How do bees choose pollen?
Collecting pollen is no small task : bees therefore use several senses and different techniques to choose it.
Evaluation
Nature offers a multitude of flowers of varying shapes and colours, each associated with a wide range of pollen types. Each pollen has a distinct nutritional value and composition ! Pollen is also crucial for larval feeding, adult growth, and sexual maturation. Bees therefore face a major challenge in selecting and assessing pollen in order to choose the most nutritious and abundant sources while minimising energy expenditure. But how do they manage this? While some aspects of their strategy were already known, researchers at the University of Exeter sought to understand the full set of “mechano-sensory” mechanisms (both mechanical and sensory) involved.
A multisensory and mechanical approach
“It appears that bees do not respond to a single component of pollen, such as crude protein content, but rather to a range of sensory signals within the pollen and the flower,” explains Dr Natalie Hempel de Ibarra, an expert in insect neuroethology. While it is relatively easy to determine the nutritional value of nectar, a sugar-rich secretion produced by flowers, assessing the nutritional quality of different pollens is far more complex due to their high variability. As bees rarely feed directly on the flower and instead collect pollen in so-called pollen baskets (corbiculae) on their legs or body hairs, they must rely on other senses to analyse it. Why do they not taste pollen more frequently? According to scientists, this is due to the limited number of taste receptor genes bees possess compared to other insects (10 versus 23 in bumblebees and 76 in fruit flies). When pollen rich in amino acids—representing about 20 % of pollen composition—was placed on cotton, bees exhibited a reward-related behaviour characterised by the extension of their proboscis. When the experiment was repeated with a sucrose solution, another pollen component, no food–reward association was observed. Bees therefore appear to be more sensitive to amino acids, while their gustatory perception of other components is very limited.
Fortunately, their sense of vision seems to be more developed. Although their eyes are small and have low spatial resolution, their visual abilities are highly effective. Bees appear to have preferences for certain flower colours and consequently forage on these more frequently. To test whether this preference is linked to pollen quality, researchers presented two coloured discs, one associated with highly nutritious pollen and the other with non-nutritive pollen. Initially, the better pollen was associated with the bees’ preferred colour, after which the association was reversed. The result was that the insects adapted and shifted to the other colour, clearly demonstrating that their colour choices are not random. However, due to the limited resolution of their vision, when the corolla and the anther (the pollen-bearing organ of the flower) are similar in colour, bees tend to land more on the corolla, likely because it is larger and therefore more visible.
Correlation test between bees’ preferred colour and the nutritional value of pollen – © University of Exeter
If taste and vision provide only partial information, researchers believe that bees use a third sense to assess pollen: smell. Bees can smell (and taste) using their antennae, which are equipped with chemoreceptors. The British research team observed that bees are able to distinguish the odours of different flower types, and thus different pollens, and that they appear to prefer strongly scented pollen. In natural conditions, however, this process is more complex, as flowers are intermixed and their scents overlap with those of surrounding elements.
Finally, it is also known that bees use vibratory movements to shake the anthers and release pollen. They adjust the duration and amplitude of these vibrations in response to the condition and type of flower in order to optimise the cost–benefit ratio of this action. In addition, bees are capable of detecting electric fields, which stimulate their mechanosensory hairs. Once pollen has been collected, the electric potential of the flower is altered, allowing bees to determine whether a flower has already been visited.
Cognitive and social processes
And that is not all. “Bees also remember the locations and types of flowers they have visited, and this experience influences their foraging decisions,” the researcher continues. Experience thus plays an important role. A bee that has become accustomed to collecting pollen rich in amino acids will tend to return preferentially to this type of pollen in the future. Pollen choice also affects nestmates, as bees, upon returning to the hive, perform a dance during which they indicate the distance of the pollen source from the nest, its direction relative to the sun, and the quality of the pollen found, by depositing a sample carried on their legs.
Bee dance
The bee dance has the shape of a figure eight. The bee positions itself according to the direction of the pollen source relative to the sun. The slower the dance, the farther the pollen source is from the hive. Finally, the greater the number of loops performed, the richer and more nutritious the pollen source.
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►see also: Identifying pollen