FUNGUS GNATS! Inundanted with them last grow. Root balls from the 5 gallon pot were the size of fists. They ate ALL the hair roots.
DE is great (Diatomaceous earth) but you can't just sprinkle it on the top. It needs to be mixed IN with the soil when you pot, so that the larvae deep in the root ball will be able to eat it and get cut apart from the inside.
If you already have an infestation, adding DE to the top will do little to nothing, plus it will get all sticky like toothpaste. YOu need a drench--mosquito dunks as Hick advised will work, but I like GNATROL which is getting harder to find, and also AZAMAX.
Best is to just mix in a few cups of DE when you make up your soil mixture to begin with and they will never get a foothold.
That Azamax works pretty good for a lot of them munching critters, but make sure to apply it twice. Do a drench one time, wait a week then do another drench, wait another week and do a light flush and then get your PH right. I didn't flush afterwards and it wacked my PH for about a week before I realized it. May be different in soil. I was in hydro.
I used teh azamax and mixed a strong batch up and it killed all the gnats with one treatment and havent seen one since. I also got the DE and pu tit on top and water from teh bottom now. but you should mix it in with the dirt next time