Propolis (yellow) lining the inside of a beehive.

Beekeepers would love to get rid of propolis, a sticky substance made of resins that bees use to line their hives, because it makes it hard to pry hives open. But propolis isn't just gluing the hive together, according to a new study published in PLoS ONE

---honeybees use it to fight off fungal infections and seek it out when their hives are infected. Bees have to invest effort in hunting down the resins that make up propolis, which like nectar is foraged from plants. That means that every minute a bee is looking for resin is a minute it's not looking for food. The trade-off is worth it, apparently, because propolis kills bacteria and fungi

lurking in the colony. In this new study, the authors looked at whether propolis helped stop a fungal infection called chalkbrood that kills larvae. When experimenters painted propolis extract on hives, these propolis-enriched hives had lower rates of chalkbrood infection. And when colonies got infected with chalkbrood, bees went looking for resins more often. That's where things get interesting because the adult bees doing the foraging are not directly affected by chalkbrood---it only lurks in larvae---so the "self"-medication happens at the level of colony instead of the individual bee. Honey bees, which are eusocial

insects, really act together to benefit the entire colony rather than just themselves.

Image via Wikimedia Commons / Abalg