Bumblebee footprints on bird's-foot trefoil uncover increasing flower visitation with land-use intensity

Wild pollinators are declining in abundance, diversity and richness and this puts the ecosystem function pollination at risk. Here, we investigated how land-use intensity and the three main components of land use used on the study sites (mowing, grazing, fertilisation) affect bumblebee visitation to bird's-foot trefoil, Lotus corniculatus, using a novel chemistry-based approach that overcomes the limitations of classical visitation monitoring. This approach uses the hydrocarbon footprints left behind by bumblebees that accumulate within the epicuticular wax of flowers. Along with land-use intensity, we investigated whether the small-scale (patch) population density and local abundance of L. corniculatus affected visitation. We found that the amount of footprints, and thus visitation, of bumblebees to L. corniculatus increased with land-use and grazing intensity while traditionally recorded visitation rates did not show such an effect. We further found that the patch density and local abundance of L. corniculatus had no effect on visitation. We conclude that bumblebee visitation to L. corniculatus increased with land use using the novel approach but emphasise that foraging decisions by bumblebees can be affected by many different parameters unrelated to the land-use intensity of a specific site.