Bee pollinators of faba bean (Vicia faba L.) differ in their foraging behaviour and pollination efficiency

Insect pollination is essential for crop production by enabling or increasing seed and fruit set in many crops. The grain legume faba bean (Vicia faba L.) is partially allogamous and benefits from bee pollination because bees transfer cross-pollen and improve seed set. Here, we study mechanisms behind bee pollination and address the question whether individual bee species differ in their efficiency to pollinate V. faba. We studied the foraging behaviour of bee pollinators in faba bean fields using transect walks, and the species-specific flower constancy based on pollen analyses. For the first time, we examined the species-specific effects of bee pollinators on seed set and degree of cross-fertilisation. We found that the two locally dominant pollinator species, Apis mellifera (56% of all observed individuals) and Bombus terrestris agg. (37%), did not provide efficient pollination service since they mostly robbed nectar. The less frequent species Bombus hortorum (4%), however, revealed to be the most efficient pollinator. It exhibited only legitimate flower visits, high flower constancy and additionally increased seed set and the degree of cross-fertilisation more than any other species. Our study demonstrates especially that less frequent and more specialised pollinator species with long tongues improve pollination and cross-fertilisation of faba bean, which is of great importance in plant breeding and seed production.