2. Site-faithful foraging by pollinators

Bumble bees are highly efficient foragers for pollen and nectar, and one of the major pollinators in temperate zones. As they gain experience, individual bees learn the locations of their favorite plants and visit them repeatedly along their favorite routes, i.e., traplines (Makino, Ohashi & Sakai 2007, see below).

Trapliners could avoid immediate returns to the same plants until nectar is refilled in flowers. This would benefit plants, too, because pollen could be delivered farther from the source plants (Ohashi & Thomson 2009). At the same time, trapliners may be a vexing problem for plants. For example, if plants reduce the nectar production to save energy, the bees will quickly learn to avoid such "cheater" plants.

In other words, bumble bees are "dependable but demanding customers" for flowers. We are interested in how flowers could effectively use them for pollination.

Bumble bees visiting a natural population of Cirsium purpuratum were individually numbered and their movements from plant to plant were recorded. We found that individual bees often followed fixed foraging routes (Makino, Ohashi & Sakai 2007). In addition, these traplining bees often incorporated plants with larger displays into their routes.

This has also been demonstrated in indoor cage experiments with artificial flowers secreting nectar at a constant rate. As bees gain foraging experience, they remember more rewarding plants and return to them more frequently (Ohashi et al. 2007).

Trapliners do not waste time moving back and forth between neighboring plants. This gives some advantages in competition with other foragers. Computer simulations have shown that these movements of trapliners also have a positive effect on the plant by delivering more pollen to a greater distance (Ohashi & Thomson 2009).

4. Anthesis timing as the determinant of arrival sequence of diverse pollinators

For flowers that bloom for more than 24 hours, the time they first open does not seem to matter much. However, many flowers open at a certain time of day, and the timing varies across plant species—some open in the morning, while others open in the evening. Why?

Adenophora flowers that open in the evening are primarily pollinated by nocturnal settling moths. Bees and hoverflies also visit the flowers during the daytime, but their pollination efficiency is much lower (Funamoto & Ohashi 2017, see below). Thus, the dusk opening in Adenophora flowers prioritizes visits by nocturnal moths and avoid the waste of pollen, nectar, and stigmatic surface by less efficient diurnal pollinators. We have recently found that a similar adaptation in anthesis timing of Salix caprea that blooms early spring. Salix flowers depend largely on diurnal pollination by bees and wind. But they open in the evening to leave the opportunity for pollination by nocturnal moths that is supposed to be infrequent but more efficient in terms of pollen transfer to stigmas (Ohashi & Jürgens 2021).

It will be interesting to see whether similar explanations apply to the anthesis timing of other species that are visited by a set of diverse pollinators varying in their activity time and pollination efficiency.

The flowers of Adenophora open just after sunset. Although the flowers last for several days, they secrete nectar only at night. The results of bagging experiments showed that pollination is carried out exclusively by nocturnal settling moths (1, 2), while diurnal bees (3) and hoverflies (4) visiting the flowers on the following day do not make much contribution, wasting pollen and nectar (Funamoto & Ohashi 2017). The dusk opening of Adenophora flowers is considered a strategy to prioritize the visits by moths while not ruling out the possibility of pollination by diurnal insects as "insurance" in case moths are infrequent.