Sodium is one of the most important and yet often most limiting micronutrients for herbivores. Leafcutters are about the only herbivorous ants, with maybe an addendum for Cephalotes. It might help your leafcutter colonies to grow better if you give them dilute salt water or give them frass. Specifically it might give them the micronutrient they're most limited by, thus letting them be healthier and grow larger.
I'm curious about how they would respond if you gave them a test tube of the stuff - still providing them water without salt, of course. The ratio would probably be around 1 tsp of salt per 4 oz of water/10g per 115 ml water. That still might be a little high, though - that's about the concentration of salt in sweat. 1/2 tsp per 1/2 cup of water might be a better ratio to try. For insect frass, offering Atta or Acromyrmex this might be intriguing. It's a lot saltier than leaves so it might be a good source of salt for the fungus and ants.
Another interesting thing might be to dip leaves/food in the solution before giving them to the ants. That could allow the fungus to be healthier (maybe, hopefully).
In the past decade or so research has shown that adding relatively small amounts of salt (NaCl, sodium chloride) to a salt-deprived can really benefit herbivores' overall health when in short supply:
https://besjournals....1365-2435.13796
https://onlinelibrar...1002/ece3.70026
https://onlinelibrar....1111/ele.13517
https://onlinelibrar....1111/ele.13270
https://esajournals.....1890/13-1274.1
https://onlinelibrar...11.2010.01209.x
This is because plants are typically very low in salt when compared to animals, and so herbivores must actively search out higher-sodium plants or find places like salt licks to get enough. Predators of these (including ants) do not benefit directly from this as they can get all their salt from their prey, other animals.
Most ants anyway. They're omnivores.
Atta and other leafcutters aren't, though. They feed entirely on their fungus, which in turn, is entirely fed by vegetation they collect. Lower attines will also collect things like insect frass but I don't know if the higher ones do.
But yeah. Just a thought I wanted to share. If anyone does try this, please keep me in the loop.
