A pot experiment, conducted under ideal growing conditions, has shown that once winter oilseed rape (WOSR) has passed through the critical emergence phase, it can cope with relatively severe defoliation.

AHDB Cereals & Oilseeds commissioned the experiment as part of a programme of work looking at the impact of the neonicotinoid restrictions.

Conducted by ADAS, the research assessed how much defoliation WOSR seedlings can tolerate to improve understanding of the impact of ‘shot-holing’ damage caused by adult cabbage stem flea beetle (CSFB).

The experiment

Pot-grown WOSR seedlings were exposed to simulated CSFB damage under controlled conditions in a glasshouse.

A number of defoliation treatments – ranging from no to severe defoliation – were applied to WOSR cotyledons, leaf 1 and leaf 2.

A hole-punch was used to simulate damage

In total, 16 treatments were applied and replicated six times (96 plants/pots in total).

When plants had reached the six-leaf stage, green leaf area (cm2) and dry matter yield was assessed compared to a control (no defoliation).

The results

In 13 out of 15 defoliation treatments, there was no significant difference in green leaf area. Surprisingly, those which were significant, were associated with an increase in green leaf area.

In 11 out of 15 defoliation treatments, there was no significant difference in dry matter yield. Only three treatments significantly reduced dry matter yield. One treatment, the removal of both cotyledons, significantly increased dry matter yield.

Caroline Nicholls, AHDB Cereals & Oilseeds research manager, said: “The glasshouse experiment showed that once oilseed rape has emerged it can tolerate severe defoliation in ideal growing conditions. This suggests that crops are most vulnerable at emergence if the growing point is destroyed.

“We are looking at the implication of these findings on the current CSFB spray thresholds.

“All the evidence points to the importance of giving crops the best possible start in life.

“In 2014, crops drilled into dry/cloddy seed beds that were slow to emerge often suffered the most damage. Once crops had grown away, they tended to withstand adult CSFB attack.”

The latest research also included a field experiment at a site in North Yorkshire with a previous history of CSFB.

Three winter oilseed rape treatments were tested (untreated seed, treated seed and a pyrethroid spray) in an experiment replicated eight times (24 plots in total).

The number of CSFB in the field experiment were relatively low and there was no statistically significant effect of the treatment on crop yield.

Resistance threats

Last September, AHDB showed that resistance to pyrethroids in CSFB was widespread in England. To limit the spread of resistance, pyrethroid sprays should only be applied where there is evidence of high pest pressure at emergence or if thresholds are exceeded post emergence.

Repeat applications of pyrethroids should not be made. If control remains poor, a pyrethroid-based product should not be used again.

For full information on AHDB’s neonicotinoid activity, visit cereals.ahdb.org.uk/neonics