AFAIK, it's completely decentralized. When a scout ant finds food, it returns back to the nest leaving a strong trail of pheromone. Ants tend to follow a pheromone trail and reinforce it on their return if they find food, so when a good source of food is found, a strong pathway is quickly established by a growing number of ants following the same route.

An ant may randomly leave a pheromone trail, which ensures that other, perhaps more attractive food sources are not missed. In general, the stronger the signal, the faster ants move with the less probability of diverging from the path.

This strategy ensures optimal area exploitation under varying conditions. For instance, if the food is concentrated in one or a few locations, you'll see a single column between the nest and the source, but when the food is scattered through the area, the ants would disperse too.

Source: an excellent book "Cells, Embryos and Evolution" where it was used as an example of how complex and seemingly organized and directed behavior could be achieved by a population of identical individuals acting under uniform rules.