Identification of a queen pheromone mediating the rearing of adult sexuals in the pharaoh ant Monomorium pharaonis

Division of labour between reproductive queens and mostly sterile workers
is among the defining characteristics of social insects. Queen-produced
chemical signals advertising her presence and fertility status, i.e. queen
pheromones, are normally used to assert the queen’s reproductive dominance
in the colony. Most queen pheromones identified to date are chemicals that
stop the daughter workers from reproducing. Nevertheless, it has long been
suggested that queen pheromones could also regulate reproduction in
different ways. In some multiple-queen ants with obligately sterile
workers, for example – such as fire ants and pharaoh ants – queen
pheromones are thought to regulate reproduction by inhibiting the rearing
of new sexuals. Here we identify the first such queen pheromone in the
pharaoh ant Monomorium pharaonis and demonstrate its mode of action via
bioassays with the pure biosynthesized compound. In particular, we show
that the monocyclic diterpene neocembrene, which in different Monomorium
species is produced solely by fertile, egg-laying queens, strongly
inhibits the rearing of new sexuals (queens and males) and also exerts a
weakly attractive “queen retinue”  effect on the workers. This is
the first time that a queen pheromone with such a dual function has
been identified in a social insect species with obligately
sterile workers.