Eighty-three per cent of the winter barley seed area under certification in Ireland in 2026 carries tolerance or resistance to Barley Yellow Dwarf Virus (BYDV). That figure, presented at the recent Crops' Forum event, signals how quickly growers have shifted to genetics as their primary defence. But Teagasc entomologist Dr Louise McNamara urged caution on over-reliance.

"There is great interest in tolerant and resistant varieties for the control of BYDV," she said. "But it is only one tool. If growers rely exclusively on one tool, it can potentially impact on the disease control plan."

Her message: tolerant winter barley varieties are risk reducers, not a cure. They lower symptom levels and protect yield, but they do not eliminate the disease.

Validation trials at Oak Park

Validation trials are ongoing at the Teagasc Crops Research Centre at Oak Park in Co. Carlow. The work matters because many of the tolerant varieties on the market were first tested in regions where the dominant BYDV species differs from the one circulating in Irish crops.

The trials confirmed the presence of three BYDV species. BYDV-MAV was the most frequent. The remaining populations comprised combinations of BYDV-MAV with BYDV-PAS and/or BYDV-PAV.

"We need to know are these tolerant varieties effective against the prevalent species in Ireland," Dr McNamara said.

To answer that, she referenced six trials carried out over four years comparing susceptible and tolerant varieties, with and without insecticide.

In unsprayed plots, the susceptible varieties clearly showed higher levels of BYDV symptoms than the tolerant ones. The tolerant varieties were not symptom-free, but their symptoms were markedly lower.

Insecticide programmes reduced symptom levels in both categories. Susceptible varieties gained the clearest benefit. Tolerant varieties started from a lower baseline, so the additional reduction was smaller.

What happens to yield

Dr McNamara also drew on Teagasc research trial data to compare yield performance across years with low, medium and high BYDV pressure.

At low pressure, differences between the variety types barely registered. At medium pressure, yield losses became apparent. Susceptible varieties showed a marked yield gap between sprayed and unsprayed plots, and every susceptible variety in the trial benefited significantly from insecticide treatment.

The tolerant varieties produced higher yields overall. But the spread between their sprayed and unsprayed plots was narrower, meaning they gained less additional benefit from the spray.

The high-pressure year told the sharpest story. Susceptible varieties showed a significant yield response to insecticide, with a large difference between sprayed and unsprayed plots. In contrast, tolerant winter barley varieties showed no significant yield difference between the two.

What it means on farm

For growers drilling winter barley, the trial results support a layered approach. Tolerance delivers the biggest payoff when aphid pressure and virus spread run high — exactly the seasons when spray windows tighten and pyrethroid resistance concerns bite hardest. In those years, tolerant varieties held yield without an insecticide response.

But in medium-pressure seasons, susceptible varieties still need a fungicide-adjacent aphid control plan, and even tolerant crops carry some infection. Dr McNamara's warning about depending on a single tool cuts both ways: abandoning monitoring and spray decisions because the variety is tolerant leaves the disease control plan exposed.

The economics are straightforward. A tolerant variety removes much of the yield downside in the worst years and reduces the reliance on insecticides across the rotation. It does not remove the need to watch aphid migration forecasts and crop emergence dates.

Watch for the ongoing Oak Park validation results as further tolerant and resistant varieties move through the system, and expect BYDV tolerance to remain the over-arching theme as the 2027 winter barley recommended list takes shape.