As I approached a clearing near a stream, a turbulent cloud of orange specks took form, erratic in motion but producing a quiet hum. Although the movements seemed random at first, it became obvious that individuals within the mass looped carefully at the edges to maintain its shape. An equally vast number of them also sat idle on the mossy buttress, making the tree appear much more orange than I remembered. This was a colony of stingless bees (Tetragonisca angustula) that I was very well acquainted with, but never had I seen so many of them out and about.
Stingless bees build nests within narrow crevices and tree cavities, cleaning and packing the space with a mixture of plant resin and wax called cerumen. The material is elastic but firm, very useful for various parts of nest’s architecture. Sheaths of cerumen surround the interior brood chamber for protection, bowl-shaped cells are filled with honey and pollen, and compact pillars give structural support to the nest. Whereas nests lie obscured from view, the entrance where bees enter and exit is visible from the exterior. The appearance of these entrance tubes are unique by species, each beautiful and intricate in their own way. Tetragonisca angustula constructs a porous waxy tube, thin enough for only a handful of bees to poke their heads out simultaneously. Of the other Costa Rican meliponine nests I’ve seen, there’s the dark funnel of Trigona fulviventris, the rusty stalagmitic tubercles of Tetragonisca ziegleri, the beige maply corals of Lestrimelitta sp., and the round mass of the honeybee look-alike, Melipona costaricensis. Beyond nest geometry, there is also substantial diversity in the humming frequencies of bees, most noticeable when colonial, and I’ve become attuned to locate and identify some nesting species by sound.
Despite the lack of a stinger, stingless bees have impressive defense against nest predators, and Tetragonisca angustula does so in perhaps the most orderly fashion. Guard bees will hover around the nest tube, leaving an open airspace corridor for incoming bees to fly through. If an intruder is recognized, hover guards will attack it from both flanks, biting down on the wings and legs resulting in a tumble away from the nest. More guards will be recruited to maintain aerial numbers, but many bees will remain perched inside and around the entrance tube as a last line of defense. Such a division of labor between hover and perched guard bees appears to be unique among social bees. Tetragonisca angustula is particularly susceptible to losses from an obligate kleptoparasitic bee (genus Lestrimelitta) that raids for food resources and nest material. These larger black bees will invade with thousands of individuals and can incur substantial casualties for both colonies. Tetragonisca angustula is chemically attuned to detect such a threat, and together with the release of an alarm pheromone, they supply the highest numbers of bees actively defending outside of the nest. Nest raids may persist overnight, and another neat behavior that T. angustula has evolved is to close the nest entrance during the early night. Discrimination between nestmates and non-nestmates is critical to survival of stingless bee colonies, and T. angustula has demonstrated the highest accuracy in recognition across all social bees studied to date.
Photographed in situ / after slight disturbance [1-2]