Right now I live in an urban environment and my balcony and windows are where I can grow plants. I have a tiny lime tree that is successfully fruiting for the first time in the three years I have had it (the first two years it produced only a handful of limes, this year it has around 15 ripening fruits/blossoms.) Every summer the ants arrive, however, and they love to build aphid farms on the underside of the leaves. I have resorted to an organic deterrent spray and even sitting and wiping down the underside of each leaf with organic soap and water when the aphid buildup reached a critical point, but I am hoping for a better, simpler solution this year. I also tried wrapping plastic wrap around the trunk but they eventually figure out how to get over it as natural buildup of city sediment on the plastic quickly allowed them to traverse the plastic wrap.

My question is... will my tree suffer if i use mint essential oil on its trunk to stop ants' aphid farms on the leaves? I applied it once to see if the ants would react to the "barrier" and they would not cross it, but I want to ensure that the application of mint oil to the tree itself will not harm the tree? Might there be a better way to stop the ants?

Maria, welcome to permies!

Would it be possible to just put the mint essential oil on the rim of the pot?

Another suggestion would be to put vinegar in a spray bottle.  Spray the ants trail on your balcony up to the pot.

The vinegar will kill the ants.  When you notice that it is no longer effective at killing them then use the spray again.

I've had good luck putting a ring of Diatomaceous Earth around the trunk of my fruit trees when they get ant/aphid farms on them.  I think the ants have to leave the tree and go down to the ground at night and when they cross the DE they start to die.  Every time I've done this the aphid farm goes away within a week or so.

I'd be careful about putting oils on the trunk of your tree.

Are the ants living in the base of the pot?  Ants LOVE to create colonies in the bottoms of potted plants.  If so, you'll need to get them out.  Pull the tree from the pot and see if you aren't hosting a colony of ants under the tree down in the soil.  You might be surprised.

When I lived in Africa, we struggled to keep ants off the table, so we'd take 4 tuna cans and put one under each of the table legs.  Then we could slide a metal bowl under each of the tuna cans -- a bowl within a bowl.  We poured a bit of kerosine into the bowl to keep the ants from swimming across and climbing the table leg.  The tuna can kept the kerosine from soaking into the wood of the table leg.  Basically, you can do the same thing with your potted tree.

If the ants are not in the pot itself, then you can ant proof your pot by getting a much larger drainage tray and setting the pot and current drainage tray inside it.  Inside that larger tray, you can pour a little bit of kerosine mixed with water, being careful to not pour it into the actual drainage tray where it would then soak up into the soil.  As long as your tree isn't touching anything else (like a wall or some other "bridge"), the ants cannot swim across the moat of water to make it to the regular drainage tray/pot.  

Interesting thread. I've observed ants running up and down my trees but not seeming to do any harm, so assumed they were providing a service. So having seen this post I did a quick search on ants farming aphids and saw a study from 2007. It involved observing aphids on a piece of paper that ants had been on, and comparing with aphids on another piece of paper that hadn't had any ants. The conclusion was that the aphids were more docile on the paper that had had ants, and the assumption was it was a subduing effect of chemicals the ants put down.

I also read some commentary that this relationship wasn;t completey symbiotic, the aphids were getting protection from other predators thanks to the ants, but were falling prey to the ants possibly more than they were being protected. The ants start off eating the secretions of the aphids but then move onto eating the aphids themselves. So I'm left wondering if the aphid farming behaviour is detrimental to the tree or not.

Anyone read anything on this that is more conclusive?

I use clear sticky window fly traps in the house to stop houseflies. A similar kind of sticky band around the trunk of the tree or the pot would keep ants from travelling up to tend the aphids, provided that the tree isn't touching other routes for ants (such as the wall or other plants). Hopefully only a few ants would get trapped and then the rest of them would get the message that the route is no longer viable. Maybe regular sticky tape such as cellotape facing sticky-side-outward would work, though might get quickly neutralised by dust, but cellotape is cheap enough you could replace it every few days till the ants get out of the habit.

One of the things I use for potted plants is spent coffee grounds, I spread them as a mulch layer and water through them, ants don't get into or if already there, they leave quickly because of this "treatment".

Oil is not good for the bark of trees, it seals the respiration qualities and that can damage or even kill a small tree.
If the ants are coming up the outside of the container, that is where you would place the oil.

Alternative would be to add a pan larger than the container bottom and oil that so the ants could not get to the tree.

Redhawk