Those little bastards are generally more of an annoyance than anything. But if the infestation gets too bad, the larvae can actually do damage to the root hairs, inhibiting their ability to intake nutrients, and delaying or stunting growth. Keeping 'No-Pest' strips near the soil surface work well to catch the adults that can fly. You can also hang them near lights where there isn't a strong draft, and the flies will naturally gravitate right into them. I'd prefer As for the larvae, there are a few things you can try.

1) Prevention. Be careful not to overwater - larva can't survive dry conditions. Also, make sure there isn't any standing water in the collection trays of your pots. This makes for a very comfortable breeding ground for them.

2) Insecticides. You need to be real careful about which chemical pesticides you introduce to your crop, and they'll only do limited good against fungus gnats, since they spend much of their time in and around the soil. Pyrethrum and neem treatments work pretty well, as do homemade recipes using hot pepper, tobacco water, etc.

3) Suffocation! Many growers advocate using sand, or a layer of perlite to block access out of the soil. I've heard mixed reviews about this, and have yet to try it myself, so I'll let someone else attest to its authenticity.

I haven't had a really bad infestation in years (knocking on wood), but the last time I had one, I just put a few of those pest strips in the room and got very conservative with my waterings. Waiting until I noticed a slight bit of wilt before watering with just enough to keep them happy. After about a week of minimal waterings, I noticed less and less of the adults flying around until one day they practically disappeared. I'd still see the occasional one fluttering about, and I'd just smash the little devils between my fingers. The soil I was using is what was causing my problem. This may or may not be the case for you, but many commercial potting soils are anything but pest-free.

Hope that helps a bit. :jointsmile: