A review of evidence and new trials work have revealed the factors most likely to influence cabbage stem flea beetle (CSFB) pressure in winter oilseed rape (OSR) – and grazing with sheep has emerged as an unlikely element.

The ADAS-led research identified 31 factors, including an estimate of the reliability of each individual effect and an indication of how open each factor is to management.

The results can be used to stack solutions in the field, as part of efforts to suppress the impact of this pest. Of the CSFB pressure factors identified, 20 decreased it, seven increased it and four, on balance, resulted in a neutral outcome.

Charlotte Rowley, who manages pest research at AHDB, said: “Effective chemistry can stop pest targets in their tracks. Without it, we need to get to grips with pest life cycles to find the chinks in their armour.

"As no non-chemical approach is completely reliable, a combination of the most reliable is needed to deliver multiple hammer blows and suppress CSFB populations.”

The research team mapped out the key factors across a growing season and used a traffic-light system to indicate those most likely to affect control. Only two received a green light – trap crops and soil conditions during sowing/early establishment.

Oilseed rape volunteers can act as a trap crop and divert CSFB away from adjacent cash crops. In trials, this reduced adult CSFB infestation by up to 88% and damage by up to 76%. It also resulted in higher plant populations, up 56% and reduced larval populations by 69%.

However, researchers concluded that benefits were variable and not always observed and it was likely that relatively large areas of adjacent trap crops were more likely to deliver benefits, especially if left in place until after the bulk of CSFB migration has occurred.

Weather strongly influences the CSFB's life cycle and crop development. If the crop has yet to emerge, or is emerging when CSFB arrives, it results in more crop damage/death. Therefore, sowing dates and soil conditions – particularly soil moisture during emergence – are critical.

On seed rate, the researchers found that, in terms of yield, increasing the seed rate beyond that needed to achieve optimal plant populations – 25-40 plants/m2 – resulted in little benefit, unless conditions were dry during establishment. Interestingly, higher seed rates were associated with greater larval numbers per hectare, which could build pest populations across seasons.

Linked on-farm trials found that sheep grazing and topping reduced larval numbers by 51% and 25%, respectively. However, researchers did not detect significant yield increases in crops with reduced larval populations.

Further research is required to refine this and the approach was marked as ‘amber’. Interestingly, defoliation also decreased the severity of phoma stem canker to levels equivalent to a fungicide spray.

Pyrethroid resistance means that effective chemical control options are limited. Pot and field experiments showed that, once emerged, OSR often has good tolerance to CSFB damage, from both larvae and adults. However, further information is required to refine current spray thresholds for this pest.