Pyrethroid resistance in aphid a concern

Growers and agronomists are being urged to provide samples of the grain aphid Sitobion avenae, so experts can determine the extent of a genetic mutation that causes knock-down resistance (kdr) to pyrethroid sprays.

"This knock-down resistance in grain aphids is a significant new discovery and there are two things that we need to find out," says Steve Foster of Rothamsted Research.

"First, how common and widespread these kdr aphids are and, second, how resistant they really are to pyrethroid sprays, " he says.

The widespread infection of cereals with barley yellow dwarf virus (BYDV) this year has caused concern that insecticide resistance could have contributed to the problem. However, Dr Foster points out that it is much too early to confirm.

"It is far more likely that the outbreak has been caused by the extrordinary weather throughtout the winter," says Dr Foster. "Although I suspect insecticide resistance could also be part of it.

"It would be wrong of me to say whether or not we should be using pyrethroids. We need to carry out the scientific research first, so receiving samples will help us gather a picture of what is happening in the grain aphid populations," explains Dr Foster.

Where growers and advisors suspect there maybe a pyrethroid resistance problem, they are urged to contact Dr Foster at Rothamsted Research before sending in any aphid samples.

Seed treatments

Work carried out at Rothamsted Research - Brooms Barn this year, in conjunction with Bayer CropScience has shown the advantages of using an insecticide seed dressing where BYDV risk is high.

"We've been looking at the efficacy of insecticide seed treatments in winter barley and also the effect of different levels of aphid inoculation densities, says Mark Stevens, head of site at Rothamsted Research - Brooms Barn.

Inoculated with infected aphids on 17 October, the trial showed a huge variation between treated and untreated plots. "The seed treatments have been very effective at keeping BYDV out of the plots, even at high inoculation densities," says Dr Stevens.

Despite the effectiveness of the treatments, which included Deter (clothianidin) and some new formulations in development, Nigel Adam, Bayer development manager for seed treatments, warns growers that they must monitor aphid movement throughout the season.

These products are designed to be finite, so follow-up sprays will be required where aphids continue to fly," he says.