few yrs back, i had an ornamental ficus. after yrs of staying in a small pot, and very spent soil, i decided to give it a good new soil. i got MG soil. as soon as i changed the soil, i noticed gnats. i took it outside and no more gnats in the house.
since about month or so ago, i noticed few gnats. they were feeding my D. venusta. in that room, i have C. follicularis, and some Huernia. and i root my fig cuttings there. i moved all my cuttings out of the room since they were more than likely to hold gnats and they were ready for the 1 gal. Huernia have about 1/2" of crushed stones on the top. and the soil mix dries super fast. but my D. venusta and C. follicularis have soil mix that will hold the water.
now. i keep my D. venusta on a dish of water, and C. follicularis in a enclosure with top and bottom. guess where the gnats are coming from? that dish of water. so if i get rid of that dish of water, there won't be any problem. but i'm keeping that in the room.. i need gnats to feed my D. venusta.
now.. if there is gnat infestation, first thing you want to do.. before start rooting the cuttings, is isolate where the gnats are coming from and get rid of the source where the gnats breed. sanitize the area and see if you see any new gnat. then once it's clear start rooting again.
my dead MP is also holding gnat larvae. i tapped the 1 gal container that MP was in and saw few flying out. so for whatever reason, the cuttings started rotting and the gnats took hold of the place.