Winter crops are mostly beyond spraying for adult pollen beetles, but may still be treated against seed weevil, pod midge and turnip sawfly. Pollen beetle larvae feed until early June. So pyrethroids used on these other pests could select out resistant larvae and exacerbate next year’s problems, warns entomologist Jon Oakley.
Resistant beetles have turned up in samples from Kent and Hampshire. “They’ve also just been found at Terrington in Lincolnshire.” They have also been pinpointed by Bayer CropScience near Southend, Essex.
About 5% of the beetles in the south-eastern populations can withstand five times the field dose of insecticide, and about 25% more could survive if not hit directly by the spray, says Mr Oakley. “At this level of resistance a single treatment would probably still seem reasonably effective in the field. “However, when the surviving beetles move on to spring oilseed rape crops the situation could be very different.”
In areas where most winter oilseed rape was sprayed with pyrethroid most beetles attacking spring sowings could be resistant, he believes. The pest flies a long way in warm weather, so the problem is expected to spread across the whole country over the next few years.
Spring oilseed rape growers in south-east England should consider switching to Biscaya (thiacloprid), advises Mr Oakley. “This will kill pyrethroid-resistant pollen beetles. “Elsewhere, treated crops should be monitored and if large numbers survive a pyrethroid treatment, Biscaya should be used for any follow-up sprays.”
TAG trials have never shown a yield response to controlling pollen beetles in the winter crop, mainly because of compensatory growth. And spring rape is a relatively minor crop, notes Ron Stobart. If pyrethroid resistance did become widespread the cost of control would increase, he acknowledges. Biscaya is more than twice as expensive as the more commonly used Hallmark (lambda-cyhalothrin), notes Mr Oakley.
“But pressure to spray routinely for pod midge and weevils, and even pollen beetle, isn’t as great as it is on the Continent,” says Mr Stobart.