Weak evidence for biocontrol spillover from both flower strips and grassy field margins in conventional cereals

Abstract

Agri-environment measures such as flower strips are considered fundamental instruments to reduce farmland biodiversity loss. Their effect on associated ecosystem services, such as biocontrol of crop pests, has received increased attention in recent discussions on the ecological intensification of agriculture, to create a win-win situation for biodiversity and production. However, studies of flower strips are rarely compared to the status quo, namely permanent field margins. We assessed the impact of two different field margin types (pre-existing grassy field margins vs. perennial flower strips) and distance decay effects from the field margins towards field centres on two major cereal pests (cereal leaf beetles and cereal aphids), their natural enemies (predators and aphid parasitoids) and crop yield on five conventional farms in Germany. Proximity to both types of field margin enhanced the enemy-to-pest ratio for cereal leaf beetles but not for aphids. However, the parasitism rate for aphids was higher next to grassy strips than next to flower strips. Yield also tended to be higher adjacent to flower strips (although not significantly) and showed no variation with distance to the field margin. Our findings paint a more nuanced picture than most previous studies: whilst field margins generally had a positive impact on one of the studied biocontrol agents, flower strips do not outperform the standard grassy margins currently in place. To promote biocontrol services, we recommend protecting existing, permanent grassy strips through the EU Common Agricultural Policy as a cost-effective alternative to flower strips.