In-hive pollen transfer among honey bees
When we think of pollination we imagine a honey bee foraging on a flower, collecting nectar or pollen, and inadvertently transferring some of the pollen to the next flower she visits.
I’ve always thought that solitary bees might be more efficient pollinators for several reasons. For example, solitary bees seem to fly faster between flowers, collect pollen more quickly, and take little nectar compared to honey bees. In addition, solitary bees carry their pollen in its natural fluffy state that is readily usable by flowers.
On the other hand, honey bees and bumble bees glue their pollen loads together with nectar, making a tightly-packed pellet that is useless for pollination. In fact, most of the pollen that honey bees transfer from flower to flower is stuck to their bodies in random places. When foraging on the next flower, some of this pollen rubs onto the stigma and pollinates the flower, but the pollen in the pellets doesn’t budge.
Floral fidelity in honey bees
Other habits of honey bees make them excellent pollinators. Honey bees have amazing floral fidelity, a trait that keeps them visiting the same kind of flower over and over. Floral fidelity is a huge boost to pollination, but it is frequently absent in other bee species. When a single bee carries many types of pollen, she is less likely to deliver the right type to the next flower.
But even floral fidelity has its drawbacks. A good example can be seen in fruit tree pollination. Varieties of fruit are often self-incompatible, which means one variety needs to be cross pollinated by a different variety of the same species. Apple, sweet cherry, and almond varieties are often interplanted in the orchard so they can easily cross pollinate.
Of course, these trees often have very dense flower clusters, so dense that a honey bee can actually walk from flower to flower. In fact, she can spend her entire foraging trip on one limb of one tree and never cross-pollinate anything. Imagine the moxie of those bees! You can find an excellent discussion of this phenomenon by Gloria Degrandi-Hoffman in The Hive and The Honey Bee (2015).
In-hive pollen transfer among social bees
In spite of this, researchers have found that honey bees still pollinate many of the flowers, even the first first flower they visit. It turns out that foragers, even those who have never foraged before, have pollen on their bodies due to in-hive pollen transfer. Inside the hive, the densely-packed bees rub against each other, transferring pollen from bee to bee.
How much pollination actually occurs this way has been studied in several crops. A wide variation has been found but, generally, pollination due to in-hive pollen transfer is greatest when the bees are in monocultures where other pollen types are scarce. So in a large apple orchard containing several varieties, pollination from this method may be substantial. In situations where one colony is foraging on several different crops at once, the amount of pollination due to in-hive transfer drops. In-hive transfer is also greater in crops that produce a lot of pollen, and less in those that produce little.
Because solitary bees live alone, they do not rub against each other, and therefore they cannot transfer pollen by this method. It is interesting to note that in-hive, or more accurately, “in-nest” pollen transfer has also been found in bumble bees, which are highly social as well.
Honey Bee Suite