BY ANTHONY KING | 15 MARCH 2024 | IMAGE CREDIT GILLES SAN MARTIN
Beetle damage drops as plants get by with a little help from their friends.
Planting companion crops helps to deter cabbage stem flea beetles (CSFB) in oilseed rape, a new study from Rothamsted Research shows. Sowing mustard, cereals, or clover, as well as mulching with straw, reduced damage from adult beetles and their larvae, field trials in the UK and Germany showed (Pest Management Science, doi: 10.1002/ps.7641).
Oilseed rape is a valuable crop that has gained in popularity on cereal farms, in England especially. ‘Many farmers found that if you grow wheat following an oil seed rape crop, the wheat crop gets a yield benefit,’ says Samantha Cook, an ecologist at Rothamsted, who led the trials.
Acreage under oilseed rape went up until 2012, when neonicotinoid seed treatments were banned and emerging resistance in the beetles to spray-on pyrethroid insecticides left the crops vulnerable. ‘Farmers were left with no alternative to control the cabbage stem flea beetle,’ says Cook. The beetles, Psylliodes chrysocephala, riddle seedlings with holes and prevent establishment.
Beetle populations ballooned and many farmers stopped planting oilseed rape. ‘This was a massive loss to farmers, but also to biodiversity, because a lot of insects love this crop,’ says Cook. Some farmers began leaving some other plants or to deliberately sow cover-crop mixtures as companion plants to oilseed rape. Anecdotally, this seemed to reduce beetle damage.
Now, four field trials show that having wheat or oat companion plants in oilseed rape fields does reduce CSFB damage. Berseem clover gave some protection in one trial. However, barley had a negative influence, possibly because it competed with the crop.
In reducing adult feeding damage, but also in significantly reducing larvae, Cook explains, ‘wheat and oats in particular, as well as straw mulch [which simulated direct drilling into stubble], worked extremely well.’
Volunteer or companion plants may make it more difficult for beetles to find oilseed rape visually or by following crop odours, Cook says. The companion plants can be removed as the crop matures, though in the case of clover, this is not always necessary.
The research demonstrates that companion plants can offer ‘substantial protection against CSFB adult feeding damage to oil seed rape,’ comments Joe Roberts, a lecturer in integrated pest management at Harper Adams University, UK – though the effect on beetle larvae was less consistent, he adds.
‘This finding introduces new, potentially effective strategies for managing CSFB damage in a manner that could reduce reliance on chemical pesticides and contribute to more sustainable pest management practices in oil seed rape cultivation,’ Roberts concludes.
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