I have not tried this but I have heard that you can catch them by using double sided scotch tape on the stems. Every kill counts....
Sounds like you may have a few fungus gnats or flying root aphids. If you dig down in the soil about two or three inches around the stalk, you will probably find larvae. Little white worm larvae is fungus gnat and they are fairly common. Root aphids are a bit more of a headache but both can hurt your finished product so its best to take care of them before they start flying and laying eggs.