Abstract:

Aims: The color change of flowers during their blooming process can serve as a cue that guides pollinators to preferentially visit pre-change color flowers. However, it remains unclear whether this color change influences nectar robbers’ behavior.

Methods: To explore whether nectar robbers prefer pre-change color flowers and the impact of nectar robbing on plant reproductive success, we used Lonicera calcarata (Caprifoliaceae) as the study subject. Its flowering period lasts for three days, during which the flower color changes from white to yellow and then to orange-red. We recorded the visitation behavior of flower-visiting insects in the field and calculated the frequencies of legitimate visits and nectar robbing. We also analyzed the nectar robbing rates of three flower colors in different populations, measured the morphological traits of normal and robbed flowers, assessed the nectar volume and sugar concentration in both normal and robbed flowers during blooming, and examined the total pollen and ovule counts in flower buds from normal and robbed flowers. Finally, we performed different artificial pollination treatments to calculate seed set.

Results: Flower visitors to L. calcarata included moths, butterflies, and bumblebees, among which bumblebees could both legitimately visit the flowers and rob nectar from flowers of different colors. The overall nectar robbing frequency was higher than the frequency of legitimate visits for bumblebees. Nectar robbing rates of L. calcarata varied across different populations, with the nectar robbing rate of orange-red and yellow flowers being significantly higher than that of white flowers. The floral traits of robbed flowers were smaller than normal flowers in general, especially the lip and anther size. The holes in nectar spurs of orange-red and yellow flowers were significantly larger than those in white flowers. In all the three phases, nectar robbing significantly reduced the nectar volume of one flower, but did not affect the nectar sugar concentration. There were no significant differences in the total number of pollen grains in the buds between robbed and normal flower, as well as the ovules. Seed sets of pollen grains from normal and robbed flowers pollinated on the stigmas of normal flowers were not significantly different. Likewise, the seed sets of pollen from normal flowers pollinated on the stigma of normal and robbed flowers showed no significant differences.

Conclusion: This study preliminarily indicates that nectar robbing bees, particularly bumblebees, do not preferentially visit pre-change color flowers. Nectar robbing reduces nectar secretion and it does not directly affect the plant's female and male fitness. The influence of flower color change on flower-visiting insect behavior requires further and more sustained investigation.

Key words: nectar robbing, reproductive fitness, pollinators, nectar robber, Lonicera calcarata, floral color change