Protecting winter oilseed rape

What is a trap crop?

A trap crop is a plant that is more appealing to a target pest than the crop you want to protect.

For instance, research shows that turnip rape can decrease CSFB larvae in winter oilseed rape when planted nearby, such as in a border. However, this approach requires additional effort during a busy period and occupies valuable land.

There is an alternative that demands far less management, with no need to buy or sow extra seeds, or use up land. All you need to do is delay controlling volunteer oilseed rape.

AHDB's Sacha White

Cabbage stem flea beetles are attracted to a juicy crop by detecting plant volatiles known as isothiocyanates (products of glucosinolate breakdown). If your field has a dense population of volunteer oilseed rape, it will emit a larger cloud of these volatiles (compared to freshly drilled oilseed rape in a nearby field), acting as a magnet for the beetles during migration.

Leaving the trap crop for a few weeks allows you to take advantage of another aspect of CSFB biology. The beetles gradually lose their wing muscles, severely limiting their escape options when the volunteers are eventually destroyed.

Testing the trap

The impact of a volunteer oilseed rape trap crop was documented as part of an ADAS-led project (see AHDB final project report 623), which included trials on six farms.

Each farm tested the effect of early (in August or early September) and late (mid-September to October) control of volunteers on CSFB pressures in nearby winter oilseed rape crops, with specific control dates set to match the trial site.

When the volunteer trap crop was controlled later, CSFB pressure was reduced in the adjacent winter oilseed rape crop at four farms – with up to 88% fewer adults, up to 76% less leaf area lost, and up to 69% fewer larvae. This was all achieved simply by delaying action for a few weeks.

Destroying the volunteers also eliminates any eggs or larvae on them, resulting in fewer pests the following summer.

At one of the two farms where no benefit was observed, the farmer removed all but two ha of the volunteers much earlier than planned. At the other, all volunteers were removed in mid-September. This suggests that small areas of volunteers may not be strong enough to attract the pest, and it may be best to leave volunteers until at least late September to ensure most CSFB migration has occurred.

Of course, this strategy must align with the broader farm management plan (e.g. its impact on black-grass control), but trap crops do seem to be a valuable element in the integrated approach to CSFB management.

Integrated approach

The lack of effective chemical options has made this beetle pest the top threat to winter oilseed rape. Managing it requires layering a developing, varied, and complex set of alternative strategies.

This is the second article in a series on CSFB IPM, designed to provide a strong foundation for on-farm experimentation, helping you to grow oilseed rape with increased confidence.

As with all cultural methods, control outcomes can vary. Please share your experiences so we can enhance our understanding of all interventions.