Intercropping with legumes can reduce insect pest pressure but does not increase yield in winter oilseed rape ( Brassica napus L.)

BACKGROUND Winter oilseed rape (WOSR, Brassica napus ) is a major food, feed and fuel crop that heavily relies on pesticides and fertilisers to obtain maximum yield. We tested the potential of intercropping WOSR with legumes (frost‐sensitive spring faba bean, frost‐sensitive berseem clover or winter hardy pea) to reduce the abundance of and damage caused by the WOSR insect pest complex, enhance biocontrol and replace synthetic inputs with ecosystem functions. Each intercrop was tested under varying crop management practices (full or reduced fertilizer and pesticide input, WOSR row spacing and sowing time of legume crop relative to WOSR). RESULTS WOSR – spring faba bean intercropping reduced the abundance of cabbage stem flea beetle larvae on average from 6.87 to 3.48 per plant compared to WOSR sole cropping, but a 10‐day sowing delay of the clover or spring faba bean intercrop increased cabbage stem flea beetle larvae per plant from 3.17 to 5.52 compared to when the crops were sown simultaneously. WOSR – spring faba bean intercropping also reduced pollen beetle bud damage by 34% (95% CI: 16%, 48%) compared to WOSR sole cropping. Parasitism rates of pollen beetle and cabbage seed weevil were not affected by intercropping or crop management. WOSR seed yield (kg ha −1 ) was 35% (95% CI: 13%, 51%) lower in WOSR‐spring faba bean intercropping compared to WOSR sole cropping, and 38% (95% CI: 26%, 47%) lower in low input compared to high input systems. CONCLUSION Our results confirmed that intercropping WOSR with frost‐sensitive legumes can reduce insect pest pressure, but also highlighted a trade‐off with WOSR yield and the importance of legume sowing time in relation to WOSR.