Foraging behaviour and pollination contribution of naturally occurring bumble bees in red clover

Pollinators optimise their foraging behaviours in response to changes in floral rewards, which may consequently affect pollen transfer and plant reproductive success, especially in plants that require cross-pollination. In a red clover field grown for seed production, we conducted a three-year experiment to compare the foraging behaviours of naturally occurring bumble bee species and their pollination contributions. Specifically, we investigated how the visitation of a flower head by one bumble bee may affect the foraging behaviours of any second visiting bee. We also measured relative abundance, stigmatic pollen deposition, and plant traits including flowering intensity, corolla tube length, and seed set. We found that bees can identify previously visited flower heads: the second visiting bees increased their rejection probabilities with the flower exploitation rate of the first visiting bees. The second bees also had a higher foraging speed than the first visiting bees, either due to the depletion of floral rewards or the reduced pressure required to trip the florets. Among the five bumble bee species naturally occurred during the peak flowering, different bumble bees showed varied preferences for the co-flowering cultivars. At the flower head level, one visit from B. pascuorum and B. muscorum was sufficient for the seed set, similar to the seed set in the open pollination. However, B. hortorum that caused high stigmatic pollen deposition did not result in comparable high seed set, this might be related to a limited number of cross-pollen transferred, caused by the abundant floral resources during the peak flowering, or an intrinsic characteristic of this bumble bee species.