I think Skylight is right with fungus gnats or what I usually calls sciarids, ( same thing)if you see them running about on the soil surface when you water then you can bet your bottom dollar thats what they are.
I had an infestation of them in my cool green house last year which seemed to target seedlings mostly but also attacked the roots of a sweet potato, I was growing in a pot.
I managed to get rid of the flies by putting the plant outside for a spell and soon discovered these critters dont like the out-doors, my carnivorous plants traped quite a few as well, especially the honey dew. Good Luck Tbird+