When we watch a bee heavy with pollen lumbering away from a flower, we may be tempted to think it is purposely doing the plant a favour. But there is nothing altruistic here; the bee will try to keep for itself as many pollen grains as possible to provide the proteins that are essential for the development of its eggs and larvae, writes Athayde Tonhasca.
Bees may take 95 to 99% of pollen back to their nests, leaving the remainder – unintentionally – for pollination. But in this liaison it is tit for tat: plants have developed adaptations to minimise pollen harvesting, such as inconspicuous anthers, narrow floral tubes, difficult flower structures, or progressive pollen release to force pollinators to make repeated visits. Some plants like orchids also cheat by attracting pollinators with scent but not giving any nectar or pollen in return (see The Trickster).
Rather than collaborating, then, bees and flowers are taking advantage of each other. Granted, this mutual exploitation has been fine-tuned by natural selection in order to avoid disastrous imbalances: overly rapacious bees and pollen-stingy plants would just collapse the pollination relationship.
Right now, chocolate mining bees (Andrena scotica) are well into the business of collecting pollen from hawthorns, blackthorns, willows, dandelions, brambles, fruit trees, and many other spring-blossoming plants throughout UK – see solitary success story. The red mason bee (Osmia bicornis) will soon be out in number too, pollinating apple trees and several other plants. Follow this link for more on the red mason bee.
These bees take away pollen attached to their scopa (Latin for “broom”) which is an area of dense, stiff hairs used specifically for this purpose. The scopa of the chocolate mining bee is on its hind legs, and in the case of the red mason bee, on the underside of the abdomen.
A mining bee scopa ©Allan Smith-Pardo, Bees of the United States, USDA APHIS PPQ, Bugwood.org, Wikipedia Creative Commons.
A chocolate mining bee with pollen attached to its legs © Athayde Tonhasca.
A red mason bee loaded with pollen in its abdominal scopa ©Jeremy Early, Nature Imaging Conservation, http://www.natureconservationimaging.com.
Other bees such as the honey bee and bumble bees carry almost all their pollen in their corbiculae or pollen baskets. From the Latin diminutive of corbis (basket), the corbicula is a shallow leg cavity surrounded by a fringe of hairs (a modified scopa). These bees, not surprisingly called corbiculate bees, moisten the pollen with regurgitated nectar and saliva, so that it can be conveniently packed for transport and easily unloaded once the bee reaches the nest.Corbiculate bees are also good at self-grooming to remove pollen attached to their hairs.
The hind leg of a honey bee. © Gilles San Martin, Wikipedia Creative Commons.
Carrying pollen in the corbiculae or the scopa makes a world of difference for pollination. Pollen tightly packed in the corbiculae is not easily stripped off by floral structures when the bee visits another plant, and it quickly loses its reproductive viability because it has been wetted. Pollen attached to the scopa on the other hand is dry and loosely attached to the bee, so it has a much greater probability of being dislodged and resulting in plant fertilisation.
Bumblebee with a loaded pollen basket © Beatriz Moisset, Wikipedia Creative Commons.
So paradoxically, bees that are very good pollen harvesters are not necessarily the best for plants, although they may compensate their over-tidiness with their large numbers and high frequency of flower visitation. Bees that are sloppy with their pollen such as the chocolate mining bee and the mason bee are more prone to the happy accident of pollination. It is not surprising then that we keep finding evidence of the importance of solitary bees for the pollination of crops and wildflowers.
Scottish pollinators 