Although reports of cabbage stem flea beetle (CSFB) activity are becoming more widespread it is important not to assume that the pest will cause problems, Dr Steve Ellis, entomologist at ADAS advises.
With more moisture being seen in oilseed rape seedbeds than last autumn, crops will be able to establish more quickly, putting them in a better position to tolerate pest attack, he says.
"As CSFB is now a relatively widespread pest there is a good chance that you will see damage on an emerging crop of oilseed rape. However, the presence of damage does not necessarily mean that there will be an impact on yield."
At this time of year, adult CSFB undergo a resting phase before migrating into this season’s crops, and pest pressure can be very variable. Dr Ellis says there are a number of ways to monitor pest levels to help with risk assessment.
Although not the most scientifically precise method, a quick look in the trailers of harvested OSR seed will give an indication of the number of beetles available to invade crops.
There is no simple way of calibrating these observations but over a number of years, experience will tell you what level of trailer infestation poses a risk.
In the first instance, if beetles are easily seen hopping around in the trailers then there is clearly a risk to an emerging crop.
Yellow water traps in proposed OSR fields will catch CSFB. Traps should also contain a drop of washing up liquid so that any insects caught will sink and drown.
Traps should be examined regularly as they will catch large numbers of insects which can make it difficult to pick out CSFB.
There are no thresholds but the presence of beetles will indicate that it is important to monitor crops as they emerge.
Data collected over a number of years will help to determine how trap catches relate to crop damage.
As with water traps these can be used to monitor beetle numbers. Regular inspection is again recommended as large numbers of different insects will be caught. The presence of beetles indicates a need to monitor crop emergence.
Even if control measures are applied against adult CSFB, this does not mean that there is no risk from the larvae.
CSFB resistance to pyrethroids has been recorded in the UK so it is important to limit further spread.
It is therefore vital that pyrethroids are used in a rational manner to minimise the development and/or further spread of resistance.
Treatment thresholds currently advise that a spray is only necessary if 25% of leaf area is lost at the 1-2 leaf stage increasing to 50% of leaf area lost at the 3-4 leaf stage.
Routine sprays at the first sign of damage must be avoided. Where resistance is present, pyrethroids are unlikely to provide effective control.
If a pyrethroid application is not effective and this cannot be explained by factors such as poor spray coverage, then it is likely that the CSFB population is resistant.
In such a case, avoid applying further pyrethroid applications as this will continue to select for resistance and will harm natural enemies.
A number of natural enemies are known to attack CSFB in the autumn, especially the egg and larval stages, and so using pyrethroids against resistant populations may result in higher larval populations than would otherwise have occurred.
As part of the AHDB funded project Integrated pest management of cabbage stem flea beetle in oilseed rape, a series of trials were done to investigate whether OSR volunteers left in situ until late September can act as a trap crop to attract migrating adult CSFB away from drilled OSR.
Results suggest that this can significantly reduce adult numbers and damage and significantly increase plant populations in nearby drilled OSR. Data (as yet unanalysed) also show large reductions in larval numbers.
Any CSFB eggs and/or larvae in the volunteers will also be killed when these are destroyed.