How companion crops reduce pest damage in oilseed rape

Companion crops in direct drilled oilseed rape could help reduce damage by cabbage stem flea beetle, say scientists.

Results from a Rothamsted Research study suggest relatively simple changes in crop management could help control a ubiquitous pest which has seen many farmers abandoning oilseed rape altogether.

Field trials, conducted as part of the EU-funded EcoStack project over four seasons at Harpenden, assessed crop damage caused by adult flea beetle feeding and larval infestation when sown with different companion plants.

Companion plants included both clovers and cereals to simulate delayed application of herbicide to kill volunteers. Application of straw mulch was also tested to simulate drilling into stubble trash.

Flea beetle infestations have rocketed since neonicotinoid seed treatments to combat the pest were banned in 2013. Implemented because of environmental concerns, the ban triggered a 50% slump in the amount of UK oilseed rape.

Researcher Sam Cook, who led the study, said: “Plant diversification – especially sowing crops with the addition of companion plants – has been shown to improve insect pest control in many cropping systems.”

The need to find similar effects for oilseed rape is acute, added Dr Cook. “We wanted to see if control methods that are aligned with regenerative agronomic practises could help provide some protection.”

Crop establishment

Adult flea beetle damages oilseed rape by feeding on cotyledons and young leaves early in the autumn. This can threaten crop establishment. The larval stages also feed in the stem causing reduced yield.

The team found significant differences in the level of feeding damage between treatments in all experiments. Rape with cereal companion plants or with straw mulch showed the strongest reduction in adult feeding damage.

A protective effect of clover was also observed in one trial. Differences in larval infestation were also observed between treatments but were not consistent and might be more related to oilseed rape biomass than to treatments.

“This study shows that companion planting can protect rape crops from both adult flea beetle damage and larval infestation,” said Gaëtan Seimandi-Corda who managed the experiments.

New techniques

“These results suggest that farmers could relatively easily adopt new control techniques, but there is a need for more research to define the best agronomical practices.”

Cereals such as oats could be sown as companion plants which are later removed, suggests the study. Alternatively, cereal volunteers could be left for longer before removal to provide protection from flea beetle.

That said, the timing of volunteer removal and the sowing date of companions is essential for efficacy – and to avoid competition between the oilseed rape and the companion crop, Mr Seimandi-Corda added.

“We are now learning that increased plant diversity can promote an increased diversity of beneficial insects within the field. That can help farmers to farm more sustainably and mitigate some of the negative effects of food production on the environment.”

New authorisation for Emerger herbicide

Pre-emergence residual herbicide Emerger – which targets a range of common broadleaved weeds – can now be used in field beans and combining peas.

The news will likely be welcomed by growers given the low number of pre-emergece herbicides authorised for use in these crops, says Richard Phillips, Bayer campaign manager for roots and horticulture crops.

Broad spectrum

“As the only aclonifen product on the market, [it] means growers have access to a herbicide with a broad spectrum of activity. Its novel mode of action – HRAC group 32 – will also support efforts to promote resistance management.”

Among broad-leaved weeds, Emerger offers good activity on fat-hen, redshank, black bindweed, mayweeds, charlock, chickweed and poppy – as well as moderate control of blackgrass from seed. It is not authorised for vining peas.

“Emerger is primarily absorbed by the shoot of germinating weeds, this is advantageous in dry weather conditions such as those that typically occur in the spring when seeking to establish spring crops,” says Mr Phillips.

To promote all-round weed control and protect efficacy, Bayer will support Emerger in mixes with one other herbicide so long as growers and spray contractors observe minimum application rates.

“It is prudent to ensure Emerger is partnered with a product belonging to another mode of action group to support effective control and protect efficacy, says Mr Phillips.

“Depending on the crop and the weed spectrum to be managed, we have identified those products and inclusion rates that give growers the greatest means possible of achieving the control they desire.”