Insect pollination is the weakest link in the production of a hybrid seed crop

Ecological intensification of farming proposes that more effective use of ecosystem services can, in part, replace external inputs allowing farmers to maintain high crop yields while reducing adverse effects on the environment. However, uptake of ecological intensification among farmers is currently hampered by a lack of realistic studies on the agronomic benefits of enhancing ecosystem services vis-à-vis the benefits of conventional external inputs. Here, we use a full-factorial field experiment to test the relative and interactive effects of fertilisation, irrigation and pollination on crop yield of three parental crop lines of leek (Allium porrum) hybrid seed production. In a commercial leek seed production field, we assessed the agronomic performance of plants receiving conventional or 50 % reduced external inputs and that were either continuously accessible to pollinators or only 50 % of the time. For all crop lines, we found that reducing insect pollination had at least two times stronger effects on crop yield than similar reductions in fertilisation or irrigation. Surprisingly, reducing fertiliser inputs by half did not negatively affect crop yield (one line) or even increased crop yield (two lines), suggesting that in this system fertiliser is an over-applied agricultural input. Reducing irrigation did not affect crop yield in two lines but reduced crop yield in the third line. However, there were strong indications that this negative effect of reduced irrigation was due to reduced attractiveness for pollinators. Effects of fertilisation, irrigation and pollination on crop yield were additive, with the exception of pollination effects being influenced by fertilisation level in one of the lines. Under real-world conditions, reductions in insect pollination consistently reduced hybrid leek crop yield while reductions in external inputs did not. This suggests that in this cropping system insect pollination is the weakest link in the agricultural production process. Our findings help explain why the relation between agricultural intensification and yield growth disappears with the dependence of crops on insect pollination. For insect-depended crops, protection or promotion of pollinators in agricultural landscapes is essential for maintaining high yields.