With severe cases of aphid-borne barley yellow dwarf virus (BYDV) potentially halving wheat and barley yields, finding the best way to spot the pests and produce a reliable and cost-effective guide to controlling them are the aims of the latest three-year project*.

Frustratingly, the first two years saw unusually low aphid/ BYDV pressure, says ADAS entomologist Dr Sacha White, who leads the work.

While this was good for growers, it was less so for the project, which relies on getting reasonable data from monitoring sites.

But this season has seen the highest aphid pressure since the loss of neonicotinoid seed treatments and this is providing us with good data for the work, he says.

What this means for virus symptoms in spring remains to be seen.

To test whether in-field monitoring could be a better guide to the need to spray aphicide than suction trap information, the project has been trialling yellow sticky traps and yellow water traps.

Field assessments last autumn, checking 100 plants on four occasions in 46 crops at sites each within 40km of Rothamsted Researchs suction traps in the North East, South West, West Midlands and the East of England, found an average of one aphid per 95 plants.

That seems quite low pressure, says Dr White.

But Im not sure its the whole story.

At each site two water traps and two sticky traps, placed on two occasions, painted a very different picture.

Aphids We found an average of three aphids per trap, although obviously some traps had a lot more aphids than others.

This indicates that putting out traps is an effective and less time consuming way of monitoring the BYDV pressure than plant inspections.

When aphids migrate into crops in autumn, only a very small proportion of the crop will initially become infested.

So, its very easy to miss plants that have aphids on them and underestimate the pressure on your field.

Water traps caught two to three times as many aphids as sticky traps, he adds.

So, it looks as though water traps are a better way of monitoring BYDV vectors.

Although many more aphids were caught last autumn than in 2019 and 2020, suction trap catches tested by Martin Williamson at Rothamsted showed the percentage of aphids carrying BYDV last autumn was lower than in 2020.

Suction trap results from five sites suggest nearby permanent grass acts as a reservoir for the pests, adds Dr White.

In developing a new decision support system to guide the need for spray applications, the project is building on earlier models developed in the 1980s and 1990s.

AHDB has already made a T-Sum DSS available, which incorporates a day degree model.

However, the T-Sum tool doesnt intrinsically take into account important factors, such as whether aphids are present or the percentage carrying BYDV, which means its likely to overestimate the number of sprays needed, says Dr White.

The new system, the ‘ADAS Crop BYDV Assessment Tool (ACroBAT), takes into account more factors to better predict BYDV pressure.

This includes aphid numbers caught in the local suction trap, the percentage of them carrying the virus, weather, grain price, and spray costs.

It uses this information to guide spray decisions and help growers understand the impact of changing sowing date and using tolerant varieties, he explains.

It carries out a cost-benefit analysis to calculate whether the predicted yield loss from BYDV outweighs the cost of applying an insecticide to control the aphids.

Impact This season the ACroBAT model recommended about half as many sprays as the T-Sum DSS.

Well be recording the impact on BYDV incidence and yield later this year.

One surprise during the work was how late parasitic wasps attacked aphids last autumn, says Dr White.

We saw this happening in November and it highlights the potential importance of these natural enemies in managing BYDV.

Unfortunately, our understanding of this is currently a major knowledge gap.

Further work is needed to assess their role in BYDV management and ways in which we can encourage their activity.

Covid-19 has also hit the development work, he adds.

Tracking down details of previous decision support system involved securing copies of these from university libraries, which closed in 2020 due to the pandemic just as we needed to access them.

Fortunately, we were able to secure the details we needed from other sources, so it didnt delay any work.

• *AHDB project 21120077a Management of aphid and BYDV risk in winter cereals
• September 19, 2019 December 31, 2022
• Funding: AHDB £200,100; Industry partners £20,000 cash and £104,500 in-kind
• Project partners: ADAS and Rothamsted Research
• Industry partners: Syngenta UK, KWS UK, Limagrain UK and BASF Digital Farming

Dr Max Newbert, Syngentas technical manager for insecticides and sustainability, welcomes the project.

We need more adaptable support tools that dont rely too heavily on assumptions, such as the current T-sum, which is triggered regardless of the aphid pressure, he says.

The T-sum tool is important in reducing foliar insecticide sprays which are the only control measures available since clothianidin left the market; but such sprays require a high level of stewardship, so any improvements via a new DSS would be invaluable.

Currently the industry relies on pyrethroids to control autumn aphids, he says.

For now, that strategy is still fully practical because only grain aphids have KDR heterozygous point mutation resistance.

Uniform

This resistance has been hovering around 20% of the population in recent years so, unlike peach-potato aphid, it hasnt become uniform in the population because theres a likely high fitness cost.

This resistance has been demonstrated as extremely unlikely to become homozygous, so reducing overexposure to pyrethroids will help ensure resistance doesnt build up in any cereal aphid populations An improved DSS accounting for seasonal pressure will allow better resistance management strategies.

Useful

The other development in this area has been virus resistant varieties, which are especially useful for early drilled high BYDV scenarios, says Dr Newbert.

But because the resistances arent extreme, foliar removal of large aphid populations should still be considered if appropriate in high pressure years.

This will protect the resistance in the long term and yields, which can still be affected in resistant crops.