Pollination increases white and narrow-leaved lupin protein yields but not all crop visitors contribute to pollination
Global biodiversity is declining under pressure of agricultural intensification and land-use change. Two-thirds of the agricultural lands is directly or indirectly devoted to the production of animal products. Replacing animal-based proteins by plant-based proteins can be an important step to a more sustainable agricultural system. Lupins (Lupinus sp.) are promising crop species due to a high protein content of up to 40 %, but crop yields are unstable in both quantity and quality. This might be due to a lack of effective pollinators, but the contribution of insect pollination to lupin crop yield is unknown. Here we studied for five varieties of two common lupin crop species (L. albus and L. angustifolius) which pollinators visit lupin flowers, whether this depends on nectar production, and what the contribution of insect pollination is to crop yield. We used a semi-experimental setup and placed bagged and open-pollinated plants in pots along an expected gradient of insect visitors and determined several yield parameters. We recorded 1355 pollinator visits of only eight bee species. None of the varieties tested produced nectar. Compared to bagged plants, protein yield increase of open-pollinated plants ranged from 3 to 11% depending on variety. Yield of open-pollinated plants was only consistently related to visitation of the large-bodied buff-tailed bumblebee (B. terrestris group; 59 % of all pollinators) with impact on seed set related yield parameters (number of seeds and pods) being generally larger than on seed filling related yield parameters (g/plant). Within the observed range, higher visitation rate of buff-tailed bumblebees increased protein yield of open-pollinated plants with 10-40 %. Visitation rates of the smaller common carder bee (B. pascuorum; 33 % of all pollinators), or all pollinators combined, were not significantly related to protein crop yield. This could indicate that only relatively large species are effective lupin pollinators. Lupins are generally considered self-pollinating, and therefore growers do not actively manage for insect pollination. Our results show that insect pollination, and in particular buff-tailed bumblebees, can contribute substantially to the crop yield, which suggests that management aimed at enhancing effective pollinator species can help to make lupin crop cultivation more profitable. Amongst others, such management should make sure that ample nectar is available in the surroundings of lupin crops, as lupin does not produce nectar.