Where are the flies coming from in my gardens?

I just took a walk around my small garden where I have planted lots of petunias, pansies, and hostas. There are lots of black flies swarming around all of the plants. I do have those ant traps down to kill a red ant problem that I noticed while I was planting the flowers... but whats with the flies? where are they coming from and how can i get rid of them??