Organic winter cereals benefit bumblebee colonies in agricultural landscapes with mass-flowering crops

Scant floral resources are a major cause of declining pollinator populations in farmland. Measures to increase floral resources on arable land include agronomically non-productive approaches, like flowering fields (seed mixture of flowering forbs sown on fallows) and production-integrated approaches like organically farmed crops sustaining flowering arable wild plants. However, little is known about their relative efficacy for bumblebee conservation. We observed weight gain, foraging activity, worker body size, queen brood cell number and analysed stored pollen types of Bombus terrestris colonies experimentally sited adjacent to arable fields farmed under four crop-use types: (i) cereal mono-crops (winter wheat) under conventional farming, (ii) non-productive conventional flowering fields, (iii) organic cereal mono-crops (winter spelt) and (iv) organic legume mixed-crops (lentil mixed-crop). Additionally, we analysed the effects of flower cover in the surrounding landscape. Colonies located adjacent to organic winter spelt fields had higher foraging activity and larger body sizes, whereas other colony performance measures were unaffected by local crop-use type. Phacelia tanacetifolia, which was only cultivated in flowering fields, accounted for about 50% of the total pollen amount irrespective of crop-use type. This indicated a landscape-wide attraction for B. terrestris to flowering fields. Flower cover in the surrounding landscape did not affect colony development, but negatively affected pollen diversity. We conclude that although the pollen supply of flowering fields in the surrounding landscape strongly attracts bumblebees, local bumblebee populations could still benefit from less intensive farming practices increasing flowering wild plant diversity, such as with flower-rich organic winter cereals.

Automated summary

Scant floral resources in farmland can lead to declining pollinator populations, but measures like flowering fields and organic farming can help attract bumblebees. Bumblebee colonies near organic winter spelt fields showed higher foraging activity and better body sizes, while the presence of Phacelia tanacetifolia in flowering fields had a landscape-wide attraction for Bombus terrestris. Despite the positive effect of flowering fields, local bumblebee populations could benefit from less intensive farming practices to increase wild plant diversity.