Given limited chemical options for controlling cabbage stem flea beetle (CSFB) in oilseed rape, AHDB-funded research is exploring alternative methods in a project due to end in December next year.
Andrew Blake reports...
Early results from the £150,000 project* suggest trap cropping and mowing pre-stem extension could be useful, according to ADAS’ Steve Ellis who is leading the work.
“Our aim is to develop an integrated pest management strategy which farmers and agronomists can use confidently to predict the risk of pest damage and make rational decisions on the need for control measures,” he says.
Research since the 1970s has created a wealth of data on the pest which affects about two-thirds of the UK crop.
“We’ve used that to guide decisions on the latest experiments and ensure cost-effective use of the new project’s funding,” says Dr Ellis.
Information gathered from adult and larval infestations on 1,610 sites with known agronomy suggests other potential cultural controls.
“I stress that these are preliminary results, but they suggest that delaying drilling by two to three weeks reduces larval pressure up to four-fold.”
Despite some significant crop failures due to attacks by the adult pests, the larvae are regarded as more damaging to the UK crop as a whole, says Dr Ellis. “Overall we estimate that the pest costs the industry £9 million/year.”
Crops sown before August 11 had an average 2.5 larvae/plant, but those drilled after September 15 had almost none (see graph 1).
Other initial findings are that dry seedbeds may increase the adults’ damage, but pre-emergence herbicides could halve it, perhaps by reducing weed competition. “And direct drilling may decrease larval damage,” Dr Ellis says.
Included in the project are the findings of an ADAS survey of 150 sites in autumn 2016 and 2017. Its aim was to understand the importance of a range of agronomic factors such as sowing date, soil type, straw incorporation, location of previous oilseed rape crops and cultivations on adult feeding, he explains.
“The results are still being analysed, but we hope to combine them with the earlier information to provide a dataset which is large enough to convey statistically meaningful messages.”
Three AHDB Recommended List trials have been assessed to compare levels of adult and larval damage between varieties. However, although there were differences between both adult and larval damage they were not statistically significant.
To help determine how the number of larvae per plant affects yield, the project has used fleece to cover trial plots two and eight weeks after drilling to try to prevent adult access and create different levels of larval infestation compared with unfleeced plots.
“Unfortunately the fleece was easily punctured when laid over some cereal stubble,” says Dr Ellis. “So the potential for using different insecticide treatments to engineer different larval numbers is being used in this year’s experiments.”
The effect of defoliating oilseed rape in late winter on yield was tested in earlier AHDB-funded work, and on average, across four experiments, it cut output by only 0.1 tonnes/hectare, adds Dr Ellis.
“So if crops could be defoliated before the larvae move from the leaf petioles to the stem, this might reduce yield losses from the pest.”
In the first season’s work under the new project, mowing plots before and after stem extension cut larvae numbers by nearly a third to over a half (55%) compared with untouched plots.
“The numbers did rise after defoliation, but it’s unclear whether this was because there was a late hatch or that larvae were able to migrate from the mown material into the mown plants,” says Dr Ellis. “Even so mowing, or even grazing, might provide effective control in high infestations, and we plan to investigate this further.”
One experiment in the 2016/17 cropping season suggests that oilseed rape volunteers may act as a useful trap crop. The numbers of adults in yellow water traps were lower in an emerging field of rape adjacent to a field in which the volunteers were destroyed late – in mid-September, in comparison to an emerging crop next to a field where the volunteers were destroyed early – in mid-August.
“The fact that volunteer oilseed rape may act as a trap crop is interesting. We have data from only two sites so far.
“At one site the farmer left the volunteers in a whole field, and at this site there was evidence that trap cropping might work. At another site, where the volunteers were left in only part of a field, the results were less encouraging. Perhaps the size of the area left is important.”