The gnats don't actually do noticeable damage, but they lay eggs in damp soil, and the larvae can damage the little seedlings.

Three things I have read about to help combat the gnats:
Let the soil dry out more before watering.
Water with 10% hydrogen peroxide in your water.
Top the soil with fine gravel

Those gnats are Fungus Gnats and their larvae are maggots which will feed on the roots of your seedlings and do serious damage. You will need to treat your soil with something to kill the maggots. There are several possible solutions. I used to use Mosquito Dunks, powdered and added to the water that I water my seedlings with. Nowadays I use a systemic insecticide to prevent them before they get started because the Mosquito Dunks aren't effective against Thrips, which also can be a problem for me on my indoor-grown zinnias.

ZM

isolate them ASAP ...

in the meanwhile sterilize some new media per link ... i do a grocery sack .... until i see steam.. or water droplets ...

then repot with all new soil ...

and non-adverse to chems or bio controls as i am.. i dont like to do it in the house .... i dont need a 7th toe on my left foot ... 6 is my absolute limit.. lol ...

and next year.. sterilize everything first.. 10% bleach on tools.. trays.. pots.. etc... reused stuff i mean ... and the media ... watering can.. etc ... an ounce of prevention is worth a pound of cure.. as they say ....

one year.. growing hosta seeds indoors... i could not see the darn hockey game thru the haze of fungus gnats ... it really didnt matter to me.. that they did no damage.. lol ... so i flea bombed the tv/light stand room ... and repotted everything ... thereby killing the flying generation.. and the egg generation ... a man has his limits .. lol ..

you were going to up pot your seedlings.. werent you??? .. you should now ....

ken

Here is a link that might be useful: link

mxk3 - don't be too alarmed. For starters, the adult gnats do no harm apart from annoying you when they fly about. Even the larvae of most species don't do anything except eat decaying plant matter and funghi. The larvae of some species will damage seedling roots but do little to bother established plants. Mandolls has give 3 proportionate responses. The main one imo is to ensure the potting medium does not get so wet.

the adult gnats do no harm apart from annoying you when they fly about.

===>>> ohhh.. flora doesnt know us.. does she ... lol ...

when i say something drives me crazy.. do understand.. that is usually ... a VERY SHORT DRIVE... lol

ken

@mandolls "Water with 10% hydrogen peroxide in your water. " Where do you find 10% hydrogen peroxide?

hey al.. one might think they meant 1 part h2o2 and 9 parts water ...

or 10% solution of h202 in your water ...

lol ... not the best use of the english language.. and i mean my explanation ...

just like i said 10% beach ... 9 parts water.. 1 part bleach ...

ken

Yep - Sorry I wasn't clear - I meant 1 part hydrogen peroxide, 9 parts water. It is usually sold in the pharmacy section in 3% & 6% strengths - I use the 6% strength. If you have the 3% strength on hand - use 2 parts with 8 parts water.

the real quandary with drenching ....that always baffled me ... was that the whole idea that the gnats are thriving because your media is too wet ...

and the only idea we can come up with.. is to completely drench too wet soil ... lol ...

if the pots arent drying properly.. whats the point of drenching them ...???

a real catch 22 ...

that is why ... i think repotting ... is the better alternative ... just get rid of the sopping wet media ... or prevent the issue in the first place ...

ken

YOU DON"T NEED DRENCHING OR PEROXIDE OR REPOTTING

Getting rid of fungus gnats is easy. Fungus gnats spend most of their life in the soil. They emerge as flies to mate and lay eggs in the soil so killing the flies accomplishes nothing. The eggs hatch and become larvae. They feed on debris in the soil and plant roots.

Take care of the eggs and larvae and you'll have no more fungus gnats. Mix a 1 teaspoon of insecticidal soap with a quart of water and water the soil of the plant with it. You don't need to make the soil sopping just water a it so the soil is damp. You can always go back and do it again. The soap kills both eggs and larvae and won't hurt the plant. I usually only have to do it once and they are gone but there is no harm in doing it again if you don't get them all

Ken, the biological remedy of Bti should not be added as extra drenching but as your normal watering when the plants need to be watered.

Yes, it's important to allow the medium to dry appropriately. Sadly, the pellets and most potting mixes are prime environments for FG breeding. It makes a great deal of sense to find a mix that won't support these little pests or to amend what you have with enough perlite or bark fines that the gnats can't breed. The coarser the better.