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Today we have a combination story-and-photo piece about pollination from reader Athayde Tonhasca Júnion. His notes are indented, and you can enlarge the photos by clicking on them.
Reluctant givers and industrious takers
For bees, pollen is an indispensable source of protein for egg production and larval development. So if they had it their way, bees would scoop up every pollen grain from a flower. And they are good at it, taking 95 to 99% of the powdery stuff back to their nests. The ‘wasted’ 1 to 5% of pollen that bees accidentally drop off or is left clinging to the bees’ hairs, is all a plant has for pollination.
Bees such as honey bees (Apis spp.) and bumble bees (Bombus spp.) carry almost all the pollen they gather 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 elongated setae (‘hairs’). These bees, unsurprisingly called corbiculate bees, moisten the pollen with regurgitated nectar and saliva, so that it can be bundled up nicely for transport and easily unloaded once bees are back at their nests.
A European honey bee’s pollen basket © Gilles San Martin, Wikimedia Commons:
A corbiculate bee grooms herself regularly to remove stray pollen grains stuck to her body: most of them will be scooped up and stored securely © Ragesoss, Wikimedia Commons:
Other bees carry pollen attached to their scopa (Latin for ‘broom’), an area of dense, stiff hairs on the hind legs (typically in the families Halictidae and Andrenidae) or on the underside of the abdomen (mostly in the family Megachilidae). These non-corbiculate bees are not as tidy as their corbiculate counterparts: they do not wet and compress the pollen, but instead take it away just like dust particles clinging to the bristles of a brush or a broom.
The scopa of a megachilid or leaf-cutter bee © Pollinator, Wikimedia Commons:
Transporting pollen on the corbiculae or 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 wet. Pollen on a scopa is kept dry and loosely attached to the bee, so it has a greater probability of being dislodged and resulting in plant fertilisation.
A load of pollen in a bumble bee’s pollen basket © Tony Wills, Wikimedia Commons:
A chocolate mining bee (Andrena scotica) with pollen loosely attached to its legs:
Regardless of how pollen is hauled away, bees’ efficiency puts plants in a jam. They need flower visitors for sexual reproduction, but the greedy blighters want it all for themselves. Pollen is metabolically expensive, so a plant can’t afford to produce lots of it and then lose most to palynivores (pollen eaters). But if it produces too little, bees may not be interested in dropping by.
To deal with this dilemma, plants have evolved strategies to keep visitors coming and at the same time not making it easy for them, thus minimising pollen profligacy. One cunning way to do this is to interfere with bees’ ability to groom themselves, so that more pollen grains are likely to be missed and end up on a receptive flower. To do this, there’s nothing better than nototribic flowers, which are built with an elaborate lever mechanism that makes stamens and style touch the dorsal surface of a visiting insect. This device is common in sage, mint and rosemary plants (family Lamiaceae), and in figworts (family Scrophulariaceae).
When a male Anthophora dufourii probes a Salvia hierosolymitana flower for nectar, its stamens are lowered to deposit pollen on the bee’s back © Gideon Pisanty, Wikimedia Commons:
Bees use their front legs to wipe their heads and antenna, and their middle and hind legs to clean their thoraxes and abdomens – you may have watched a bee or other insect doing these cleaning manoeuvres. But the space between their wings is a blind spot: think about an itch right between your shoulder blades, and you will understand the bee’s pickle. The pollen grains deposited on this hard-to-reach area are likely to escape grooming efforts and be taken to another flower.
Pollen of meadow clary (Salvia pratensis) seen under UV light on the back of B. terrestris © Koch et al., 2017:
Some flowers hide pollen at the bottom of their corollas, and visitors such as the fork-tailed flower bee (Anthophora furcata) must creep into these narrow, tubular structures that don’t allow much moving about. The bee vibrates her flight muscles to release the pollen, which gets attached to her head. She pulls out of the flower and scoops up the pollen with her front legs, but not all of it. Some grains become stuck to the thick, curved hairs sticking out between her antennae; these grains could end up on another flower.
A fork-tailed flower bee has to use her head – literally – to pollinate © Dick Belgers, Wikimedia Commons:
The common hollyhock (Alcea rosea) and other mallows (family Malvaceae) use a different tactic: they induce some bees to be less efficient gatherers thanks to their echinate pollen. Besides being prickly (echinate: covered with spines or bristles), these pollen grains are relatively large, thus difficult to handle and to mould into neat pellets. These features constitute a headache for corbiculate bees, the proficient packers, but are less of a problem for sloppy pollen harvesters such as solitary bees. As a result, more pollen grains are likely to be dislodged from bees who bother visiting these plants, increasing their chances of pollination.
Echinate pollen grains from three Malvaceae species © Konzmann et al. 2019:
Plants have developed other adaptations to minimise pollen harvesting, such as complex flower structures or progressive pollen release to force pollinators to make repeated visits. Some species hide pollen inside poricidal anthers, others produce indigestible or even toxic pollen so that only a few specialised pollinators can get to it; the palynivore hoi polloi is kept at bay. Many plants such as orchids are downright cheats: they lure pollinators with scent or visual mimicry but do not give away any nectar or pollen in return.
All these adaptations demonstrate that pollination is a negotiation between parties with conflicting interests. There is nothing altruistic here, bees and flowers are taking advantage of each other in an evolutionary give and take. Granted, this mutual exploitation has been fine-tuned in order to avoid disastrous imbalances. Plants can’t afford giving away too much pollen but can’t risk being too stingy; bees would take all the pollen they could handle, but settle for what’s available as long it’s worth their time and energy. Overly parsimonious plants and overly rapacious bees would collapse the relationship. Every plant-pollinator combination is an example of a mutually beneficial compromise; it’s natural selection as its best.