Mary Poppins taught us that a spoonful of sugar makes the medicine go down. A bumblebee's favorite sugary drink may already be laced with medicine. And bees seem to dose themselves with medicinal nectar when they're suffering from a gut full of parasites. Plants manufacture many chemical compounds to defend against attackers. Some of these are familiar to humans—like capsaicin, the potent weapon made by chili pepper plants. But not every animal enjoys painful food experiences like we do, and these defenses usually help keep hungry creatures away from a plant. Some are toxic or unpleasant for insects to eat, or protect against diseases. The compounds can also trigger incredible events in the plant's environment. For example, a leaf being chewed by a caterpillar may send out signals that attract wasps, which arrive like cavalry to attack the pest. Although these compounds are common in leaves and stems, they can also show up in nectar and pollen. If a flower relies on insects to pollinate it, this is confusing—why poison the nectar you want a bee to drink? In a study published earlier this year, Dartmouth researcher Leif Richardson and his colleagues fed bumblebees eight defensive chemicals that occur naturally in nectar. All of the bees were carrying an intestinal parasite called Crithidia bombi. Richardson found that some of these nectar compounds (including another human favorite, nicotine) knocked out more than half the parasites from a bee's system. "These compounds also can have costs for consumers," just like a human drug can have harmful side effects, says Richardson, who's now at the University of Vermont. So it would make evolutionary sense for bees to seek out these chemicals when they're carrying parasites, but avoid them when they're healthy. Richardson tested this idea in the wetlands of northern Vermont. He used one type of plant, a flower called turtlehead or Chelone glabra, and one set of defensive chemicals called iridoid glycosides. In turtlehead, these compounds are thought to deter nibbling by many types of insects, as well as larger animals such as deer. (Richardson notes that some other insects have adapted to live only on plants containing iridoid glycosides; the insects store these poisons in their bodies to use against their own enemies.) Richardson sampled the flowers, leaves, nectar and pollen of turtlehead plants, measuring their concentrations of iridoid glycosides. Then he snipped some stems of turtlehead and carefully dripped a little artificial nectar into each flower. This solution was either plain sugar water (which would dilute the compounds in the flower's existing nectar) or sugar water spiked with a small or large dose of iridoid glycosides. All of the resulting nectar mixtures had iridoid glycoside concentrations within the range Richardson had found in natural flowers. Richardson placed his manipulated blossoms within a patch of normal turtlehead and watched as bumblebees came to visit. Whenever a bee drank from a manipulated flower, he captured the bee and examined her gut for Crithidia bombi parasites. Infected bees spent more than three times as long drinking from flowers with the most iridoid glycosides, compared to flowers with diluted nectar. Unparasitized bees spent the same amount of time on all flowers—and after visiting a flower high in iridoid glycosides, they were less likely to visit another blossom on the same stem. In other words, healthy bees are either indifferent to iridoid glycosides or try to avoid them. But bees that are sick with parasites seem to self-medicate by seeking out nectar that's rich in these compounds. What's in it for the plant? Richardson also did a pollination experiment, using blue and green dye to mark the pollen from manipulated flowers and watch where bees carried it. He found that flowers with high iridoid glycosides spread their pollen more widely. So in nature, the turtlehead plants that make nectar higher in these compounds may get a boost from pollinators. It's also possible, Richardson says, that the medicine gets into the nectar accidentally. As plants manufacture defensive chemicals, some of them may unavoidably wind up in nectar and pollen. Either way, bees that visit these flowers seem to use the chemicals to their advantage. The nectar might not taste as appealing as usual, but it treats their ailments in—as Mary might say—the most delightful way.
NOTE: This post has been updated to clarify Leif Richardson's university affiliation.
Image: bumblebee visiting a turtlehead, courtesy of Leif Richardson.
Richardson, L., Bowers, M., & Irwin, R. (2015). Nectar chemistry mediates the behavior of parasitized bees: consequences for plant fitness Ecology DOI: 10.1890/15-0263.1