Queen influence on worker reproduction in bumblebees (Bombus terrestris) colonies

The bumblebee Bombus terrestris is a good model in which to study the regulation of worker reproduction, because the onset of queen-worker conflicts regarding male production is constrained by the annual life cycle of the colony. Worker reproduction in this species is inhibited until late in colony development. The underlying proximate mechanism suggested for this delayed worker reproduction is queen control using combined pheromonal and behavioural determinants. The volatile queen pheromone hypothesis was tested by monitoring the length of time necessary for worker reproduction to occur when workers were separated from the queen by a double screen. These workers always reproduced before the onset of the competition phase in the queenright compartment (QRC), after the time lag required for egg maturation. Since the double-mesh-separated compartment was genuinely a queenless compartment, the volatile queen pheromone hypothesis was refuted. The possible involvement of a non-volatile pheromone and/or of physical intimidation by the queen was then tested using a queen excluder, which allowed the workers, but not the queen, to travel freely between two nest compartments, creating a "refuge from the queen". Although worker reproduction in the queen-excluded compartment (QEC) was delayed compared to the queenless situation, it still occurred before the onset of the competition phase in the QRC and after the onset of queen production. These results indicate that workers autoregulate their reproduction rather than it being controlled by the queen (pheromonal or behavioural). The possible factors affecting this autoregulation may be 1) the workers' perception of the queen and, possibly, her status through pheromonal emission and 2) the workers' perception that the larvae are committed to become queens.