Many pollinating animals visit a variety of flowering plant species. Rare plant species pollinated by such generalists may experience a low quality or quantity of pollination, depending on the pollinators’ foraging behaviour. How plants cope with this rarity disadvantage is not well understood. One possibility would be to offer a higher floral reward, for example, a higher nectar sugar concentration. However, since nectar production is costly, rare plants may only be able to increase their nectar concentration for a limited time and offer little reward afterwards. In this study, we performed a laboratory experiment with bumblebees (Bombus terrestris) foraging on artificial flowers of two colours to investigate whether the bees’ foraging behaviour produces a rarity disadvantage and if so, whether the rare flower type could improve its pollination success through temporal variation of its nectar sugar concentration, i.e. a temporary increase of nectar sugar followed by a period with low concentration. We found that when both flower colours offered equal rewards, the rare colour received only slightly fewer visits per flower, but had a considerably lower expected pollination success based on the bumblebees’ visitation sequences. Temporal variation of the rare colour’s sugar concentration increased both the quantity and quality of visits it received. This positive effect was reduced when there were fewer rare flowers or when two bumblebees foraged simultaneously. Our results suggest that temporal variation of floral rewards can alleviate, but not completely eliminate the rarity disadvantage.

许多授粉动物会拜访各种开花植物。由此类通才授粉的稀有植物物种可能会经历低质量或低授粉数量，具体取决于传粉者的觅食行为。植物如何应对这种稀有劣势尚不清楚。一种可能性是提供更高的花奖励，例如更高的花蜜糖浓度。然而，由于花蜜生产成本高昂，稀有植物可能只能在有限的时间内增加花蜜浓度，而之后几乎没有什么回报。在这项研究中，我们对熊蜂（ Bombus terrestris）在两种颜色的人造花上采食进行了实验室实验，以研究蜜蜂的采食行为是否会产生稀有劣势，如果是的话，稀有花型是否可以通过时间来提高其授粉成功率其花蜜糖浓度的变化，即花蜜糖暂时增加，然后是低浓度的时期。我们发现，当两种花的颜色提供相同的奖励时，稀有颜色每朵花的访问次数仅略少，但根据大黄蜂的访问序列，预期授粉成功率要低得多。这种稀有颜色的糖浓度随时间的变化增加了其访问的数量和质量。当稀有花朵较少或两只大黄蜂同时觅食时，这种积极作用就会减弱。我们的结果表明，花卉奖励的时间变化可以减轻但不能完全消除稀有性劣势。