Taking the guess work out of Barley Yellow Dwarf Virus (BYDV) control

The disease is spread by an aphid vector throughout the main planting seasons.

A recent spring crop walk hosted by CAFRE on the South Down farm of Shaw Hanna featured a discussion on the continuing challenge posed by winter barley crops: specifically in terms of the final yields achieved by growers.

Teagasc tillage specialist, Shay Phelan, took part in the discussion.

He explained: “We have an unfolding scenario within which many growers apply inputs to winter barley crops, which should see final yields coming in at around 4t/ac. However, in most instances this potential is not realised.”

The Teagasc representative went on to confirm the yield robbing power of BYDV infestations in this context, adding: “BYDV can reduce yields by to 1.5t/ac.”

However, there is now growing hope that new science, both from the perspective of plant breeding and better knowledge of aphid behaviour, can provide arable farmers with much more tangible control measures.

BYDV tolerance

Significantly, claims made with regard to the development of BYDV tolerance by a number of plant breeding companies actually hold up under trial conditions.

This follows the completion of independent research carried out by Teagasc scientists. This development should mark a significant step change in terms of arable farmers’ commitment to the planting of cereal crops, particularly the early autumn period.

What’s more, it holds out the prospect of genuine BYDV resistance becoming a reality in the not too distant future. However, it is now accepted that extremely complicated processes are at play when it comes to the spread of BYDV, all of which associated with the relationship that develops between aphids, the virus they carry and the host cereal plant. And each link in the chain has a number of factors that can work through to the final impact – if any - that BYDV can manifest within a crop. For example, there are different forms of the BYDV virus, with differing capabilities of interfering with crop growth patterns.

In addition, the timing of the initial infection and the stage of plant growth can have a key impact on subsequent crop performance.

Sowing dates

This is why sowing times are so critical in terms of the disease pressure that cereal crops are exposed to. But in terms of practical advice to farmers sowing spring cereals in 20206, the route one BYDV prevention measure comes back to sowing date.

Shay Phelan further explained: “March sown spring barley crops will not need an insecticide to control BYDV: it’s too early in the season. On the other hand April sown crops, will, invariably, require an insecticide to control the issue. Where autumn sown crops are concerned, the late-early scenario also kicks-in: but this time in reverse. Later sown crops are less exposed to the threat posed by aphids. And, of course, all of this comes down to grower-choice. Some farmers may not wish to use an insecticide under any circumstances.”

Teagasc research has also confirmed a number of general trends, where BYDV is concerned. First off, fields with a history of BYDV should be identified. All cropping areas should be inspected for aphids prior to planting. Before applying an insecticide, it is important to walk crops and search sheltered spots, particularly near headlands. The best control of BYDV is achieved from a single aphicide application at growth stage 14. It has been shown that two applications at leaf two and leaf four or at leaf four and first node are not significantly different, in terms of their overall impact, relative to a single application of insecticide at the leaf four stage.

Aphid monitoring

Teagasc’s current aphid monitoring network comprises three suction towers, located in Carlow, Cork and Dublin. Each is 12.2m in height. In essence, flying insects are caught at the top and then dragged down to collection units at the bottom. The information gathered courtesy of each tower is representative of a zone that stretches out a distance of 80km in all directions from that point. ​

The suction tower data is confirming that aphid migrations commence around the middle of March and will continue through until November. Where aphid flight is concerned, it kicks-off when temperatures are in the range 11° to 15°C. But they can also fly at sub optimal temperatures.

Teagasc research is also confirming that that weather factors, in addition to temperature, impact on grain migrations. When rain levels are low and temperature is higher the largest numbers of grain aphids are recorded. Conversely, where rain levels are high and temperatures are low, the numbers of aphids caught in suction towers falls-Back.

AHDB

Meanwhile, the Agricultural and Horticultural Development Board is to launch a new BYDV management tool. The web based facility will be rolled out in time for the 2026 autumn planting season. According to AHDB agronomists, when neonicotinoid seed treatments were withdrawn in 2018, it set back aphid management by decades and saw a return to the routine use of foliar pyrethroid sprays in the autumn. Although pyrethroids are relatively cheap, every spray puts pressure on non-target organisms and increases insecticide-resistance risks. This means they need to be used carefully.

Current BYDV tool

The current BYDV tool has been available for several years. It predicts the presence of the second aphid generation in crops, which is most associated with BYDV spread. It only needs two inputs: a start date (usually crop emergence or spray date) and accumulated daily air temperatures (from the start date). Although it's a simple and effective way to time sprays, it overlooks many nuances that influence virus-transmission risks. To compensate for this, it is conservative and tends to suggest more sprays than may be required to prevent unacceptable yield losses.

ADAS research

The scope of the updated version scope reflects AHDB-commissioned research undertaken by ADAS. This work has led to the development of a new model that accounts for additional measures of risk and provides comprehensive information on BYDV. For example, although bird cherry-oat aphid, rose-grain aphid and grain aphid all transmit BYDV, the ADAS work confirmed that the former aphid is by far the most significant virus vector across the UK. Rose-grain aphid is rarely seen in winter crops (it overwinters on roses), and grain aphid flies in relatively low numbers in the autumn. This has been confirmed by recent in-field monitoring work and results from the Rothamsted Insect Survey (RIS) suction-trap network (across England and Wales).

Although grain aphids can spread BYDV, this only happens in limited situations. It is also known that grain aphid transmits BYDV less efficiently than bird cherry-oat aphid.

Suction trap data is representative of aphids flying over a radius of about 80 km, according to RIS. ADAS found that the traps provide a good indication of aphid migration trends, crop infestations near traps (especially up to 10 km away) and the proportion of aphids carrying virus up to 40 km away.

The new model

In addition to the number of aphids carrying BYDV, the new model considers many other data sources. These include minimum, maximum and mean temperatures as well as crop-specific information such as: cereal type, sowing date, plant populations, treatment costs, predicted yield, estimated grain price and location risk factors. Initial validation work found the model accurately predicted BYDV risk, based on symptom development in untreated crops. It also guided control as good as or better than the current BYDV tool in treated tramline and plot trials. Moreover, this was achieved with fewer insecticide applications and provided yield benefits: specifically where BYDV was present.

The tool only needs a few farm-specific inputs, as several data sources are embedded. For example, it automatically uses five-year growing-season (1 September to 31 August) data sources. This includes local temperature patterns. The model also uses forecast temperature data for the next fortnight. Based on virus screens over several autumns, the model assumes about 20% of aphids carry virus. This might seem low, but the virus isn’t passed from parent to nymph. New aphids need to feed on infected plants to take it up.

Farm-specific inputs include: location information, which tells the tool where the nearest suction trap is and identifies other key area characteristics, such as proximity to the sea and other arable land; Crop type (wheat or barley); plant population; sowing dates; predicted grain yields and grain price.

Tool outputs

The tool crunches the data to show the daily risk level based on the estimated percentage of the crop infected with BYDV. It also: indicates optimum spray timings; accounts for diminishing returns from follow-up sprays; considers pyrethroid insecticide persistence; runs cost-benefit analyses for every potential spray and factors in any loss of ‘no-insecticide’ payments. The tool also supports cultural control decisions ahead of drilling (such as field selection and drill date).

In high-risk situations, drilling crops after mid-September is particularly effective, as is opting for a variety with BYDV resistance or tolerance. Due to the complexity and diversity of resistance/tolerance, the new tool does not yet account for it when determining treatments. ​

So what next?

The optimal BYDV control solution is the development of spring and winter cereal varieties that have genuine resistance to the disease. To date, a number of winter barley varieties have been developed that demonstrate a degree of tolerance to the disease. This still leaves the issue of developing true – and long term resistance across the entire scope of cereal types grown in the UK and Ireland.