Pollen beetle resistance to pyrethroids is worse than this time last year, early findings from Bayer CropScience suggest.

Three of 40 samples analysed so far (all from Suffolk), found that just 10% control was achieved from the field rate of lambda cyhalothrin, compared with close to full control in some cases this time last season, the firm’s Eileen Bardsley says. “There was virtually no activity at all at any dose rate. Even at five-times the field rate, we’ve only seen 70-80% control, which suggests there are some very resistant individuals out there.” By comparison, in Cambridgeshire last year, 80% pollen beetle control at less than 20% of the field rate was achieved, she says.

Control

While the tests were based on lambda cyhalothrin, she warns that if any resistance is detected, it affects all pyrethroids – the neo-nicotinoid Biscaya (thiacloprid) is the only non-pyrethroid form of control approved in the UK.

Mrs Bardsley says there were clear signs the resistance problem was getting worse last year, when the company carried out 25 resistance tests. “We tended to find that as the season went on, there were more samples with resistance. The areas affected mirrored those identified by the HGCA, with East Anglia, north Cambridgeshire and Kent worst affected, although we did find resistance around the Wash as well.”

Resistance

The problem for growers is knowing whether beetles are resistant or not, says Frontier’s Bob Mills. “If they are, and a pyrethroid’s used, it’s likely a Biscaya treatment’s going to be needed to control survivors. If you’re close to one of last year’s outbreaks it would be wise to go with Biscaya instead because you know it’s going to work.”

Pollen beetles start to fly when temperatures reach 15C and crops are vulnerable through the green to yellow bud stages and official advice is to only treat where thresholds are exceeded. Mrs Bardsley says recent cold weather has reduced many populations below this, but there are still localised cases where numbers have exceeded thresholds.

CSL pollen beetle monitoring found that of 15 sites tested by Monday (14 April), none exceeded the 15 beetles a plant spray threshold, with an average of just 1.49 beetles a plant across all sites and an average maximum population of 10.2 beetles a plant.