Mild weather could heighten aphid risk

The risk of aphids spreading diseases in cereal crops is rising with the prolonged mild weather and big increase in autumn-sown wheat and barley.

Growers are being warned to prevent a repeat of the situation two years ago when mild conditions led to a big increase in the aphid-borne disease barley yellow dwarf virus (BYDV).

In the South West, where the disease can be more prevalent, Devon agronomist Neil Potts says if temperatures stay unseasonably high, growers could be caught out again.

“Two years ago we had a similar sort of autumn and early winter and even Deter-treated crops that year suffered from BYDV in the spring,” he says.

“I would say the weather is a lot milder than in 2011, which starts alarm bells ringing,” Mr Potts adds.

Shortages of the clothianidin-based seed treatment Redigo Deter may also have increased the pressure of BYDV infections, leaving crops more vulnerable to attack.

Growers unable to use the seed treatment – which contains an insecticide – may have to rely on a one- or two-spray autumn programme.

Mr Potts suggests those early-drilled crops and ones that didn’t receive a seed treatment will be in a high-risk category and probably need an aphicide as soon as possible.

“Even those drilled relatively early with Deter will be at risk, as the product will probably be running out of steam,” he says.

Richard Harrington, head of Rothamsted Research’s insect survey, says the number of aphids caught in traps is below normal level for the time of year, after a cold, wet spring and dry summer.

“In reality, we have not seen many aphids flying, but if the weather stays really mild aphids will do very well,” he says.

Dr Harrington advises growers to watch the HGCA newsletter and monitor aphid activity in their crops.

Although aphids are often a problem in the milder western areas of Britain, the latest update from the HGCA says aphids can readily be found in southern and eastern England on untreated crops.

Dr Harrington warns one of the main vectors of the disease, the grain aphid, has shown widespread resistance to pyrethroids, but if full rates of these insecticides are used, they should give good control.

ADAS entomologist Steve Ellis says the optimum timing for aphid sprays is late October and early November, but urges growers to use the weather to dictate their plan of action.

“The aphid flight thresholds are about 11-12C. However, development and reproduction is still possible above 4C and walking between plants above 1C,” he says.

“If you find any aphids it will be best to go on as soon as possible, because there will need to be several frosts on the trot to damage the aphids and reduce the threat,” he explains.


Twitter: @lukecasswellfw

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