Yes they can do some serious damage around the base of the stem. I finally knocked my infestation out with the Microbe-Lift BMC soil soaks once a week and lots of yellow stickies. I still treat the media once every week or two to prevent any future infestations. I had a hard time knocking mine out because of the pots i was using ( Radicle bags). I finally was able to get a good media soak by lowering the pot into a 5 gallon bucket to get a good media soak with the BMC.Ya man all I'm saying is I didn't have gnats before the quick fill bags came into my flower room. And they're pretty bad, when I go in there I constantly have them flying in my face and see them all over strong the bases of my plants. I'm at war right now lol.