Halictus sweat bee visiting Helminthotheca echioides (prickly ox tongue). (c) 2012 MRS All rights reserved.
Whenever I do education outreach, especially with small children, I like to paint a rosy picture of pollination. The bees and the plants help each other; the bee collects food from flowers, and in return they transfer pollen from the male parts of flowers to the female parts of flowers, which results in the production of seeds. We break out the cute fuzzy bee hand puppet and use velcro yellow balls (simulating pollen), which the kids can transfer from felt flower to felt flower.
When viewed in the visible light spectrum (left), this Mimulus flower looks uniformly yellow. However, when viewed in the UV spectrum (visible to bees- right), a dark arrow-shaped wedge points to the nectaries at the base of the flower.
source: wikipedia.org (Plantsurfer 2009)
When I talk with older audiences, I might portray a slightly racier scenario. After all, what is pollination other than flower sex mediated by a third party? In the photograph above, look at how tenderly the Halictus
hooks its hind leg around the seductively curved stigma of the flower, at the profusion of pollen she has so carefully collected on the hairs of her hind legs to carry between flowers. Despite the more adult tack I use, I still tend to emphasize that this is a mutualistic
engagement; the flower has evolved nectar and enticing patterns to seduce the bees, and in return the bees transfer pollen between plants. A nice example of this are something called nectar guides
, or patterns in the UV spectrum (visible to bees but not to us) that guide bees towards the nectar reward in the flower.
And so the flowers grow. And so we get fruits and nuts and seeds. And so this green planet fluorishes.
As idyllic and parabolic as this picture may be, it brushes over a Machiavellian but more accurate version of how pollination (and evolution, for that matter) really works. In a way, I feel guilty teaching this airbrushed version of pollination because in some ways it may misrepresent how evolution and symbiotic relationships work in real life: as an escalating arms race.
In this darker world, flowers evolve to attract pollinators in order to increase reproductive fitness. In parallel, bees evolve to collect sugar rich nectar and protein rich pollen from flowers. As a byproduct, flowers feed bees and bees pollinate flowers, but this is not the purpose of the plant or bee, merely a byproduct of their concurrent actions.
And thus we enter the dark underbelly of the world of pollination, full of cheats and robbers and generally bad behavior. After all, it takes energy on the plant’s part to produce nectar and nutritious pollen, and pollen that is transfered by bees between plants is pollen that the bee cannot eat. If a bee can find a way to eat without pollinating, or a plant can find a way to be pollinated without feeding the bee, this may give them an edge over their competitors. One basic example of this that we see are bees that fastidiously clean all excess pollen off themselves before leaving a flower. Especially for a small bee, pollen can be quite heavy and make it more difficult to fly. By cleaning itself before takeoff, a bee can travel between flowers more easily but does not pollinate the next plant it visits.
Let’s take a look at the aspects of pollination I leave out of my classroom visits: the sex, the drugs, and the violence.
Sex and Drugs: The Cheating Orchid
The Bee Orchid preys on the sexual urges of male bees and wasps, luring them with drugs called pheromone mimics into a senseless orgy of failed mating. The flower copies in its shape, color, and texture a female bee or wasp. Furthermore, it entices males by releasing an intoxicating perfume: the smell of a female who is ready to mate. Hapless males converge on the flower and fight each other for the chance to attempt to mate. In the process, pollen packets called pollenia attach to the male. The next time it tries to mate with an ersatz bee, the pollenia make contact with the female part of the flower. In this scenario, the cheating orchid reaps all the benefits of pollination while the males get nothing but frustration and lost mating opportunities. For a great video of bee orchids in action (not to mention a rockin’ synthesizer soundtrack), check out this neat excerpt from Wild Orchids of Israel, filmed by Doron Hirschberg:
And Now for some Violence: Big Bad Carpenter Bees
Bees are no strangers to cheating at the pollination game, either. In my Xylocopa post, maybe you noticed that the gargantuan carpenter bees, rather than venturing into the trumpet-shaped structure of the sage flower, clutched onto the outside of the flower. Perhaps you even noticed a dark, drinking straw structure unfurled from the head of the bee and piercing into the flower through the side, rather than the front entrance.
Xylocopa robs the nectar from the an Autumn Sage (Salvia gregii) flower
The sage flower is designed for a pollinator to enter the main opening of the flower in order to reach the nectar. In the constricted petal tube, the bee must crawl past the anthers (pollen bearing structures) to reach the nectar, thus getting covered in pollen. This carpenter bee has other plans, however.
Using its heavily armored mandibles (used to chew through rotting wood to build nests), the carpenter bee rips a hole in the base of the petals, next to the nectar reward. It then sticks its proboscis in through the hole it has ripped, like a kid in a Tropicana orange juice commercial, and drinks the nectar without ever touching the anthers of the flower. The defenseless flower, once ripped open, is now vulnerable to other smaller bees that can use the hole made by the carpenter bee to access nectar directly. This process is called nectar robbing, because the flower is robbed of its nectar but reaps no pollination reward.
So the next time you go out to enjoy a pleasant Spring day, as you watch the industrious bees trundle from flower to flower, take a moment to reflect on the dark designs of both flower and bee. Although salacious and exploitative, I don’t like to think of this interplay in a negative light. To me, the complexity of coevolution makes this relationship more interesting, more beautiful, more strange than the boring stories we tell our children. This intricate biological world we live in operates by its own set of rules separate from Western human morality, and we have much more to gain by learning and appreciating these rules rather than placing value judgments on them.