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.
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%.
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.
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 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.
Scottish pollinators 