Agronomy tricks to beet beetle damage
Leaving a sufficient area from the previous year’s OSR to green up with volunteers could attract adult Cabbage Stem Flea Beetles away from emerging seedlings in neighbouring fields, according to ADAS entomologist,Dr Sacha White.
The CSFB IPM project had identified the potential for a number of techniques to protect new crops from attack, he told the latest iOSR grower group meeting.
The key is to have volunteers at a stage of emitting isothiocynates – a metabolite of the naturally occurring gluconsinolates in brassica crops - that attracts CSFB at the time of the key August migration. Importantly, once beetles are established in the volunteers, they tend to lose the muscle strength to fly and migrate into the emerging crop, Sacha advised.
Trials where the volunteers had been left into the autumn resulted in a significant reduction in adult CSFB (up to 80-90% less) in the growing crop, along with significantly less damage and significantly higher plant populations.
He indicated one of the benefits is to leave the volunteers for long enough for eggs to hatch and larvae to enter the plants, before destroying the green material and breaking the life-cycle.
The project has also investigated the opportunity to physically remove the CSFB larvae harboured in leaf petioles over the winter. Research trials, by ADAS and on the iOSR Focus Site at Rougham in Cambridgeshire, has mechanically removed the green material.
Meanwhile, iOSR grower, Chris Eglington (above), has been involved in the research by grazing a hectare of OSR known to be affected by CSFB larvae with sheep for eight days.
“There is definitely a practical issue if getting sheep onto and across the OSR crop at the right time to target the larvae, but not adversely affect the crop,” he suggested.
“Fortunately with low plant numbers established early, we generally have good root structures that should recover quickly.”
Grazed at the beginning of January, he reported the effects initially looked harsh, but by early February was clear signs of some new growth.
"One thing we have noticed, is that any Charlock in the field has been completely nibbled off by the sheep (below), and no sign of any regrowth, which could be a further benefit," he added.
The iOSR growers also questioned the practicality as to whether a topper could chop low enough and evenly enough to remove larvae in the lower leaves, particularly in a prostrate crop or uneven field. It also raised options for selecting variety hybrid vigour, for autumn growth to get going and spring recovery.
Understanding of the Cabbage Stem Flea Beetle life cycle could help growers utilise natural predators to reduce pest pressure. When adult CSFB emerge in early summer, they generally move out of the crop and into a resting stage. .
Then, when they move back into the new crop, parasitic wasps attack the adults. Ground beetles and rove beetles will then predate on both CSFB eggs laid in the soil surface and emerging larvae as they make their way into emerging seedlings. Furthermore, in spring, parasitic wasps can still pierce the plant petiole to lay their eggs inside the larvae and result in its death.
“Beneficial’s could be extremely useful for reducing CSFB numbers. However, they can be adversely affected by indiscriminate pyrethroid use,” advised Dr Sacha White.
“If a large proportions of the CSFB population is already pyrethroid resistant, their use could make the situation worse by reducing the background number of beneficial insects.”
Furthermore, since most of the beneficial predators are active after dark, the night spraying for CSFB could be further affecting numbers, he pointed out.