Close Encounters

When foraging for pollen from flowers, timing can be crucial

By timing their visits to twilight, just as nocturnal flowers are opening, some bees can grab most of Pseudobombax longiflorum's pollen.
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Some bees have evolved to visit flowers in twilight. New research by Priscila De Cássia Souza Araújo and colleagues finds that the shift to dawn and dusk allows the bees to operate without competition for a brief period of the day. The freedom allows them to remove over fifteen times as much pollen per minute from Pseudobombax longiflorum flowers than day or night time visitors. The study, published in the Biological Journal of the Linnean Society, is the first quantified measurement of how effective twilight foraging is for bees.

Most bees fly during the day due to the limitations of their eyesight. However, some bees have learned to use the first light of the morning or the dying light of the evening to exploit flowers that open for nocturnal visitors. As day and night exchange places, there’s a period where the nocturnal flowers are briefly open without many visitors. It means there is a period when the flowers are accessible with little competition. How useful is this period to a bee?

Pseudobombax longiflorum. Image: Mauro Halpern / Flickr.

To find out, De Cássia Souza Araújo and colleagues examined visitors to Pseudobombax longiflorum, a plant pollinated by bats. The team wanted to see if bees contributed to pollination and how effective they were at collecting pollen.

The scientists determined the bees’ contribution to pollination by bagging some of the flowers during the night and releasing them at dawn. This kept bats away from the flowers, leaving only the bees capable of pollinating them. Other flowers were left open for a comparison. The bagged flowers did not produce seeds. From this experiment, the team found the bees were not pollinating the flowers.

Nevertheless, the team also found that the bees took a large amount of pollen from the flowers. In their article, De Cássia Souza Araújo and colleagues write: “In the short twilight periods, when crepuscular bees foraged without competitors, they collected > 40% of the pollen content of a flower of Pseudobombax longiflorum in visits at dusk and dawn and > 30% of the pollen in flowers available to these bees only at dawn. Averaging both periods of exclusive crepuscular bee access (dusk and dawn), these bees collected ~550 pollen grains per anther and minute when there were no other visitor groups. This value of pollen removal is much higher than that for other visitors. This high efficiency of pollen collection by crepuscular bees is a consequence not only of their individual ability to remove pollen, but mainly because they visit the flowers when these are still rich in pollen.”

The bees are thoughtful about how they collect pollen. The scientists watched Ptiloglossa and carpenter bees inspecting floral buds at dusk before they opened. The unopened flowers were valuable as, when they opened, they held fresh pollen. Being first to the flowers made foraging easier. After the bees had visited bats, arriving later, found flowers with almost a quarter of the pollen gone.

The biologists think that the Ptiloglossa bees’ effectiveness at gathering pollen means the bees can collect all the pollen their larvae need in the short window of twilight before honeybees swarm over the landscape. The results prove the bee equivalent of the phrase the early bird catches the worm.

RESEARCH ARTICLE

De Cássia Souza Araújo, P., De Araujo, F.F., Mota, T., Schlindwein, C., 2021. The advantages of being crepuscular for bees: major pollen gain under low competition during the brief twilight period. Biological Journal of the Linnean Societyhttps://doi.org/10.1093/biolinnean/blab137

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