Putting aphids on the map

The aphid control exerted by seed treatments helped reduce the spread of yield-limiting barley yellow dwarf virus (BYDV). Critically, the guard was kept up during the vulnerable early stages of the crop, until the chemistry ran out of steam (usually about four to six weeks after drilling). At this point, weather then became friend or foe, with low temperatures needed to dampen down aphid activity. 

With the loss of neonicotinoids, the virus and its aphid vectors need to be understood more than ever. The aphid species responsible are known but the amount of virus they carry is unclear. However, only a small proportion of aphids need to carry BYDV to result in crop losses. It’s why aphids are treated as if they are infected. But this is not a licence to spray. A great deal is known about aphid biology and this knowledge can be used to reduce the number of sprays required.

BYDV transmission

Winged aphids are the most common initial source of BYDV infection. It’s one of the reasons AHDB supports the Rothamsted Research/SASA suction-trap network. The 12.2 m tall traps suck in air continuously. Emptied frequently, the aphid species are identified and counted, and the results are promoted via AHDB Aphid News. ahdb.org.uk/aphid-news

Another source of BYDV is from wingless aphids that move from grass, grass weeds (especially annual meadow-grass) or volunteer cereals from within the field. This is known as the ‘green bridge’ effect. Aphid colonisation is generally lower on fields with minimum tillage. Where fields are cultivated, aphids can survive under the ground, feed on crop roots and transmit virus.

BYDV does not pass to aphids’ offspring. However, they can pick it up from feeding on infected host plants and transmit it again (in as little as 12 hours). In terms of control, it is the second generation that should be the target. This is the generation that tends to move away from the plant originally colonised. The speed at which this generation develops is driven by air temperatures. As part of the AHDB WeatherHub, a BYDV management tool was launched last autumn to help people calculate when the second generation is likely to appear (following crop emergence or insecticide treatment).

The longer the autumn resists the winter, the higher the virus risk to crops. If temperatures stay above 3°C, aphids remain active. However, virus inoculation efficiency decreases to 23–35% when average temperatures drop to 6°C. 

BYDV symptoms

Leaf yellowing and stunting symptoms appear in the spring. Initially, confined to individual plants scattered throughout the crop, eventually, distinct circular patches develop. Sometimes, these patches merge to form extensive areas of infection. Red tipping of upper leaves can also occur. In the case of severe infections, BYDV can cause losses of up to 60% in winter wheat and 50% in winter barley. However, the occurrence of these levels of infection is rare.

Aphid vectors

In the South of England, the bird cherry–oat aphid (Rhopalosiphum padi) is the principal vector. In the Midlands and the North of England, the grain aphid (Sitobion avenae) is usually more important. Each species has its own behavioural quirks and AHDB research is helping to reveal these.

In the autumn, bird cherry–oat aphids usually fly in much greater numbers than grain aphids. This is because the former flies to bird cherry trees to overwinter. The latter aphid, however, overwinters in cereals and grasses. Because of this, grain aphids often get a head start in the spring. It’s also essential to note that moderate levels of pyrethroid resistance are widespread in the UK’s grain aphids.

As the amount of aphids varies on a field-by-field and within-field basis, AHDB has funded the Game and Wildlife Conservation Trust to map movement. They use yellow sticky traps to catch aphids in the field. They also look at how the local landscape and prevailing wind direction influence results.

Results show aphid numbers are much higher towards field edges (up to 35 m in), with grain aphids more likely to hug the boundaries (up to 10 m in). Trees, understandably, provide a barrier to flying aphids. The research team also works with farmers to improve trap designs and placement and looks at the relationships between catches and observed BYDV levels. The influence of natural enemies, such as predatory beetles and web-spinning spiders, is also being studied.

This article appeared in the Summer 2019 edition of Grain Outlook