[KarenO]

Ants don't generally harm or eat plants so I would first check to see what attracts the ants to your lemon. I had ants in an indoor grapefruit tree that I used to put outside for summer and bring in in the fall. I thought it was so odd that they had moved in until on closer inspection, I discovered that the ants were there because of the honeydew produced by a bad scale infestation on my little tree. Ants are easy to get rid of with a product called "ant be gone" scale insects are a real difficult critter to get rid of. I never did get the plant cleaned up and wound up getting rid of it. aphids also produce honeydew which attracts ants. If I was you I would have a look at the lemon tree first because there must be a reason the ants decided to set up housekeeping in your pot.
KO