Michael Simone-Finstrom of North Carolina State University and Maria Spivak of the University of Minnesota have found that honey bees self-medicate with plant resins to ward off fungal parasites. This may not seem that unusual until you consider that the bees aren’t doing this by ingesting or applying the resins to themselves. Instead, they incorporate the resins into the structure of their hive, thereby medicating the entire nest.
Over three summers, the researchers exposed colonies of honey bees to fungal parasites that cause the rather unfortunate sounding chalkbrood disease. As the name suggests, this disease affects bee larvae, not the adult bees that do the foraging. Nevertheless, adult bees living in infected hives collect more plant resins than those residing in uninfected hives. Thus, the adult bees responded to a health threat for the whole colony by changing their habits.
Bees collect plant resins and turn them a substance called propolis, known to have anti-bacterial and anti-fungal properties. Wild bees often line and repair their nests with propolis. However, the honeybees that Simone-Finstrom and Spivak were studying did not simply incorporate the same resins into every nest as a prophylactic measure against infection. They actively foraged for protective resins as they needed them. This is the very definition of self-medication, even though it’s applied to the colony as a whole.
To be clear, it’s only a very small subset of bees that collects resins regardless of the state of the nest. The vast majority of foragers continue to bring back pollen and nectar. I’d love to know how those few bees 'decide' to switch over to making propolis.