How and why do honeybees pollinate flowers?

When you watch honeybees at work, visiting flowers from dawn to dusk, it’s easy to assume that pollinating flowers is their job — a job they seem very keen to do. The truth is, honeybees have no innate desire to pollinate flowers; they are really just out for themselves gathering resources from flowers with no idea that they are pollinating them. 

Yet, honeybees are the most prolific pollinators on the planet. They are not only responsible for pollinating thousands of naturally occurring plant species around the world, but also ensuring that crops (with an estimated annual yield value of between $235 to $577 billion) are pollinated every year. But, how can honeybees be such great pollinators if they have no desire to be good at it?
As honeybees drink nectar from flowers, they inadvertently pollinate the flowers.
A honeybee drinks nectar while the flower places pollen on the bee in hard-to-reach spots. Also, note the collected pollen on the bee’s hind leg (known as “pollen baskets”); this collected pollen, which is mixed with the honeybee’s saliva, has become sterile and will be used to feed young honeybees. (Photo credit: Adobe stock [64096194])
Outsourced mating
The answer to this question lies in why flowers exist in the first place. Because plants are literally rooted to the ground (unlike mobile animals), they face a unique problem — mating is difficult when you cannot move. Flowers are plants’ solution to this problem. Plants lure honeybees (and other pollinators) with colourful, scented flowers. When a bee lands on a flower to collect nectar or pollen, the flower sneakily places pollen onto the body of the bee. It’s like a transaction: the bee gets food, and the plant gets to place pollen on the bee’s body to deliver to other flowers. 
Pollen grains are placed on a honeybee's wing. As the honeybee forages, pollen is transferred to other plants, making the honeybee an inadvertent pollinator.
Golden bean-shaped pollen grains on a honeybee’s wing. These pollen grains come from a South African plant called a butterfly lily, and they are particularly sticky. (Photo credit: Corneile Minnaar)

As the bee moves from flower to flower, it transfers the pollen from one flower to stigmas (the female part of the flower that captures pollen) of another flower. This ensures pollination and eventually fruit formation. Bees are therefore the unwitting pollen couriers of the plant world. It doesn’t matter that honeybees don’t know that they are pollinating flowers. As long as flowers provide food, they will keep visiting them and move pollen around for the plants.

Honeybees pollinate flowers by moving pollen from one flower to the stigmas of another flower.
As a honeybee transfers pollen from the anther of one flower to the stigma of another flower, the plant is pollinated.
(Illustration: Willem Augustyn-Goussard)
Flowers are pollen placement devices
While pollen couriers solve the immobility problem for plants, it is far from efficient; typically, less than 5% of a plant’s pollen grains end up being transferred to other flowers. So, plants use all kinds of strategies to tip the odds in their favour. For example, flowers place pollen on specific spots on a bee’s body, to decrease pollen loss. Rosemary flowers place pollen on honeybees’ backs since they find it difficult to reach and can therefore not groom the pollen off of their bodies. This ensures that the pollen stays on bees — resulting in better pollination. Some plants even place pollen on the wings of honeybees (an even harder spot for honeybees to reach).   
A rosemary flower places pollen on the bee's back. When the bee visits another rosemary flower, it will pollinate that flower.
A bee visiting a rosemary flower. Take note of the pollen on the honeybee’s back. Rosemary flowers place pollen here since honeybees find it difficult to reach and can therefore not groom the pollen off of their bodies. (Photo credit: Adobe stock [22155671])

This give-and-take relationship between flowers and honeybees works best when the flowers provide good rewards to the bees. The more pollen and nectar a crop provides to honeybees, the more likely they will be to keep visiting that crop. Pollination is likely to be better for these crops than less-rewarding crops.

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