They are among the most industrious creatures on the planet, but honeybees still struggle when they’re ill. Once a disease takes hold inside a hive, the bees can become sluggish and disorientated, and many may die.
Now it seems honeybees may have a way of helping to keep their workforce healthy - by employing bees that feed "medicinal honey" to other members of the hive.
A group of worker bees called "nurse bees", if they are infected with a parasite, selectively eat honey that has a high antibiotic activity, according to Silvio Erler of the Martin Luther University Halle-Wittenberg in Halle, Germany and his colleagues.
These bees are also responsible for feeding honey to the larvae and distributing it to other members of the colony. So it's possible they are the hive's doctors, prescribing different types of honey to other bees depending on their infection. If that is true, it could be a big part of how bees fight disease.
In Erler's study, nurse bees infected with a gut parasite called Nosema ceranae were given a choice of honeys. Three were made from the nectar of plants - black locust, sunflower and linden trees - while a fourth was honeydew honey made from the secretions of scale insects or aphids. Each of the honeys was known to have antibiotic activity.
Bees with greater levels of infection tended to eat more of the sunflower honey, which had the strongest antimicrobial activity. It reduced the level of infection in the bees that ate it by 7%, compared to the honey from the linden trees.
"Honeys are full of micronutrients, alkaloids and secondary plant compounds that are good for both bees and humans alike," says Mike Simone-Finstrom of North Carolina State University in Raleigh. One study suggested they can increase the activity of honeybees' immunity genes, boosting their ability to fight disease.
A separate study from September by Erler's group suggests that different honeys are effective against different diseases. While sunflower honey is good at preventing the growth of bacteria that cause American foulbrood in bees, it is less effective against bacteria associated with European foulbrood. However, linden honey was more effective against these bacteria.
"The in-hive worker bees might be in an exceptionally important position to distribute honey selectively in the colony that affects their own health but potentially also that of other nestmates," says Erler.
His team is now investigating whether nurse bees select honeys from different sources depending on the infection they are fighting. If this turns out to be the case, it will reveal a level of medical care within honeybee hives not seen before.
With honeybees under threat from disease, climate change, pollution and new farming techniques, Erler says their medicinal abilities could prove invaluable. "Apiculturists might take advantage of specific honey flows to protect their colonies against specific diseases," he says.
But we mustn't overstate the medicinal role of honey, says Francis Ratnieks of the University of Sussex in Brighton. "If after six days of feeding just one type of honey you only get a 7% effect on infection, I would reckon that the effect in a hive would be less. Bees collect honey primarily as a food supply, not as medication."
Honeybees do have other sources of medicine besides honey. For example, they collect resin from plants and incorporate it into their nests, where it may help combat fungal parasites. In 2012 Simone-Finstrom and a colleague showed that bees infected with fungal spores collected more of the resin.
Honeybees, along with other insects like ants, also display "hygienic" behaviour: workers carry dead members of the colony far away to avoid an infection spreading. Ratnieks is trying to breed honeybees that do this more often, to produce colonies that are more resistant to disease.
Bees are far from the only animals that can self-medicate. While humans reach for an aspirin to combat a headache, many primates including chimpanzees eat bitter bark and rough leaves that may help kill off parasites in their guts. Goats eat vegetation high in tannins when they are suffering from intestinal worms. Woolly bear caterpillars fight parasitic flies by eating plants rich in toxic chemicals, while wood ants incorporate antimicrobial resin from conifer trees in their nests.