The majestic horse chestnut flowers in April–May and offers bees mainly pollen. Which cue helps bees recognise the flowers that have not yet been pollinated?
- The flower emits a long-lasting pheromone that attracts bees that have already visited.
- The pollen contains an addictive molecule that forces the foragers to return.
- Once pollinated, the flowers change colour.
Show answer
Correct answer: 3.
Once pollinated, the flowers change colour.
Why?
In the horse chestnut, unpollinated flowers usually bear a clearly visible yellow patch. After pollination, this patch turns pink or red.
For the bee, this colour change acts as a floral guide. Flowers with a yellow patch are more worthwhile to visit, since they indicate a flower that is still useful for foraging and for pollination.
Since bees perceive red poorly, flowers that have already been pollinated become less attractive. The forager thereby saves time and energy, while the tree increases the efficiency of its pollination.
What to understand
This is an elegant example of communication between plant and pollinator. The horse chestnut does not “speak” to bees, but provides them with an exploitable visual signal.
Bees use numerous cues to choose flowers: colour, smell, shape, warmth, availability of nectar or pollen, and sometimes scent marks left by other foragers.
In this specific case, the main cue is neither a pheromone emitted by the flower nor an addictive substance in the pollen, but the colour change of the floral centre after pollination.
Key points
In the horse chestnut, a yellow patch often indicates a flower that is still attractive to bees.
After pollination, this patch turns pink or red, which makes the flower less attractive to foragers.
This mechanism improves the efficiency of foraging and pollination: the bee avoids flowers already visited, and the plant directs pollinators towards the flowers still available.