I honestly don't know where they come from,they just seem to show up one day unannounced.
I don't think they live in the soil like fungus gnats. I just noticed some on my cuttings the other day,and sprayed with incecticidal soap,it was just a few,but I seemed to have caught them in time,as I have not seen any lately,but I am spraying a few more times just to make sure all the young and eggs are killed.Come on spring,so they can go outside !!!