Ants will not hurt some plants depending on what they are. If you have shallow rooted plants or newly planted shrubs, the ants can become numerous enough that their extensive (and I do mean extensive...) tunneling can expose roots to air drying them out, keeping them from having contact w the soil-they are not able to asbsorb moisture or nutrients. Also then when it rains the tunnels can collapse, causing the roots to be exposed. So, no the ants do not directly harm plants, but yes I have had them reek havok on some things before. Also if you let them continue on around the foundation of your home they will become numerous and you will have gobs of ants show up in your kitchen/bath. I have been battling ants here for 3 years. I hate them.
