Why do stingless bees (Melipona subnitida) leave their nest with resin loads?

Abstract

Stingless bees (Apidae, Meliponini) collect plant resins and deliver them to their nest for use as construction material. In some species, however, workers can be observed leaving their colonies with resin loads on their corbiculae. The present study investigated the role of these "resin bees" in the Brazilian stingless bee Melipona subnitida. In 7 nests, we observed a total of 424 workers departing the nest under undisturbed colony conditions. One tenth (11.3%) of these outgoing bees carried resin on their corbiculae. The average weight of the loads was 5.2 mg, approximately, 10% of the bees' body weight. The round-trip duration of resin bees (1.3 min) was significantly shorter than that of nectar foragers (12.3 min). When the colonies were experimentally disturbed, the majority of the agitated bees that left the colony in response carried resin loads (90.5%). Our results suggest that resin bees in M. subnitida are nest defenders. Under undisturbed colony conditions, these bees may perform surveillance flights close to the nest, watching out for potential intruders and even mummify them with resin. Additionally, the deterrent effect of plant resins on many bee predators may reduce the workers' risk during these flights.