IT MIGHT be ideal weather conditions for sowing and growing oilseed rape, but it's also been ideal 'growing' weather for the young crop's biggest enemy, cabbage stem flea beetles.

Monitoring of beetle numbers on the Syngenta iOSR Focus Site and across the eastern counties of England has already seen a significant increase in cabbage stem flea beetle (CSFB) pressure in recent days.

Early sown crops are now well established and have, for the most part, grown away from limited initial beetle feeding damage. But, where there has been protracted emergence in dry weather, or new sown seedlings are now emerging, plants at the one to three leaf stage are especially susceptible to attack.

That warning comes from Syngenta's technical manager, Dr Max Newbert, who pointed out that at the iOSR site in Norfolk, assessed on September17, had shown that one well-timed application of Hallmark Zeon had given 50% reduction in beetle damage. A second application in trials had added an additional 12% to control of damage.

“The initial treatment was applied just as beetle activity started to take off on the emerged crop 10 days after drilling. Monitoring showed beetle numbers doubled in three days in the trial field,” he said. The repeat application, in the first week of September, coincided with a second wave of beetle numbers, which had multiplied three-fold in the past week.

Dr Newbert also noted in trials where OSR was sown with a buckwheat and berseem clover cover crop, the Hallmark Zeon treatment had shown a 74% control of beetle damage as an effective ICM programme. The iOSR trials will continue to assess impacts on future larvae numbers and final yield.

“Successful early establishment has now seen a surge in renewed interest among growers to plant more oilseed rape for its value as a break crop in the arable rotation,” added Dr Newbert. “For many, it’s a relatively low cost option to see what can be established, with the chance to revert back to a later drilled cereal if the oilseed rape fails.”

However, with CSFB numbers now on the rise, investment in insecticide treatment could help to get the crop well established from later sowing this autumn, he believed, but with the caveat of following thresholds for applications and to monitor CSFB populations both pre- and post-application.

“If the results of treatment are effective, it gives the crop a great chance. But if beetles remain in numbers after application, there is a good chance the population is showing pyrethroid resistance and so further treatments should be avoided,” he advised.

Growers should use Hallmark Zeon at the full label rate and applied appropriately to get the best result and minimise the risk of resistance. Furthermore, controlling adult CSFB now could serve to reduce egg laying and potential larvae infestations, even in crops that are already well established.