How do workers of the primitively eusocial wasp Ropalidia marginata detect the presence of their queens?

Queens in primitively eusocial insect societies are morphologically indistinguishable from their workers, and occupy the highest position in the dominance hierarchy. Such queens are believed to use aggression to maintain worker activity and reproductive monopoly in the colony. However, in the primitively eusocial wasp Ropalidia marginata, the queen is a strikingly docile individual, who interacts rarely with her workers.