From WIKI:
Workers cannot mate; however, because of the haplodiploid sex-determination system in ants, workers of a number of species can lay unfertilised eggs that become fully fertile haploid males.

From http://blog.taragana.com/science/2010/07/01/worker-ants-can-lay-eggs-in-queens-absence-17522/

Worker ants can lay eggs in queen’s absence

WASHINGTON - Worker ants separated from the queen develop large ovaries in preparation for laying eggs, Danish researchers have demonstrated.

For the study, researchers from the Centre for Social Evolution at the University of Copenhagen identified a candidate queen pheromone in the black garden ant.

Thereafter, they made a synthetic copy of the pheromone to definitively test its function.

They found worker ants could also lay eggs in the absence of the queen. However, if the orphaned ants were given a glass model queen coated in synthetic queen pheromone, they remained infertile.

The team also noted that the queen’s eggs are covered in pheromone, and that sick queens produced less pheromone.

Together, these results suggest that the queen pheromone lets the workers know that the queen is laying many eggs and is in good health. (ANI)