Reluctant givers and industrious takers

By Athayde Tonhasca

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’), which is 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, and 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 Malvaceae and other 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.

A sloppy but efficient pollinator

By Athayde Tonhasca

We hear a lot about the pollination services provided by the European honey bee (Apis mellifera), so you may be surprised to know this bee is not that competent at its job. A honey bee moistens the pollen she collects and carries it tightly packed on her corbicula, or pollen baskets, so pollen grains are not easily dislodged when the bee visits another flower. Moreover, honey bees learn quickly to collect nectar with minimal contact with the flower’s anthers, so reducing the chances of pollen transfer. They are also good at flower constancy (the trait of visiting the same type of flower over and over), which is not good for plants that need cross-pollination between different varieties, such as apples. Thus, paradoxically, honey bees’ efficiency as food collectors reduces their efficiency as pollinators. These shortcomings are offset by the huge numbers of bee workers per hive and the fact that they are so amenable to management.

In comparison with the tidy honey bee, the red mason bee (Osmia bicornis) is a messy flower visitor. Females have low flower constancy, flying all over the place, and carry dry pollen loosely attached to their scopa (a mass of hairs under the abdomen). This means that pollen grains have a greater chance of becoming detached from the bees’ bodies and ending up on a flower’s stigma.

A red mason bee with her scopa loaded with pollen © Jeremy Early, Nature Conservation Imaging

What’s more, the red mason is one the most polylectic bees in Europe, that is, it collects pollen from a variety of flowers from unrelated species: 18 plant families altogether, including willows (Salix spp.), maples (Acer spp.), birch (Betula spp.), oaks (Quercus spp.) and several fruit trees in the family Rosaceae such as apples, pears, plums, cherries and peaches. Unsurprisingly, this bee is an excellent orchard pollinator; 500 or so female red masons can pollinate as many trees as 2-4 honey bee colonies. 

Like other Osmia species, the red mason is a cavity-nesting bee; it makes itself at home in preexisting holes and fissures in soil banks or dead wood, abandoned insect burrows, hollow stems, or cracks and holes in walls – which explains the common name, ‘the mason bee’. It may also excavate soft mortar, hence the reason for another common name: ‘the mortar bee.’ The red mason readily occupies man-made structures such as ventilation bricks, the space beneath roof tiles, even inside door locks. So this bee is the most likely tenant of bee houses.

Once a female occupies a cavity, she will construct a series of compartments (brood cells) and stock them with pollen as food for her offspring. She will then close the nest entrance with a mud plug. But she’s not done once the nest is finished: if conditions are right, she may build another six nests before the season is over. The larvae will eat the pollen and emerge as adults the following year to start the cycle again. See red mason bees in action here.

A session of a mason bee nest. Each cell contains one egg and a provision of pollen 

Mason bees tend to nest close to each other in aggregations of 50 to 250 females. And they are diligent pollinators, as demonstrated by these facts and figures:

• A female bee may construct 16 cells per nest, 1 cell/day.
• She will fly 300-400 m on average, up to 600 m, in search of flowers.
• Nineteen foraging trips are needed to collect the pollen and nectar for each cell.
• Her pollen load weighs 100-250 mg, up to 300 mg.
• She may visit 75 flowers each trip, up to 25 flowers/min, and she will stock up each cell in about 3.5 h.
• A cell with an egg that will develop into a female bee may contain 8 million grains of pollen. Fewer for male bees (they need less food): 4.6 million.

This hardworking bee is good news for wild flowers, and also for crop production. The red mason is an effective pollinator of rapeseed oil and a number of crops grown under polytunnels and glasshouses, such as strawberries and raspberries. Other mason bees have been managed as orchard pollinators in Japan and USA for many years; there is growing evidence that the red mason can play a similar role in orchards in Britain and other European countries.

A female red mason bee and sealed nests in a bee house

The red mason bee is common throughout most of the UK from late March to June/July. During this short time as an imago (the adult stage), this bee will contribute to the pollination of countless wild flowers, crops and fruit trees. The red mason bee deserves to share the spotlight with the honey bee.     

A bitter-sweet medication

By Athayde Tonhasca

Most flowering plants need to keep pollinators happy. If not, the flow of pollen from flower to flower is interrupted or reduced, which will impair or prevent plant fertilization. To avoid such a disaster, many plants entice flower visitors with an irresistible reward: nectar. This solution contains 15% to 75% (by weight) of sugars – mostly glucose, fructose and sucrose – free amino acids, proteins, minerals and lipids. Bees, wasps, hover flies, mosquitoes, butterflies, moths, hummingbirds and bats are among the most enthusiastic consumers of this energy-packed drink. Not all nectar-eaters are pollinators, but nectar pilfering is a price plants have to pay to get pollinated.

A hummingbird hawk moth (Macroglossum stellatarum) taking a sip of nectar © Charles J. Sharp, Wikipedia Creative Commons

But nectar is more than nutrients dissolved in water. It contains a variety of secondary metabolites (compounds that are not directly involved in an organism’s development) such as tannins, phenols, alkaloids, flavonoids and terpenoids. The role of these chemicals are not completely understood. Some of them are indigestible, unpleasant (too bitter) or toxic to animals, so they defend plants against plant eaters, pollinators included: bees and other insects can be poisoned by secondary metabolites in nectar and pollen. But some nectar-diluted chemicals have positive effects: caffeine enhances pollinators’ memory, while other substances act as addictive stimulants, attracting insects or inducing them to stay around for longer, thus increasing the chances of pollination. 

These metabolites play another part in plant’s lives, one which importance is being increasingly recognized: as antimicrobial agents. The evening trumpet flower or yellow jessamine (Gelsemium sempervirens) provides a nice example of nectar’s medicinal qualities.

This garden plant, native to warmer parts of the Americas, is loaded with the alkaloid gelsemine. This strychnine-related chemical makes the whole plant toxic to humans, livestock and to honey bees. Bumble bees however not only are immune to it, but they also get some protection against Crithidia bombi, a widespread gut parasite that reduces the development and survival of colonies. Gelsemine-laced nectar may lower the rate of C. bombi infection by 65%.

The evening trumpet flower © Kenpei, Wikipedia Creative Commons

Gelsemine is not the only natural prophylactic against C. bombi. Callunene, found in the nectar of common heather (Calluna vulgaris), reduces the parasite’s infectivity against the buff-tailed bumble bee (Bombus terrestris), and in this case we know how. Crithidia bombi is a flagellated protozoan, that is, a single cell organism with a whip-like appendage called a flagellum. Callunene induces the loss of the flagellum, which the parasite uses to attach itself to the bumble bee gut.   

Crithidia bombi © R. Schmid-Hempel, ETH Zurich

Heather, together with white clover (Trifolium repens), marsh thistle (Cirsium palustre), and bell heather (Erica cinerea), are responsible for about 50% of all the nectar produced by flowering plants in the United Kingdom. We can only imagine the protective effect of heather on bumble bee populations. As virtually all plans secrete some secondary metabolites with their nectar, certainly there is much more to be discovered about their medicinal properties and consequences for pollination services.

A field of common heather, bumble bees’ pharmacy © Rasbak, Wikipedia Creative Commons

A greater understanding of nectar pharmacology may benefit us directly. Various alkaloid, terpenoid and phenolic compounds are lethal to other protozoans related to C. bombi (family Trypanosomatidae). Some of these trypanosomatids are responsible for awful diseases, like Trypanosoma brucei, which causes human sleeping sickness, and T. cruzi, the agent of Chagas disease. So, who knows: a healthy bumble bee may be a clue for reducing human suffering.

The writing on the pollen

By Athayde Tonhasca

Palynology, from the Greek palynō (‘sprinkle’), is the study of microscopic organic particles such as spores, planktonic organisms and pollen. The branch of palynology focussed on pollen contained in honey is called melissopalynology – the ‘melisso’ part refers to bees, from the Greek mélissa for ‘bee’. This tongue-twister sounds as arid and academic as a study subject could be, but this is not the case. By examining the pollen in a sample of honey, it is possible to determine its purity, geographical location and floral sources. Melissopalynology is a tool to combat fraud and inaccurate labelling of honey, which is handy if you want to buy pure Elvish honey from Turkey, Manuka honey from New Zealand, or other expensive honey bee products. 

Morphology of different pollen from honey samples © Obigba, S.O. 2021. IntechOpen 10.5772/intechopen.97755

Melissopalynology has other applications. It can be used to assess climatic conditions such as rainfall and temperature at the honey’s place of origin, or tell us about environmental changes. This is particularly useful in light of the rapid losses of natural habitats around the world, mostly because of agricultural expansion and intensification.

Half of the world’s habitable land is used for agriculture © OurWorldinData.org

By comparing pollen in honey samples from 1952 and 2017, researchers recorded a profound shift in the most important flower sources for honey bees in the UK. White clover (Trifolium repens) was present in 74% of the 1954 honey samples, but it decreased to about 30% in 2017. Brambles (Rubus fruticosus agg.) and related species were found in 5% of the 1954 samples, but in 2017 they became the most foraged plants.

Predominant and secondary honey bee pollen sources in 1952 (left) and 2017 © Jones, L. et al. 2021. Communications Biology 4, 37

These changes in honey bee preferences have reasons: with the intensification of managed grasslands, crop rotation decreased, and the use of fertilizers and herbicides increased. So clovers became less abundant; countryside surveys revealed a 13% decrease of white clover distribution from 1978 to 2007. Changes in the rural landscape favoured opportunists like brambles and invasive species such as Himalayan balsam (Impatiens glandulifera) (a great source of nectar), which increased their distributions from 1978 to 2007 by 21 and 100%, respectively.

A change of main course for honey bees; from white clover (L) to brambles © Harry Rose (L) and H. Zell, Wikipedia Creative Commons

A shift in sources of food may not sound too bad; honey bees still have their flowers. But not all flowers are equal: brambles have lower levels of proteins and essential amino acids than white clover. Honey bees may compensate for this nutritional deficit, but that requires more time and energy spent on foraging.

Pollen is the main source of proteins, fat, minerals and vitamins for honey bees and many other pollinators. But it is also a rich source of information: pollen attached to museum specimens has helped us understand changes in the distribution and abundance of bumble bees and other pollinators. Taxonomists, earth scientists, climatologists, archaeologists, palaeontologists, and criminal investigators – those examining suspect honey, or those tracing a body, dead or alive, to specific surroundings – have also tapped into the data extracted from pollen. So, a shout out to palynologists, who understand so well the value of those grains of plant dust.

Palynologist Dr Kat Holt doing climate research © Phys.org

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.

Smorgasbord or Spartan: the consequences of pollen diets

By Athayde Tonhasca

There is nothing visibly remarkable about the mining bee Andrena florea. This bee, one of the 67 Andrena species in Britain, is found in open scrubby areas, grassland and woodland edges of south-east England. But one thing makes this bee unusual; it only takes pollen from white bryony (Bryonia dioica).

Andrena florea, which is commonly and unsurprisingly called the white bryony mining bee, is a rare British example of a bee that forages on a single plant species. This dietary restriction is circumstantial, because white bryony is the only plant of this group occurring in Britain. In continental Europe, A. florea has other Bryonia species available. So in a wider geographical context, this bee is oligolectic (or an oligolege) that is, it collects pollen from a few related plant species (from the Greek oligo: few, scant; and lect: chosen, picked).

A white bryony mining bee and its pollen source, white bryony © Aiwok (L) and H. Zell (R), Wikipedia Creative Commons

Pollen specialisation can be a considerable drawback for a bee because food may be scarce even in a landscape full of flowers, and this may limit populations of some species. For example, until recently the white bryony mining bee was rare and threatened in Poland. This has changed with the spread of white bryony into the country’s urban areas. And yet, a considerable number of species are pollen specialists; in some habitats, they make up the majority of the bee fauna. So pollen specialisation must have its advantages, for example by allowing more efficient flower visitation and pollination rates, which benefits bees and plants.

Polylectic bees are at the other end of the spectrum: they collect pollen from various unrelated kinds of flowers. The advantages of being a pollen generalist seem evident: there is more food to choose from and it’s available for longer, as flowers blossom at different times. But these bees must also have an array of physiological adaptations to overcome a variety of chemical and physical barriers to different types of pollen. This could be too costly for a bee’s metabolism.

Pollen is a rich source of protein, lipids, vitamins and minerals. But it also contains secondary compounds that may be noxious to some bees, and pollen grains are often protected by indigestible coating. These barriers explain why few insect taxa rely on pollen alone for food, and could also explain why most polyleges (polylectic bees) exhibit a degree of pollen specialisation: for example, heather (family Ericaceae) and legumes (family Fabaceae) make up over 70% of the pollen collected by British bumble bees, despite local abundance of other pollen sources.

Experiments with the closely related red mason bee (Osmia bicornis) and horned mason bee (Osmia cornuta) show the effects of different types of pollen. Red mason bee larvae develop well on buttercup pollen (genus Ranunculus), but fail to do so on pollen from viper’s bugloss and related plants (genus Echium); the reverse happens for the horned mason bee. Both bees do well on field mustard pollen (genus Sinapis), while neither develop on pollen from tansies and related species (genus Tanacetum). But the story is a bit more complex: neither bee shows any negative effect as long as they are not restricted to ‘bad’ pollen. In fact, unsuitable pollen is part of the bees’ natural diet. Other bee species show similar patterns.

Viper’s bugloss (1), creeping buttercup (2), field mustard (3) and tansy (4): nutritious/poisonous food for the right/wrong bee. © Wikipedia Creative Commons

So what can we conclude from all this?

Oligolecty and polylecty are both successful evolutionary strategies. Some bees depend on a few plants, others have diversified pollen diets. The range of hosts can be narrow or wide, depending on the species, but setting aside a handful of exceptions, bees need pollen from different plants to complement nutritional imbalances or to mitigate the effects of harmful secondary metabolites. But even pollen of low nutritional quality or digestibility is taken, as long as it’s a portion of a balanced diet.

These aspects have important consequences for the conservation of bees. They need a diversity of flowers, and plenty of them. Habitats such as semi-natural grassland, hedgerows, field borders, cover crops, brown sites, road verges, wild gardens and weedy parks are all suitable. Planting is helpful, but except for the honey bee and some bumble bees, we know little about what plant species to use. The safest action is to let our wild plants go wild, so that we have bigger, and more diverse flower-rich habitats. That’s not much or too difficult a task to assure the future of our most important pollinators.  

A healthy diet for fussy eaters

By Athayde Tonhasca

Pollen, the fertilizing agent that carries the male gametes (reproductive cells) of flowering plants and grasses, is packed with protein, starch, sugars, fats, vitamins, and inorganic salts: carotenoids and flavonoids add the colouring. This rich resource wouldn’t go untapped by many insects and mites. Among them, bees are the ultimate palynivores (pollen eaters).

To us humans, one pollen grain is indistinguishable from the next: it’s that granular yellow stuff that may cause seasonal allergies such as hay fever. But pollen of different plant species comprises a smorgasbord of chemicals. Protein, by far the most important nutrient as the source of vital amino acids, ranges from 2 to 60% of pollen dry mass. The composition and amount of other essential nutrients vary as well. Some pollen contains secondary metabolites such as alkaloids and glycosides, which are harmful to some bees: buttercups and related species (Ranunculus spp.) for example are toxic to honey bees. Pollen grains of some plant families are coated with a sticky substance called pollenkitt, which probably helps pollination. But just as some people can’t digest lactose, some bees can’t digest pollenkitt.

Miscellaneous pollen grains © Dartmouth College Electron Microscope Facility, Wikipedia Creative Commons

Bees have adapted to the range of pollen quality by adopting diversified diets: most species are polylectic, that is, they collect pollen from various unrelated plants (as opposed to oligolectic species, which specialize on a few related plants). By taking pollen from many sources, bees get a balanced diet and reduce the relative intake of harmful chemicals. When polylectic bees are fed pollen from a single source, they often fail to reproduce or die. The need for nutritional diversity has deep implications for bee conservation. 

Agri-environment programmes throughout Europe have promoted the creation of flower-rich habitats to reduce the impact of agriculture intensification on pollinators. Field margins and other non-crop areas are planted with seed mixtures, and the practice has made a difference: bumble bee declines have slowed or sometimes reversed in recent decades. As a bonus, honey bees and butterflies have benefited as well. However, most solitary bees (which make up about 90% of the approximately 250 species of bees in UK) have been unintentionally left out.

Two of our solitary bees: a miner bee © Pauline Smith, and a leafcutter bee © Saxifraga – Pieter van Breugel

It turns out that seed mixtures comprise a high proportion of legumes (family Fabaceae) such as red clover, white clover and vetch. These plants are good for bumble bees, but are not the best or not suitable at all for many solitary bees. Most species get their pollen from plants such as smooth hawk’s-beard (Crepis capillaris), scentless mayweed(Tripleurospermum inodorum), field bindweed (Convolvulus arvensis), rough chervil (Chaerophyllum temulum), meadow crane’s-bill (Geranium pratense) and dandelions (Taraxacum agg.). Species from the families Asteraceae (daisies, marigold, snakeroot, tansy, thistles) and Apiaceae (cow parsley, wild carrot, ground elder) are also important. 

Weeds or food for pollinators? Smooth hawk’s-beard (L) © Michael Becker, Wikipedia Creative Commons, and wild mustard (R) © Hectonichus, Wikipedia Creative Commons.

These plants grow naturally in and around arable fields, but some of them are not welcomed by farmers because of their invasiveness. Wild mustard (Sinapsis arvensis) and wild rose (Rosa canina) for example are excellent sources of pollen for solitary bees, but the first is a serious weed of oilseed rape fields and other crops, and the latter is a climbing shrub, not suitable for field margin management. 

The inclusion of weeds in seed mixtures may not be an option, but a more tolerant attitude towards them would be beneficial and safe. A wild plant does not become a weed until it starts competing with crops, and this threshold may take a while – or it may never be reached. The same principle applies to our gardens: we don’t need to kill weeds willy-nilly for questionable aesthetic reasons.

As in so many areas of conservation, the answer lies in finding a middle ground. We need to cultivate an appreciation for wildness over manicured fields and gardens because just as a varied diet is best for human health, a diversified flora represents an essential buffet for bees and other pollinators.