Many species of Atta have enormous colonies with more physical worker subcastes than any other group of ants. This may be associated with the habit of growing fungus.
The diverse requirements of the colony include:
This range of tasks means that a diversity of worker castes is favoured.
The size of the nest that some species create is astounding. You can watch a video showing some very dedicated people excavating an entire colony: ant colony excavation - YouTube.
Studies of the co-evolution of the Attini and the fungi they grow shows that some fungal lineages have been cultivated for at least 23 million years (Chapela et al 1994).
However, parasitic fungi also exist, which infect the gardens and feed from the cultivated fungus itself (Geraldo and Caldera 2007). These parasitic fungi can be highly virulent for the ant colony and its fungal cultivar.