A triple pest threat could threaten oilseed rape crops, according to experts from SAC and Syngenta.

The threat from cabbage stem flea beetle was likely to be high this season, SAC entomologist Andy Evans said. The high numbers of beetles identified in oilseed rape crops at harvest indicated a high risk for establishing crops this autumn, particularly for fields close to last season’s crop.

“Increased oilseed rape in the rotation inevitably leads to higher flea beetle damage,” he warned.

Control of adults disrupted egg laying in the bare soil around rape seedlings, and reduced the amount of larvae that burrowed into stems.

Later treatment thresholds for the larvae were three to five per plant, he said.

Aphids carrying beet western yellows virus were also a threat, according to Syngenta’s Iain Hamilton. “The increased area sown with low-cost broadcast systems will have no insecticidal seed treatment, making them vulnerable.”

A third of all oilseed rape crops tested in 2005 were infected with BWYV. Yield losses can be up to 15% even if plants show few visual symptoms.

“If aphid pressure is low, a Hallmark Zeon application of 50ml/ha for cabbage stem flea beetle will control aphids,” Mr Hamilton said. “But if pressure is high the rate needs to be increased to 75ml/ha for greater persistency.”

Control of aphids might need needed until the end of November, he said. “Many crops were drilled early this year so the effect of insecticidal seed treatments may have worn off while aphids are still prevalent, so be prepared to add an insecticide to your phoma or light leaf spot treatment.

Late aphid sprays can also give useful control of rape winter stem weevil, Dr Evans said. “It has been a recognised problem across southern and eastern England for years, but for the past two seasons we have seen serious damage further north into Scotland.

There was a three to four week window for controlling adult weevils in early autumn before they start egg laying, he said. “This year SAC will be monitoring weevil populations to advise on optimum spray timings.”