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.