Very high numbers of bird cherry-oat aphids have been caught by Rothamsted suction traps the past two weeks, putting untreated cereal crops at risk from barley yellow dwarf virus infections, ADAS Crop Action reports.

Insecticide seed treatments should give six to eight weeks protection, but untreated crops are at risk now, it says, and pyrethroid sprays will need careful timing. Optimum control is achieved when the immigrant aphids are about to produce a second generation.

This means insecticides should be applied when the T-sum for the crop has reached 170. The T-sum is calculated by taking the daily mean temperature, subtracting three from it and adding the result to the running total. Start calculating the T-sum on the day of crop emergence, as this is the earliest aphids can colonise plants, the report says.

In the West Midlands, higher than average temperatures at the end of September means that untreated cereals emerging before 22 September will already have reached T-sum 170 and should be treated as soon as possible.

Crops emerging between 23 and 30 September will reach T-sum 170 in the next 10 days.

T-sums should continue to be calculated as if the autumn remains mild there could be further aphid migration, requiring a second treatment at T-sum 340.

For later emerging crops, there is more flexibility in spray timing as falling daily temperatures slow aphid development.