptible counterparts. For growers, that is good news; the spread of BYDV is unlikely to accelerate simply because aphid populations carry pyrethroid resistance.”

Examining aphids at Teagasc Oak Park. Photo credit: Andrew Downes

Assessing trade-offs

The researchers then carried out a second study, examining the life history of resistant aphids. Using detailed life-table analysis, they compared a resistant clone (SA3) with a susceptible clone (SA27).

The results revealed a stark biological penalty. Compared to susceptible aphids, resistant aphids took longer to develop, meaning slower population build-up. In addition, resistant adult aphids live shorter lives and produced fewer nymphs. Overall, reproduce fitness in resistant clones was reduced by nearly 50% compared to the susceptible clone.

“In other words,” Louise explains, “while resistance helps aphids survive insecticide sprays, it also slows their growth and reduces their competitive ability in the absence of chemicals. This fitness trade-off may help explain why resistant clones do not always dominate in the field, even where pyrethroid use is high.”

A double-edged sword

So, what does this mean on the ground, and what are the key takeaways for growers?

Resistant aphids still spread BYDV just as well as susceptible ones, explains Maximilian Schughart, a postdoctoral fellow at Teagasc. “You cannot rely on their weakness to reduce virus pressure. While resistant clones are less fit overall, even small numbers can maintain virus circulation.”

Overuse of pyrethroids will continue to select for resistance, despite these costs. IPM remains the cornerstone of control: regular monitoring of aphid flights, use of virus-tolerant or resistant varieties, later sowing, and spraying only when thresholds are reached.

By combining cultural, genetic, and chemical tools, farmers can manage both aphid pressure and virus risk while slowing the spread of resistance.

Knockdown resistance in the grain aphid is a double-edged sword, Maximilian concludes.

“On the one hand, knockdown resistance reduces the reproductive success of resistant clones. On the other, it leaves their ability to spread BYDV untouched. For cereal growers, the message is clear: resistant aphids are weaker, but no less dangerous. Sustainable management will depend on vigilance, careful monitoring, and the adoption of integrated approaches that look beyond chemistry alone.”

Funding

This project has received funding from the European Union’s Horizon Europe Research and Innovation programme under the Marie Skłodowska-Curie grant agreement no. 101106698 – MONET. This work has also emanated in part from research conducted with the financial support of Taighde Éireann – Research Ireland under grant agreement 22/FFP-A/11049.

Acknowledgements

We would like to acknowledge Emmanuel Jacquot (INRAE, France), Ali Guncan (Ordu University, Turkey) and Tom Wilkinson (UCD, Ireland) for their collaboration on this project.

Contributors

Louise McNamara, Research Officer, Teagasc Oak Park.

louise.mcnamara[at]teagasc.ie

 Maximilian Schughart, Postdoctoral Fellow, Teagasc Oak Park.

Stephen Byrne, Research Officer, Teagasc Oak Park.

Munir Mostafiz, Postdoctoral Fellow, Teagasc Oak Park.