Bumblebees Shift Foraging to Antimicrobial Nectar When Colony Faces Fungal Threat

A colony of buff-tailed bumblebees faced with a fungus that kills their larvae made a striking choice: they began foraging more intensively for nectar laced with quercetin, a natural antimicrobial compound. Over five days of free foraging, the infected colonies increased both their visits to quercetin-rich flowers and the time they spent feeding there. Healthy colonies showed no such preference. Neither did colonies exposed to a different pathogen—a bacterium that infects adult bees' guts. The pattern suggests something unexpected: the bees were not simply medicating themselves, but rather gathering compounds to protect their brood.

The study, published in Royal Society Open Science by researchers at the University of Valladolid and KU Leuven, used robotic flowers to track which nectar sources bumblebee colonies preferred under different conditions. The researchers offered colonies a choice between plain sugar nectar and nectar spiked with quercetin, a flavanol known to fight a broad spectrum of bacteria, fungi, and viruses. Some colonies remained healthy throughout the experiment. Others were exposed to Ascosphaera apis, the fungus responsible for chalkbrood disease in bee larvae, or to Serratia marcescens, a bacterium that attacks the digestive systems of adult foragers.

The results diverged sharply by pathogen type. Colonies infected with the fungus showed a clear behavioral shift as the experiment progressed, seeking out the antimicrobial nectar more often and spending more time there. The healthy colonies maintained their original preferences. The bacterium-exposed colonies also showed no change, despite being infected themselves. The distinction points to how different pathogens shape a colony's response. Chalkbrood harms larvae, not the adult foragers who do the gathering. The increased interest in quercetin among fungus-exposed colonies may therefore represent what researchers call social medication—worker bees collecting compounds that benefit the brood rather than providing direct benefit to themselves.

Serratia marcescens operates differently. It infects the adult foragers directly, the very bees doing the foraging. Yet these colonies showed no shift toward antimicrobial nectar. The researchers suggest this may be because any medicinal benefit from quercetin against a gut infection would be indirect or delayed, making the behavioral response less pronounced or less adaptive than when larvae are under direct threat.

The findings reveal a form of adaptive plasticity in social insects—the ability to modify foraging behavior in response to pathogen pressure. Zoopharmacognosy, the field studying how animals use natural compounds for medicinal purposes, has long documented self-medication in individual organisms. But demonstrating it in a social insect, where the decision to seek medicinal compounds may benefit others rather than the forager, adds a new dimension to the phenomenon. The researchers note that medicinal compounds often carry costs—they may be toxic or nutritionally suboptimal for healthy individuals. Theory predicts that organisms should seek them out only when infected, and avoid them otherwise. The bumblebees appeared to follow this logic, but with a twist: they sought the compounds when their offspring were threatened, not when they themselves were sick.

The study also highlights the value of robotic flowers as tools for behavioral research. By automating the presentation of nectar sources and tracking visits and feeding time, the researchers could observe how colonies made foraging decisions under controlled conditions. This approach opens new possibilities for studying how insects respond to pathogens and how social structure shapes individual behavior in response to disease.