Rape winter stem weevil heads south

03 October 2021,  England: Significant damage to oilseed rape crops from the effects of rape winter stem weevil (RWSW), which has hitherto been an issue for growers primarily in Scotland, was reported on sites across England last season.

The effects were exacerbated last season by the exceptional cold weather in spring defoliating crops. But with extreme weather conditions occurring more frequently due to climate change, growers across the UK should be alert to the risks and management strategies in future, advocates Syngenta Technical Manager, Dr Max Newbert.

“RWSW lay their eggs on the leaf petiole, where emerging larvae burrow direct into the plant, unlike cabbage stem flea beetle where larvae emerge from the soil and climb onto the plant,” he said.

“That means autumn control actions can only be targeted at the adult weevil to prevent egg laying and larvae getting into crop,” he advised.

Adult RWSW invasion typically starts in late September and through October. The adults are believed to require four weeks feeding on the crop, before they lay their eggs. In most instances, the adults’ feeding has limited effect on the crop, but it’s the subsequent larvae that cause the damage (larvae infested plant, below right, compared to unaffected). They often co-exist alongside CSFB larvae.

Monitoring of RWSW adult numbers on the Syngenta iOSR site in Norfolk last year identified the initial infestation on October 4, then rising to peak numbers by the third week of November.

“The activity of the adult weevils on the crop can coincide with Hallmark Zeon applications to target CSFB larvae movement.”

However, new research into optimising timing for CSFB larvae could mean earlier treatments, which would be applied ahead of RWSW activity.

“Growers and agronomists will need to assess the potential damage from the two pests and create appropriate management strategies,” suggested Max (below).

Research at the iOSR site had indicated later drilled crops, which were smaller at the time of RWSW migration, suffered more intense invasion of the pest, compared to larger plants from earlier drilling that experienced later and less intensive attack.

“As such, early drilling, usually undertaken to overcome CSFB damage, would have the side benefit of making the crop less attractive to adult RWSW,” he pointed out.

Partial defoliation of the crop, with sheep grazing, had also proven effective in reducing pest larval loading. The challenge has been to manage the level of grazing or mechanical defoliation, such that the crop is not hampered by cold or dry conditions in the spring.

Last year’s trial, with sheep stocked at 40 head per hectare for one week in mid-September, had demonstrated a 14% reduction in larvae numbers whilst the crop had fully recovered by the spring.

This season the iOSR site will be investigating tandem resilience strategies for reducing larvae impacts on the crop, involving combinations of PGR treatments, biostimulant applications and Hallmark Zeon insecticide interaction trials.