Planting turnip rape around oilseed rape crops could decrease pollen beetle infestation, recent trials by Rothamsted Research show.

When used as a border, turnip rape plots develop more quickly and become infested with pests earlier than the main cash crop, says project leader Sam Cook, an invertebrate behavioural ecologist at Rothamsted.

That helps protect the oilseed rape crop as it passes the damage-susceptible yellow bud stage.

Experiments conducted on small and field scale plots suggest that pollen beetles are attracted by the yellow flowers and stronger floral compound of turnip rape (Brassica rapa), a close relative of oilseed rape (Brassica napus).

The results indicate that turnip rape has good potential as a trap crop to protect oilseed rape from pollen beetle attacks, notes Dr Cook.

“Trap cropping could also reduce the number and area of insecticide applications and slow the development of insecticide resistance,” she says.

While growing two different crops alongside each other may be impractical for many growers, trials are ongoing looking at whether fast-developing, early flowering winter oilseed rape varieties could be used as border trap crops in a two-cultivar oilseed rape strategy.

Dr Cook conducted a series of experiments looking at colour and smell to try to understand the mechanisms of attraction from pollen beetle towards turnip rape and oilseed rape.

The trials found beetles were attracted to flowering plants, regardless of species. “When both plants were in flower there was no preference at all.”

Pollen beetles have an amazing attraction for the colour yellow, she says, which partly explains the attraction to flowering plants.

The experiments also showed that turnip rape produces higher quantities of odour-producing chemicals, which attract pollen beetle, says Dr Cook.

“Phenylacetaldehyde and indole are very attractive to pollen beetle,” she says.

“Phenylacetaldehyde is not present in oilseed rape buds, but it is present in turnip rape buds. It is present in oilseed rape flowers and it is present in quite large amounts in turnip rape flowers.”

That explained why pollen beetles showed a clear preference for turnip rape when both plants were in bud.

“That means it will attract and retain beetles,” says Dr Cook. “The beetles will remain there and any new infestations will prefer turnip rape as it flowers earlier than oilseed rape.”

The trials have been scaled up to 1ha field plot trials as part of the Pollen Beetle LINK Project, a three-year study ending this May.

Results show that at the damage susceptible stage of oilseed rape – green to yellow bud stage – the plots surrounded with a turnip rape border had fewer beetles.

Plant breeding partners involved in the project have been looking at their lines and data sets to look for a highly attractive oilseed rape crop to use as a trap crop.

“Four very early flowering lines have been selected and are now being trialled,” says Dr Cook.

“Hopefully, we will be able to plant them on the same date, as you do with turnip rape, which just grows faster.

“Turnip rape can be harvested as an oilseed rape seed, but as it grows faster, there are problems with seed shed.

“So we are also looking for an early flowering turnip rape line that matures late as a trap crop.”

• Pyrethroid resistance in pollen beetles is increasing, says Rothamsted Research entomologist Kevin Gorman. For the full story, go to www.fwi.co.uk/pollenbeetle