Bob Mills, Technical Manager from Frontier Agriculture considers what farmers can learn from last season to protect next year's crop against BYDV infection.
Last year saw the highest level of Barley Yellow Dwarf Virus (BYDV) infection in cereals for many years. The lack of stale seed beds and high levels of volunteer cereals in drilled crops, together with very mild weather conditions ensured very high aphid pressure. Even crops which were sprayed with aphicide last autumn were often 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 among growers who drilled a variety that had a pre-applied seed treatment followed up with an insecticide spray.
BYDV is carried and transmitted into emerging winter cereals by aphids. The major 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 but is uncommon but likely to 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 aphid can be an effective vector of BYDV if present in crops.
As with most technical aspects of farming, much will be dictated by the conditions of any particular season. However, growers' prospects for successful BYDV control in the year ahead can be improved by employing a range of tactics:
- Use of seed treatments: products containing clothianidin or imidacloprid (both neonicotinoids) will suppress viruliferous aphid populations for up to six to eight weeks after crop emergence, subject to seed rate and growing conditions. In most years this will be sufficient to cover the susceptible period i.e. until the aphid migrations cease, normally by early November. However, if conditions are similar to autumn 2011 where migrations persisted into December, then an additional foliar spray might be necessary.
- Effective grass weed and cereal volunteer control: particularly in stubbles prior to seed bed preparation (to reduce the 'green bridge' risk).
- Time of sowing: crops drilled during September are most at risk from BYDV. Late sown crops (i.e. post mid-October), in most years will not need a seed treatment. However, it is worth noting that seed treatments active against BYDV may still be part of a robust crop protection programme as their spectrum of activity will include other targets e.g. slugs and seed-borne diseases).
- Monitoring of aphids migrating into cereal crops prior to any foliar treatment in the early autumn: keep an eye on the results from the Rothamsted Insect Survey suction traps which will give an indication of the general level of aphid activity (www.rothamsted.ac.uk/insect-survey), as well as 'Aphid News' at www.hgca.com/pests. Monitor emerging crops carefully for aphid immigration, and make sure you know the difference between bird cherry-oat aphid and grain aphid. Information on aphid identification can be found on the Rothamsted Insect Survey website (see above). 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.
- Effective timing of foliar insecticide applications: where insecticidal seed treatments have not been used, spray applications may be required once aphids have migrated into crops but before they have had a chance to spread 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 black-grass 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.
- Effective application: the resistance factor identified so far in grain aphid is not high (c30 fold) so it is very important that growers use the full recommended rates, not lower doses. It is also important to ensure good crop coverage because pyrethroids only have contact activity against aphid pests. These actions will maximise the prospects for successful control, and minimise the selection of resistant individuals.
- Continued aphid monitoring post-treatment in the late autumn: this is particularly 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.
Aways take professional agronomy advice before making technical decisions on your crop. Frontier has a team of over 100 agronomists, covering the arable area of the UK.