Brassica and field vegetable growers must take account of insecticidal seed treatments when choosing foliar aphicide applications this season, if they are to minimise the risk of resistance developing, industry experts advise.

With high numbers of Myzus persicae being picked up by aphid monitoring this season - 40 days earlier than normal in Scotland, for example - growers should already look to adopt anti-resistance strategies, according to Steve Foster of Rothamsted Research and the Insecticide Resistance Action Group (IRAG).

He advocates that where neonicotinoid seed treatments have been used, such as imidacloprid, it is good practice to delay the use of foliar insecticides with the same mode of action, such as thiacloprid, for as long as possible. “There are other alternatives, such as pymetrozine (Plenum), which can be used first where Myzus persicae are present and there is a risk of resistance,” he said. “Foliar neonicotinoid insecticides could still be used later in the season, if required.”

Foster highlighted lower sensitivity to neonicotinoids has been identified among Myzus persicae populations, although it does not appear to have become progressively worse over recent seasons. “There is evidence that strains with reduced susceptibility to neonicotinoids could reproduce faster than fully susceptible strains later in the season, when the effects of seed treatment are reduced, allowing the potential evolution of potent resistance capable of causing control failures.

“The principles of good stewardship dictate growers should be rotating modes of action to minimise the risk,” he added.