A social, stingless bee species in Brazil feeds their larvae a special fungus that they grow on the walls of their own nest. Without it, most of the young will die, according to findings published in Current Biology this week. 

Other social insects, such as ants and termites, have been known to farm microorganisms that provide nutrients or protection against pathogenic microbes. In return, the fungus and other symbionts get to enjoy a comfortable microenvironment with stable conditions and a rich source of food. But this new work is the first time we’ve seen a relationship between social bees and a cultivated fungus. 

Cristiano Menezes of the Brazilian Agricultural Research Corporation first noticed the white fungus when he was attending to beekeeping tasks in the lab for a different study. Under humid conditions, it was growing out of control along the internal walls of the wax cells used to rear larvae; and after larvae started dying, he thought maybe the fungus was a pathogen. That is, until he saw that it was also growing – but much less profusely – in natural brood combs. Under these less humid conditions, the survival rate of larvae improved dramatically.

So, Menezes and colleagues studied 30 colonies of Brazilian stingless bees (Scaptotrigona depilis). Their brood cells are provisioned with a semi-liquid food mass regurgitated by nurse workers. The queen lays an egg on top of the food, then workers close off the cell, which isn’t opened again until adult bees emerge. The fungus began proliferating just as the three-day-old egg was about to hatch, and its growth was intense until the third day of larval development. Two days later, it looked as if the fungus had vanished from the brood cells. Recordings showed that larvae devour the fungus, which the team identified as a member of the genus Monascus. 

To see if the fungus is critical to survival, the team harvested larval food from brood cells and placed in acrylic brood cells in the lab. Larvae raised on sterilized food supplemented with fungus filaments survived 76% of the time. Meanwhile, only 8% of the larvae without the fungus survived: They grew slowly, developed darkened guts, and died in large numbers in a week. Additionally, larval food from cells without the fungus smelled bad and seemed sticky and spoiled. 

^{Bee larvae in vitro. Cristiano Menezes}

The fungus originates from the building materials used to construct brood cells. These bees recycle and transport “contaminated” material between mother nests and new colonies. Unlike known fungus-farming insects, these bees don’t tend to their crop besides providing a suitable substrate. In this way, the authors say the system is more of a proto-farming one. 

The team has yet to figure out the main function of the fungus. If it’s nutrition, they’d expect to find smaller bees or deformed ones in its absence. So it’s possible that the fungus helps protect the larvae and larval food from other microbes. "This is amazing because it opens up a new field in bee research to understand the role of this and other microorganisms in colony health,” Menezes says in a statement. “On the other hand, it is frightening because fungicides and bactericides are widely sprayed during blooming periods of several crops and may affect bees' symbionts and consequently their health."