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Crop Advice & Expertise

Get your T-sum calculator out to tackle BYDV in cereal crops

With the impending ban on neonicotinoid seed treatments approaching in 2019, knowing the best time to apply pest control treatments is going to be key to controlling the aphids that spread barley yellow dwarf virus (BYDV). There is a calculation that can help growers to pinpoint this.

Claire Matthewman, Campaign Manager at Bayer, says: “The T-sum calculation, based on air temperature and establishment timescales, can help growers to work out the optimum time to spray. Aphids only breed at certain temperatures and the T-sum calculation is a reliable predictor of when that will be.”

Optimising BYDV spray timing

Mrs Matthewman says that if it’s necessary to use a pyrethroid such as decis (deltamethrin), growers should use the full recommended dose and optimise the timing of the spray, instead of increasing the number of sprays. This will reduce the chance of aphids developing resistance to the insecticides – and lessen the harm to beneficials in the crop.

To calculate the T-sum, daily average air temperatures should be noted from:

  • Six weeks after sowing for seed-treated crops
  • A week after application for pyrethroid-treated crops
  • The date of emergence for other crops

The T-sum is calculated by subtracting 3oC from the average temperature each day and adding the result to the running total. When the T-sum reaches 170, it’s an indicator that the second wingless aphid generation could be starting to emerge, so growers should be walking their crop daily and only spray if aphids are present. When the T-sum reaches 340, significant aphid activity is likely and BYDV spread will accelerate, so a pyrethroid treatment should be applied as soon as possible.1

We have produced a wallchart to help you with your T-sum calculations. You can download it here.

Minimising aphid resistance

“Using the T-sum calculation will help both minimise the use of foliar insecticides, and maximise their effectiveness when they need to be used,” says Mrs Matthewman. “T-Sum is very widely used, not just in Europe but also in other parts of the world where cereal crops are grown.

“Making sure any insecticide treatment is slotted in at the optimum time is the best way to avoid getting into a situation where pyrethroids are overused. We will all need to work together to stop aphids building up further resistance to current effective treatments.”

Cereal crops sown this autumn will be the last that can use neonicotinoid seed treatments like Redigo Deter (prothioconazole + clothianidin) to protect establishing plants, which are still available to growers until the end of 2018.

Keep checking our website for the most up-to-date advice on protecting your cereal crops after the neonicotinoid seed treatment ban.


1 AHDB Wheat Disease Management Guide, Winter 2014/15 – Updated February 2016