New research found a non-chemical control method resulted in 30-60% larvae reductions. Try it on your farm.
With the loss of neonicotinoid seed treatments, oilseed rape growers across the country have become concerned about the lack of effective chemical control options for cabbage stem flea beetle (CSFB). Foliar pyrethroid sprays are the only chemical control option available. But there is widespread resistance to pyrethroids in CSFB, and where present control with these insecticides is likely to be poor.
Come along to a kick-off meeting to hear more about the research to date and the field lab:
From 11am, 11 Dec '18 | ADAS, Boxworth, Cambs | Book Now
Focusing on the larval stage
Myself and colleagues at ADAS are an AHDB-funded three-year IPM project looking at improving control of CSFB. Part of this has investigated non-chemical control of the larval stage. CSFB larvae invade the crop in October or November and feed within the petioles all winter. As they grow, they move to the stems to feed behind the growing point, usually in March.
In recent years much of the focus has been on damage from adult CSFB rather than the impacts from larval feeding. This is understandable as adult feeding is easily seen and severe pressure could result in the loss of the crop. In contrast it is difficult to gauge larval pressure and damage. However, given that larvae spend about six months in the plants at a critical growth stage, we believe that they can have a significant impact on crop development and yield, and may be in part responsible for preventing rape varieties from achieving their true yield potential. Recent warm autumns and winters have heightened the problem by allowing adults to continue egg-laying, resulting in larger larval populations. As larvae are protected within the petioles and stems, insecticidal control can be variable - even where they are not resistant - so growers can struggle to minimise their impact on yield.
One control method that has proved promising in our trials is defoliating the crop during the winter to remove or knock off leaves containing larvae. Doing so increases larval mortality by exposing them to cold conditions and predation. Previous research has shown that defoliating the crop during the winter has a negligible impact on yield, as long as it occurs before stem extension. We used a mower to cut the crop down to 2-3 inches in December, January or March to understand the effects of timing on larval density and effect on the harvest crop. Mowing resulted in reductions in larval numbers of 30-60% compared to unmown plots (see graph). The highest yields occurred in plots mown in December, with the lowest in those plots mown in March (most probably down to mowing after stem extension).
Figure 1. Impact of defoliation in December 2016, or January or March 2017 on numbers of CSFB larvae in oilseed rape plants at ADAS Boxworth.
Try it on your farm
Could this reduction of CSFB larvae happen at field scale? A new field lab is being set up to run alongside the replicated, controlled research plots at ADAS to see if this technique can significantly reduce CSFB burden and improve yields in a real farm setting.
If you are interested in getting involved in a trial this winter, you could choose the defoliation method that best suits your farm, such as topping, grazing with livestock, or rolling the crop, and carry it out from December to stem extension (usually around mid-February to mid-March).