BYDV: Lessons from 2026 on how to reduce risk

Last season proved to be a perfect storm where earlier drilling, higher aphid numbers, milder conditions and increased virus levels all combined in one of the worst seasons for barley yellow dwarf virus (BYDV).

A much higher incidence of the virus was seen in winter barley and wheat crops across all parts of the UK, not just in the usual hotspots.

Fortunately, there are some lessons that, combined with a new forecasting tool launched by the AHDB, could help farmers manage the risk in the coming season.

See also: How nature-friendly practices built farm’s climate resilience

Hutchinsons’ technical manager and agronomist, Will Foss, has been investigating the reasons ahead of the new cropping season.

Reasons

Aphid numbers and activity

Reviewing last season, there were the ideal weather conditions for the vectors – grain aphids and the bird cherry oat aphid.

Will points to suction trap data showing a lot of bird cherry oat aphids last autumn. Grain aphids tend not to be seen much in the autumn.

AHDB virus testing also showed higher levels of infected aphids.

In autumn 2025, 34% of aphids tested carried the virus, which is the highest in the past seven years.

So not only were there more aphids, but more were carrying the virus.

It’s not just numbers, aphid activity is important with temperatures being a key driver.

Looking at weather data in Suffolk, he says it never really got cold enough to kill aphids over winter and early spring.

“It needs to go down to -4C for bird cherry and -7-8C for the grain aphid.”

In fact, temperatures were high enough for them to fly and spread the virus.

Will explains that they need a minimum of 11C to fly and 6C to transmit the virus to the crop.

“In his area, data for winter/spring showed a 6C minimum, so they were able to transmit virus and there were days when they were flying, too.”

The milder winter also saw several generations of aphid, as there were eight T170C (T-sum) T170 accumulations from September to early April. T-sum is a predictor of aphid development, he explains.

“So lots of vectors plus more virus meant conditions were conducive for the virus to spread in spring.”

More habitat

One underlying issue is the opening up of the landscape, making it more favourable for insects.

This is increasing aphid numbers, resulting in a greater reservoir of virus.

This is a combination of Sustainable Farming Incentive (SFI) habitats (any grass-containing habitat) and crops that have not been sprayed with an insecticide.

Some of this is down to more farmers choosing not to spray, perhaps they have an SFI no insecticide option. However, he warns this puts crops at greater risk.

Early drilling

The final reason was the early harvest and subsequent earlier start to drilling.

“The earlier you drill, the more challenges to BYDV crops have.

“Now we have seen another early harvest and the concern is that we may see earlier drilling again.”

However, Will says one good thing about this summer is the lower quantity of forage. All the grass died off, so there was less habitat to support aphids.

However, while the starting point may be lower this autumn, numbers can build quickly again.

Actions

Don’t rely on crop observations

Will advises growers not to rely on crop observations.

“With aphids, you are working on risk. The suction trap network is a good resource, and the data can show if you are likely to have aphids in crops.

“However, they work on an 80km radius, so there isn’t full country coverage.”

Therefore, farmers may be tempted to look for aphids in crops themselves, but they are getting caught out, warns Will.

“It is not a reliable way to assess risk, as you can’t check every plant.”

Grain aphids tend to move to the top of the plant while bird cherry oat aphids are at the bottom.

Bird cherry oat aphids even hide below soil level, so are easy to miss, he explains.

Remove green bridge

Later drilling is an obvious measure, but an early harvest and fears about drought next year could see earlier drilling. So what else can farmers do?

Will recommends removing any direct green bridge transfer – preventing the direct movement of aphids from grassweeds or cereal volunteers.

“If there is green stuff at drilling, there will be direct transfer of aphids.”

This year, the drought has delayed the flushes of volunteers, so the concern is that will appear around drilling or soon afterwards.

“If there is a late flush and are sprayed off, there is still a risk even if they are yellowing.

Will explains that’s because yellowing plants are more attractive to aphids due to the colour change and the higher concentration of sugars after being sprayed off.

Ideally, farmers need a wider gap between spraying off and drilling, with 10 days better than two.

Use of resistant varieties

The use of resistant varieties of winter barley is a complex area due to the terminology used.

Kestrel is the first winter barley described as resistant, before that they were termed tolerant.

Will says there is a perception that anything described as resistant will not get BYDV. However, this is not the case as seen earlier this year.

Resistant varieties can still get BYDV, and farmers can still see a yield loss when they are under high disease pressure.

“Varieties are a useful tool, but they have to be used along with other measures, you can’t just rely on genetics,” he says.

Resistance gives farmers some leeway, such as on outlying land that they can’t get an insecticide on as quickly.

It gives farmers a bit of time, but they should not assume they don’t need spraying and won’t be affected by BYDV in higher pressure situations.

Chemistry

The key problem is the reliance on just one group of chemistry – pyrethroids. They work by contact, therefore, good coverage is essential.

Furthermore, not being systemic means any new growth is not protected.

One observation from last season was a lot of late infection with upper canopy symptoms that tested positive for the MAV strain.

This strain tends to be carried more by the grain aphid, suggesting an increase in grain aphids.

There is known pyrethroid resistance in grain aphids. Therefore, a key concern is this late infection may be the result of increased resistance.

Case study: James Freeth, Cornwall

Like many farmers across the UK, James Freeth had one of the worst seasons for barley yellow dwarf virus (BYDV) in 2026 on his farm in Cornwall.

Farming right up to the Cornish coast, field selection is a major risk factor with the disease.

James says BYDV has always been a challenge, but there were more chemical options including seed treatments.

However, recent years have seen it become more of a problem.

He explains that it is mainly a problem with winter barley in the more sheltered fields.

“In the more exposed fields near the cliffs, we don’t see it. Last season [2024-25], most barley was grown in the exposed fields and we didn’t see it.”

James says it tends to be more a problem in the valleys and other sheltered areas and this season turns out to be the worst with patches in many fields.

He points to one field – the top is exposed and there was no virus, but where it dipped down into the valley, there were the patches.

He estimates that yields were down by one-third in affected fields.

His agronomist, James Whatty, recalls there were four to five T-sums (170C) by the new year and it didn’t get below 12C in Truro last winter, so few aphids were killed.

This coming season, James Freeth is cutting back his winter barley area from the normal 40-60ha to 10-20ha. He is growing more second wheat instead.

The risk will be lower as wheat is drilled later – he is aiming for the second week of October onwards, with the barley in lower risk fields.

James Freeth also looks to minimise the green bridge, spraying off stubbles before drilling and he tries to keep stubbles clean.

However, being in a mixed farming area with grass fields, controlling aphids is a challenge.

He has used pyrethroids in the past, but the problem is not being able to travel to get more passes on later in the autumn.

He has tried diatomaceous earth, which can kill aphids by scratching their soft bodies and drying them out, but results were limited.

Looking to the future, he is considering trialling a resistant variety when the barley returns to the more sheltered fields.