Last year saw the highest level of barley yellow dwarf virus (BYDV) infection in cereals for many years and even crops receiving an insecticide treatments still showed symptoms.
The lack of stale seed-beds and high levels of volunteer cereals in drilled crops, together with very mild weather, ensured very high aphid pressure.
Even crops that were sprayed with aphicide last autumn were often still left with high levels of infection due to the early infestation of aphids.
The unusually mild weather also ensured that aphid pressure continued right the way through the autumn. The result was that, as pyrethroid chemistry has a very short persistence, a single application was often insufficient to give complete control.
Best results last season were obtained by growers who drilled treated seed and followed up with an insecticide spray.
BYDV is carried and transmitted to emerging winter cereals by aphids. The main vectors in the UK are the grain aphid (Sitobion avenae) and the bird cherry-oat aphid (Rhopalosiphum padi). The corn leaf aphid can also transmit BYDV in small-grain cereals, and while still uncommon, it could increase as the area of maize increases and winters get milder.
Traditionally, bird cherry-oat aphid is usually considered to be the more important vector, but there is much variation and this assumption may not hold true in all parts of the country. Past experience has shown that grain aphids can be an effective vector of BYDV if present in crops.
Growers’ prospects for successful BYDV control in the coming year can be boosted by employing a range of tactics. In most years, the seed treatment will be sufficient to cover the susceptible period – until the aphid migrations cease, normally by early November. However, if conditions are similar to last autumn, when migrations persisted into December, then an additional foliar spray might be necessary.
Crops drilled during September are most at risk from BYDV. Late-sown crops (after mid-October) will not need a seed treatment in most years.
Monitor emerging crops carefully for aphid migration and make sure you know the difference between bird cherry-oat aphid and grain aphid. If the predominant species is grain aphid, then pyrethroids may not provide effective control. Last season agronomists reported difficulties in finding aphids early on, when infection was not present, and later on, when the BYDV symptoms started to appear. It is normal not to find aphids once the symptoms appear.
Monitoring aphids continuously post-treatment, this is critical if the aphid migration period has been extended (and hence more aphids arrive after the first treatment has been made) and/or the late autumn is mild, allowing surviving or late-arriving aphids to multiply in crops and spread the virus further.
Where insecticidal seed treatments have not been used, spray applications may be required once aphids have migrated on to crops, but before they have had a chance to spread the virus from their initial colonisation point (usually mid- to late October). However, many BYDV infections reported in 2012 occurred because application timings were more linked to blackgrass or other weed control. Although this is a cost-effective approach and other agronomic considerations may point to this, it may not necessarily provide the most effective method for BYDV control.
The resistance factor identified so far in grain aphid is not high, so it is important growers use the full recommended rates. It is also important to ensure good crop coverage, because pyrethroids only have contact activity against aphid pests.