Quite possibly. The ants secrete Formic acid which does the roots of some plants no good at all. Dianthus especially, which like an alkaline soil, really hate the acidity. Try to kill off the ants if you can.

Could be that the ants' nets are drying out the soil around the plants' roots. So, I would try Ant Stop if you don't mind the expense. Otherwise, try and keep the soil well watered and this might deter the ants from nesting.

A cheap way of getting rid of ants is to cover a jar with a small amount of honey or jam in the bottom and wrap in cling wrap. Make a very small hole in the top. The ants climb in and get trapped in the honey. We find this much more effective and a lot cheaper than any commercial product.