Hi Pam,
I’m not aware of proven tactics for deterring leaf-cutter ants from entering new areas. Perhaps mulching plants would deter them slightly from harvesting leaves, but they are aggressive and determined organisms. Perhaps increasing soil organic matter through composting or mulching over time would bring more balance to the soil community and bring in potential predators such as specific nematodes.
There has been a lot of research looking at control methods including application of parasitic fungi that kill the fungus that the ants are feeding/cultivating. The ants are really smart though and have been able to detect the parasitic fungi and remove it.
Here’s a pretty good article reviewing the above
I just looked in the book “Producing food without pesticides: local solutions to crop pest control in West Africa” by Lowell Fuglie in our library. It mentioned caster oil plant (Ricinus communis) as an effective control. It lists options for applying the plant parts. A spray can be made from soaking green leaves and seeds in water for a full day. After soaking, the solution is filtered and sprayed on target pests. Alternatively, the green seeds and leaves can be crushed into a powder that is then applied. Or seeds, leaves or oil cake can be mixed with the soil.
Caster oil may not be available in your region though, I am not sure.
I also found this open source resource that lists other tropical crops including neem and jack bean that negatively effect leaf-cutter ants. I have also heard that if you find a dead nest without ants in it anymore, you can take the soil from there and put it on an active nest and they will leave. I don’t know if it’s true or not. Has anyone else heard of this or tried it?