Pollination, a game of hide and seek

By Athayde Tonhasca

For bees, pollen is an indispensable source of protein for egg production and larval development. So if a bee had it her way, she would scoop up every pollen grain from a flower. And she’s good at it, storing pollen securely on specialised transport structures, usually on her legs or under her abdomen. She also grooms herself regularly to remove stray pollen grains stuck to her body. As a result of this meticulous work, some bees take about 99% of the powdery stuff back to their nests. The ‘wasted’ 1%, which accidentally drops off or is left clinging to the bees’ hairs, is all a plant has for pollination. 

A bee covered in pollen grains: most of them will be scooped up by the bee © Ragesoss, Wikipedia Creative Commons

Bees’ efficiency puts plants in a jam. They need flower visitors to transport pollen and 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 several strategies to keep visitors coming and at the same time minimizing pollen loss. Some species hide pollen inside their anthers (poricidal anthers), others produce indigestible or even toxic pollen so that only a few efficient, specialised pollinators can get to it; the palynivore hoi polloi is kept at bay. Another clever approach is to induce bees to be less efficient at grooming, so that more pollen grains are available for deposition on a receptive flower. And one way to accomplish this is through nototribic flowers. This term applies to flowers built in such way that their stamens and style come in contact with the dorsal surface of the bee’s body. They are common in the group of sage, mint and rosemary plants (family Lamiaceae) and figworts (family Scrophulariaceae). 

A honey bee on a meadow clary (Salvia pratensis) flower cut open laterally, and a schematic drawing showing the stamen touching the bee’s back © Reith, M. et al. 2007. Annals of botany 100: 393-400

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 grooming itself). 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 problem. The pollen grains deposited in this unreachable area are then taken to another flower.  

Pollen of meadow clary on the back of Bombus terrestris under UV light
© Koch, L. et al.  2017. PLOS ONE 12(9): e0182522

Some flowers hide pollen at the bottom of their corollas, and bees 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 are stuck to thick, curved hairs between the antennae; these grains can’t be groomed, so become possible pollination agents.

A fork-tailed flower bee has to use her head – literally – to pollinate © Nederlands Soortenregister, Wikipedia Creative Commons
Facial hairs of a fork-tailed flower bee © Muller, A. 1996. Biological Journal of the Linnean Society 57:  235-252

A few plants resort to making life difficult for bees whose habits are not the best for their interests.  And these could be corbiculate bees, that is, bees that carry pollen in their pollen baskets (corbiculae) such as honey bees and bumble bees. Corbiculate bees use regurgitated nectar to stick the pollen together so it can be bundled up nicely for transport. Few pollen grains detach from a corbicula, and the moisture quickly reduces their viability. Most plants live with that, but some would rather save their pollen for bees that transport it on their scopae, which are elongated setae (‘hairs’) on their legs or under the abdomen. These non-corbiculate bees are not as tidy as their corbiculate counterparts: they do not wet and compress the pollen, which is taken away just like dust particles clinging to the hairs of a brush or a broom (scopa, in Latin).

Pollen tightly packed on a bumble bee’s pollen basket (corbicula) (L) and loosely attached to the scopa (fringe of hairs in the abdomen) of a megachilid, a solitary bee © Tony Wills (L) and Vijay Cavale, Wikipedia Creative Commons

To discourage corbiculate bees from making off with their pollen, plants such as the common hollyhock (Alcea rosea) and other mallows (family Malvaceae) produce pollen covered with spines. These echinate (prickly; covered with spines or bristles) pollen grains are relatively large, difficult to handle and to mould into neat pellets. Echinate pollen is a headache for corbiculate bees, the efficient packers, but not a problem for messy pollen harvesters such as solitary bees. As a result, more pollen grains are dropped off from bees, increasing the chances of pollination. 

Echinate pollen grains from three Malvaceae species © Konzmann et al. 2019. Scientific Reports 9: 4705

All these adaptations illustrate the wonderful complexities of an evolutionary give and take: insect pollination is a negotiation between parties with conflicting interests. Plants can’t give away too much pollen but can’t risk being overly 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. Every plant-pollinator combination is an example of a mutually beneficial compromise. It’s natural selection as its best.