Flowers and Bees: A Story of Chemistry and Attraction
I am in charge of maintaining 14 research beehives based at the Cambridge University Botanic Garden, home to 8000 plant species from all over the world. In summer, the Botanic Garden research plot is surrounded by a wild meadow. All flowers pop almost at the same time around mid-June to create a small prairie, with splashes of white dotted by blues, yellows, and little freckles of red. On a summer evening, just after finishing my weekly beekeeping visit, I wandered through the meadow. Hundreds of honeybees and wild bees were busying, flying from one flower to the next, and rushing to get as much pollen and nectar as possible for their last foraging trips. Without flowers bees would not exist; and without bees, flowers would not have evolved into the wide diversity of colours and shapes I was observing in the meadow.
Wild meadow on a summer evening
The story of flower and bee co-evolution started 130 million years ago, when plants needed a better way to ensure their reproduction. As Dave Goulson amusingly describes in his book, A Sting in the Tale: My Adventures with Bumblebees, “Sex has always been difficult for plants, because they cannot move.” As you can imagine, having feet rooted in the ground does not make it easy for the male sex cells of a plant (pollen) to meet the female reproductive part of another plant. To facilitate this so-called pollination act, plants needed a third agent to come into play. At first, many of them relied on wind to disperse pollen particles to the stigma, the female part of a plant. As of today, around 12% of the world’s flowering plants use wind mediated pollination, called anemophily. These include nut flowering trees, such as pistachio, pecan, and walnut, but also many of the cereal crops like wheat, rice, corn, rye, barley, and oats. Wind pollination is extremely inefficient and only 0.01% to 1% of pollen particles will be delivered to the right flower recipient. To compensate for the heavy loss, wind-pollinated plants must produce millions and millions of pollen particles; this production requires a lot of energy.
Other plants decided that this process was too wasteful and felt the pressure to evolve to secure their survival. The solution came in the form of the most inconspicuous of creatures: insects. Small beetles started to feed on the very nutritious and protein rich pollen grains from flowers. By doing so, they inadvertently carried pollen from one flower to another, and unknowingly became the new pollinators – aka sex-assistants – that plants were waiting for. With this more efficient means of pollination, flowering plants started to spread on the surface of the Cretaceous world and began to diversify. To increase the odds of attracting insect pollinators, some of them produced nectar, a sweet and high-energy reward for the insects that visited them. This upgrade in plants gave insects the opportunity to specialise in nectar gathering and for new groups of insects to emerge. Among flies, butterflies, and other nectar sucking insects, bees became the most successful insects specialised in nectar and pollen gathering. Bees and flowers have developed a mutualistic relationship where both species benefit and safeguard the survival of the other. Unlike other flying insects, bees derive from plants the only food resource they need to develop. In exchange, bees provide flowers with the most reliable means of reproducing and spreading.
From this literally fruitful relationship, flowering plants called angiosperms have dominated the landscape and have become the most important group of plants on the planet. They produce most of our food resources, either directly through food crops or indirectly by feeding livestock. As angiosperms diversified and spread, so too did the fierce competition between them to win the bee’s heart. Plants learned how to develop more and more sophisticated chemical weapons of seduction.
The first flower, which appeared 130 million years ago, was small, unimpressive, and undoubtedly difficult for pollinating insects to spot in the green and brown tones of the lush Cretaceous vegetation. To become more noticeable against the forest background, and to advertise their positions to bees, flowers developed petals and showy shades of blue, purple, and yellow. As very skilful chemists, flowers blend complex mixtures of pigments to create their colours. Flower pigments are organic molecules mainly including flavonoids, carotenoids, anthocyanins, and betalains. Yellow, orange, and red flowers contain different concentrations of carotenoids, while blue, purple, and red flowers mainly have anthocyanins pigments.
However, these are only the colours that the human eye can perceive, as many flower pigments will emit or absorb UV light. This is of no interest to us but very useful for bees, who can perceive UV light. The simple yellow marsh marigold will appear to bees as a very bright flower with a dark centre directly marking the location of nectar and pollen. Flowers like lupines and lantanas can also modulate their pigments and change colour to signal that they have already been visited by a bee. A very clever way to increase the efficiency of the pollination process.
Another seductive weapon of flowers – and one that has a longer lasting effect on bees compared to colour – is scent. Flowers release hundreds of volatile organic compounds into the air to make complex and alluring fragrances destined to signal their position to bees. But how can one flower scent be successful at attracting a bee in the myriad of other pretendants? On that matter, flowers have developed an interesting strategy. Instead of competing against each other, they collaborate to attract bees in numbers. Flowers of totally different species, but who share the same colour, will emit a similar scent profile. They together send a stronger and more reliable scent signal to pollinators that is more powerful than each flower species trying to emit its own unique scent.
As with many relationships, however, the one between bees and flowers is far from perfect. Partners do not necessarily receive equal benefits or make equal sacrifices. Bees have evolved on a plant-based diet made of pollen and nectar not only to feed themselves, but also their larvae. The latter are particularly demanding for a protein-rich pollen diet. Depending on the species, bees can collect the whole pollen content of 30 to 1000 flowers to rear a single larva. This leaves only few pollen particles for the flower pollination. In some flower species, as little as 3.7% of the pollen contributes to pollination while the other 95.5% are collected by bees. Bees not only collect a huge amount of pollen, but do so very efficiently thanks to their specific anatomy. Bees have hair that can accumulate a considerable amount of pollen. Most bee species have also developed special body parts to transport pollen from flowers back to their hives. For example, honeybees and other members of the Apidae family transport pollen on their corbiculae, a flat plate situated on their hind legs.
Flowers have adapted their morphology to prevent excessive pollen harvesting from bees. Heteranthery is a curious word that describes the property of flowers to display two types of anthers, the parts where pollen is produced. The first anther is very visible and attracts bees, while the second anther is hidden and becomes visible at a later stage in the season, to gradually release pollen to bees. Another strategy to balance pollen harvesting by bees is to produce pollen that lacks some essential nutritional elements, forcing the bees to forage on other flowers to achieve a balanced diet. A more extreme approach is found in flowers producing pollen that can only be digested by one or a few species of bees, but which is toxic to other species. The so-called specialist bees are often solitary, and will emerge from their nest at the same time the plants they feed on are flowering. In the play of evolution, specialist bees have developed a dependence on their host plants that make them particularly vulnerable to decline and extinction.
In the UK alone, three bumblebee species have become extinct and 52% of solitary bees species have declined in recent decades. The global population of wild bees and other insect pollinators is following a similar trend. Climate change, loss and fragmentation of habitats, pesticides, emerging pests and diseases, are the main threats to wild and domesticated bees. We are now aware that it is our interest to preserve the long, complex and, oh so fragile relationship that bees and flowers have created. If UK wild pollinator populations were healthier, British Gala apple trees could produce an extra £5.7 million worth of apples. In the Sichuan Province of China, renowned for its production of pears, the heavy use of pesticides has led to the disappearance of wild pollinators. Local beekeepers were also driven to relocate their hives far from the cultivation areas. Left with no other means of pollination, farmers are doing by hand what bees have been doing for millions of years. Is homo sapiens going to be the next plant-pollinator, and is our species up for the job? I am not sure how the story between bees and flowers will continue but I hope it never ends.
