High numbers of Cabbage Stem Flea Beetle identified in oilseed rape crops at harvest indicate an 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 CSFB damage, warns SAC entomologist Dr Andy Evans.
Control of the adult CSFB aims to disrupt their egg laying in the bare soil around the base of oilseed rape seedlings, reducing the numbers of larvae attacking plants, whilst later application targets the larvae that have burrowed into stems. "Treatment thresholds in November are three to five larvae per plant, but reducing adult numbers in the autumn could avoid the need for a later spray," adds Dr Evans.
Syngenta Technical Manager, Iain Hamilton, also highlights that growers need to be on the lookout for aphids carrying Beet Western Yellows Virus. "The increased acreage of oilseed rape sown with low-cost broadcast systems will have had no insecticidal seed treatment, making them especially vulnerable to insect attacks," he says.
A third of all oilseed rape crops tested in 2005 were found to be infected with BWYV, with yield losses typically up to 15% - even where plants show little or no visible symptoms of infection.
"If aphid pressure is low, a Hallmark Zeon application rate of 50 ml/ha recommended for Cabbage Stem Flea Beetle will control aphids," advises Mr Hamilton. "But if aphid pressure is high, with large numbers are migrating in from desiccated potatoes or senescing sugar beet, the rate should be increased to 75ml/ha for greater persistence and repellant activity."
Mr Hamilton reminds oilseed rape growers that with many crops drilled early this year, the effect of insecticide seed treatment may have worn off whilst aphids are still prevalent and threatening damage. "Typically sprays may still be required right through to the end of November, so be prepared to add in Hallmark Zeon with a Plover treatment for Phoma and Light Leaf Spot if insects are still active," he advises.
Dr Andy Evans adds growers' experience last year revealed the application timing for late aphid control also gave useful control of the increasing scourge of Rape Winter Stem Weevil, which is causing ever greater levels of damage to oilseed rape crops in Scotland, he warns.
"Rape Winter Stem Weevil has been a recognised problem across southern and eastern England for years, but for the past two seasons we have seen serious damage occurring in northern counties and Scottish crops too," he reports. "Larval feeding in the stems over winter leads to damaged or stunted growth, and even plant death in the spring, when it's too late to take any action."
Dr Evans highlights there is a three to four week window for controlling the adult weevils in early autumn before they start to lay their eggs on leaves; this year SAC will be monitoring weevil populations to advise on optimum spray timings.
