Continuing mild weather is complicating the decision whether to top up the aphid activity from Bayer CropScience’s new insecticidal seed treatment Deter.

With no prolonged cold spell to kill off aphids so far this autumn, growers have a tricky decision on whether to over-spray crops with a pyrethroid insecticide as the seed treatment runs out of steam.

Crops drilled late September onwards should not need treating with a pyrethroid insecticide, unless there is extreme barley yellow dwarf virus pressure, says Bayer seed treatment manager Adrian Cottey.

Harder call

Mid-September-drilled crops are a harder call, but in lower risk BYDV areas there should not be any need to over-spray – as long as seed rates are reasonable, eg above 125kg/ha, he says.

Evidence suggests the efficacy of insecticidal seed treatments drops at lower seed rates because the concentration of the insecticide is reduced.

But early September-drilled crops will need over-spraying, if they have not already been treated. “Especially those drilled at 100-125kg/ha or lower. However, the timing does remain flexible – growers probably have this month to treat. Once you get into December the evidence suggests control gets a bit more dodgy.”

Cold weather

A week of really cold weather would end the debate. “If we had frosts down to -6C then the risk for everyone would be gone.”

But without that cold weather Bob Mills, Frontier’s technical manager, believes it might be a good idea for growers, particularly those going through with a herbicide, to top up all September-drilled crops. “A lot of seed rates were quite low this year, many were nearer 100kg/ha.”

Combined with the exceptionally mild weather, high numbers of aphids this autumn and last year’s bad experience of BYDV, the sensible decision is to treat, he says. “That’s particularly so if you are going through with a herbicide. If not, then you have to consider previous history and other factors, but most growers ought to be seriously considering treating.”

Use of insecticidal seed treatments this autumn has increased, he notes. “A lot was used – maybe 60% of my September-drilled crops were treated – partly because of their slug activity.”