Inter-colony fights in Tetragonula stingless bees result in temporary mixed-species worker cohorts

Two stingless bee species, Tetragonula carbonaria and Tetragonula hockingsi, engage in extreme inter-colony fights, both within and between species. Inter species fights can result in one species taking over the nest of the other. Following successful takeovers, brood from the previous colony could be retained and become workers, but this has yet to be tested. We first tested whether T. carbonaria callows are accepted when introduced into a different conspecific nest and found that they are indeed accepted as normal workers. This suggested that mixed species cohorts might be expected after a hive takeover. We assessed this in a managed T. carbonaria hive, fitted with an observation window that was subject to multiple takeover attempts. Using microsatellite genotyping, we established that multiple different colonies (of both species) attacked the observation colony over 63 days, resulting in a catastrophic collapse of the nest. Our observations inside the hive during these inter-colony invasions revealed no evidence of resource robbing, adding further support to the hypothesis that fighting behaviour in these species has the primary goal of taking over the nest. Newly emerged callows were ejected by invaders, predominantly during the initial few days of fighting. The ejection of callows ceased; however, shortly after each takeover attempt, and, following this, newly emerged callows were accepted as workers. These findings add new evidence that the invading colony co-exists with workers from the previous brood following a takeover, but only until the last of that cohort dies.