Archive Article: 1997/10/18
THIS season could host the biggest epidemic of barley yellow dwarf virus (BYDV) since the late 1980s. Learn by the mistakes made then, say leading entomologists.
Drilling started early and proceeded apace this year, helped by the same fine weather which has favoured the bird cherry aphid. The result is a fit and active aphid population reproducing merrily in time to infest young cereal plants.
During the last years of severe infection, most of the crops at risk were sprayed with insecticide but hindsight showed the applications came about a fortnight too late. Growers waited until the end of October/early November when they could put their insecticide into a tank mix with herbicides.
The result was that infection was already being transmitted into the young plants and infection was later spread by other aphids moving within the crop.
"The aphids we are picking up now in the live traps are looking very fit," says Steve Tones, of ADAS Starcross, Devon. "They have been steaming this year."
Mr Tones is working on the HGCA-funded project to produce a computer-based decision support system for advising farmers when to spray. This is not, however, far enough advanced to give definitive advice this year, and he bases his predictions for this autumn on field observations and past experience.
"My judgement is that this is a high risk year," he says, warning that by waiting until the end of the month many growers could be caught out by aphids.
Dr Richard Harrington, of IACR-Rothamsted, backs this concern about a high risk year. "There are certainly a lot of aphids out there in early sown crops around here, many more than at the same time over the last couple of years," he says.
"There seems to have been enough of the various grasses left in good condition to provide aphids with good food between the harvest of the last cereal crop and the emergence of the present one.
"It looks as though it will be necessary to spray early-sown crops this year."
His colleague, Dr Adam Burgess, who is monitoring the suction traps, says a mixture of cereal-colonising aphids with those that will only settle on bird cherry trees is being found. However, he is also finding many more aphids in crops in early October than in previous years.
Below 11-12íC and in heavy rain, aphids will not fly. However, Dr Burgess points out that September in southern England has been the sunniest and driest for some years, with only light winds.
Garth Foster, of the Scottish Agricultural College at Auchencruive, Ayrshire, says the scientists are not "crying wolf" this year. Up to 2,000 bird cherry aphids are turning up each week in his suction traps in mixed arable and livestock areas with plenty of grass.
"Mid-October is the cut-off point for spraying in Scotland," adds Dr Foster. Currently far fewer in number, but no less important in the spread of BYDV, are grain aphids which made a major contribution to the late 1980s epidemics by their steady development over winter to spread the virus.
This will be one of the key factors to be input into the decision support system being developed for the HGCA and MAFF. Although the system is well advanced there are still gaps to be filled in knowledge of aphid behavioural patterns which could affect when and where to spray.
Mr Tones hopes the DSS system will allow growers to pinpoint for spraying only those fields which have the highest risk of being infected by BYDV. Historic data suggests some valleys, coastal and estuarine sites have the most disease but there are also possible links to the altitude of fields.
Information is still being gathered on what happens once the aphids are in crops and the young nymphs start to move around. A severe infection year may be a worry for farmers but it could help scientists fill in the gaps in their aphid knowledge so that spray forecasting is an accurate tool within a very few years.
The bird cherry aphid is having a good year and so will BYDV. Spray timing will be crucial, reports David Millar.