Bumblebee flower constancy and pollen diversity over time

Foraging bumblebees often focus on certain flowers. By studying pollen foraging, we show that this focus is not long-lasting in the field, and that the visited flower species vary over time. Only in less than a quarter of the cases, buff-tailed bumblebee individuals focused on certain flowers during foraging trips. Instead, most individuals collected several types of pollen. The more pollen types each individual collected, the more pollen types were collected to the bumblebee colony. Bees often focus their foraging effort on a few or even a single flower species, even if other equally rewarding flower species are present. Although this phenomenon-called flower constancy-has been widely documented during single foraging trips, it is largely unknown if the behavior persists over longer time periods, especially under field conditions with large temporal variations of resources. We studied the pollen diet of individuals from nine different Bombus terrestris colonies for up to 6 weeks, to investigate flower constancy and pollen diversity of individuals and colonies, and how these change over time. We expected high degrees of flower constancy and foraging consistency over time, based on foraging theory and previous studies. Instead, we found that only 23% of the pollen foraging trips were flower constant. The fraction of constant pollen samples did not change over the study period, although repeatedly sampled individuals that were flower constant once often showed different preferences at other sampling occasions. The similarity of pollen composition in samples collected by the same individuals at different occasions dropped with time. This suggests that the flower preferences change in response to shifting floral resources. The average diversity of pollen from single foraging trips was around 2.5 pollen types, while the colony-level pollen diversity was about three times higher. How rapidly preferences change in response to shifting resources, and if this differs between and within bee species depending on factors such as size, should be the focus of future research.

Automated summary

Bumblebees often focus on certain flowers for pollen foraging, but this focus is not long-lasting in the field, with the visited flower species varying over time. Most individuals collect several types of pollen, and the more pollen types they collect, the more diversity is contributed to the bumblebee colony. Flower constancy, focusing on a few or even a single flower species, is a widely observed behavior during single foraging trips, but its persistence over longer time periods is largely unknown. Only 23% of the pollen foraging trips were flower constant, and repeatedly sampled individuals showed different preferences at other sampling occasions. The preferences change in response to shifting floral resources.