Posted: 30/06/2014 at 12:31
It indicates that your pots are on the dry side. You can put down poison traps as indicated by Lesley but you need to stop them coming back too.
Give the pots a thorough soaking several times to make sure the compost absorbs the moisture instead of it just running through. Keep them watered well in future. I read somewhere that one inch of water on the top of the pot will soak down 9 inches. Adjust your regular watering accordingly and water every couple of days in the growing season.
Once the compost is thoroughly moistened you can also water on a solution of one small bottle of essential oïl of cloves diluted in 10 litres of water. The ants can't stand the smell and move on.
If you can, raise your pots onto special feet or even just bricks in order to ensure they drain well and aren't waterlogged, especially in winter. Give your plants a feed or two of liquid rose or tomato feed to help them recover as their roots will have been damaged by the ants' nesting activities so they'll be having a hard time taking up enough moisture and nutrients.