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How do I deal with fungus gnats? I've had SO many suggestions.

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How do I deal with fungus gnats? I've had SO many suggestions.

Growth 91 Replies 40,089 Views
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Growth

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#1
This is my 4th grow and first time getting fungus gnats. I've tried letting the soil dry up it's barely "moist" knuckle deep at this point but I don't want to under water them.

I've tried crushed eggshells on top of the soil.
I've tried fruit fly traps with apple cider vinegar.
I've tried spraying the soil and leaves down with diluted alcohol spray.
I have fans blowing on them.

I'm growing auto-flowers and I'd say they're almost halfway done. Are these things going to kill my plants? How do I get rid of them?
 
#5
I read something recently that sounds like it might work. That is to cover the top of the soil with about a half inch of sand. The article said they can't survive in the sand. I didn't do it because I top dress nutrients and recycle my soil. It does sound like it would work, though.
 
#6
At some point in my growing experience, I started spraying the surface of the soil with plain water before watering because the water is supposed to penetrate more evenly. Then I noticed that sometimes when I sprayed, a lot of gnats would show themselves. So, I started spraying with a hydrogen peroxide solution instead of water. It's supposed to kill them on contact, and it seems like it does. I only sprayed enough to wet about the top 1/4 inch of soil. I didn't want the solution to penetrate so deeply that it would affect the soil microbes. After spraying, I wait before doing my normal watering because hydrogen peroxide breaks down quickly under light and normal air pressure. If I'm having a problem with gnats, I spray the soil more often.
 
#7
encladd said:
I had enough last night and used my shop vac to suck up probably 100 adults lol.

Top dressing with neem seed meal helps keeps the larvae under control, maybe some diatomaceous earth as well. I don't think there's a magic bullet.
Click to expand...
Be careful with the neem seed meal, it has NPK and you can overfertilize. You have several suggestions. I also use diatomaceous earth, its mostly silica which will benefit your plants and its very forgiving in terms of overfertilization. If you cover all the top soil and specially the borders of the pot (they hide there when you use something on the substrate) they will all die. You just gotta make sure they dont have anywhere else to hide and put eggs. If youre using plastic pots you might wanna put some below them aswell since they can get inside using thse holes while the diatomaceous earth slowly goes away with each watering. Its also only effective when dry so dont be watering every day if youre gonna do it, and mix it well with the top layer soil, you dont need a layer of diatomaceous earth just a slighty white and uniform layer of substrate with diatomaceous earth, not missing any crack.
 
#9
This is a simple solution solved with the combination of Entonem [nematodes] & yellow sticky strips.
This is a failproof and chemical free solution with no harm to flowering plants.
Do not spray oil based insecticides in flower.
 
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#10
I find more simple and much cheaper to just make sure I have all my top soil with diatomaceus earth. 10 euros 10kg on Amazon, I got silica for my plants and gnat killer for years to come. That said nematodes will work, they will kill the larvae and the sticky traps will kill the adults, but so does diatomaceous earth. Theyre tiny crystals that stick to the larvae or adults on contact and dry them to death. You will get thicker branches too, so its a win-win situation for me. Cheap, effective if you do it correctly and gives slow release silica.
100% organic too, no harm to flower or plants nor to beneficial microbes, quite the contrary, you make them more resillient to pests and help them get thicker.
 
#11
If you use perlite, putting an inch of perlite at the bottom of your pots below the soil, as well as another inch of so covering the top of the soil will knock them back big time. Let your pots dry completely between watering as well. I add diatomaceous earth to my soil mix as well. Like, 10 gallons of soil will get 4 big red cups full of DE that I will mix in real good. If you let the soil dry out well between watering, the DE will really do a number on the larva.
 
#12
mosquito bits...simple cheap and effective. sprinkle this around the top of pot. Sand and DE work too but this is less messy and watering does not affect it. Use it liberally. Will not harm the plant or you.
 
#13
Growth said:
This is my 4th grow and first time getting fungus gnats. I've tried letting the soil dry up it's barely "moist" knuckle deep at this point but I don't want to under water them.

I've tried crushed eggshells on top of the soil.
I've tried fruit fly traps with apple cider vinegar.
I've tried spraying the soil and leaves down with diluted alcohol spray.
I have fans blowing on them.

I'm growing auto-flowers and I'd say they're almost halfway done. Are these things going to kill my plants? How do I get rid of them?
Click to expand...
I dry my soil out for a couple weeks and then I use rice hulls as a mulch layer to keep things up top dry. I've never had enough that they bothered a plant, I just pick up yellow sticky traps and made sure to dry it out as much as I could in between waterings. 🤷‍♂️
 
#14
yo bro i had a bad infestation last run and tried all the same things you did, and all the time i was killing adults they were laying eggs in the soil and more and more appeared. they were in every room in the house. the ole lady went ape shit. Don't worry about the adults kill the eggs in the soil, and use yellow sticky traps for adults and to see where population numbers are. What i did was research and found mosquito bits, and they worked. its not overnight so be ready for a 2-3week fight, depending on how bad it is. Stay deligent it works. I put 4 tbs of mosquito bits in a mesh bag placed in a gallon of water and let it sit for at least 30 min u can stir it or just let bag sit. after 30 min drench soil slowly and completely. do this once a week. i had to do it 3 times(3weeks), and havent seen a gnat since. i do it now every 3-4 weeks just as an ipm. you also dont need to use bag u can put bits right in water then water onto plants, its messier doing it that way. and it doesnt hurt to let the bits stay on soil either. hope this helps. and depending on how old your plants are, gnats are harmless to us and plants. the larvae can eat the roots of young plants if theres no organic matter left in the soil for them to eat, but on older plants the roots are established and the larvae live in top 2-4 inches of soil.

oh and i used the mosquito bits, not the mosquito dunks. Don't know about them. they probably work the same, just have to let them sit in water longer.
 
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#16
Growth said:
This is my 4th grow and first time getting fungus gnats. I've tried letting the soil dry up it's barely "moist" knuckle deep at this point but I don't want to under water them.

I've tried crushed eggshells on top of the soil.
I've tried fruit fly traps with apple cider vinegar.
I've tried spraying the soil and leaves down with diluted alcohol spray.
I have fans blowing on them.

I'm growing auto-flowers and I'd say they're almost halfway done. Are these things going to kill my plants? How do I get rid of them?
Click to expand...

ArtfulCodger said:
Diatomaceous earth, top dressed and worked in a little bit to kill the crawlers. Yellow sticky traps to get the fliers. That's what I do, anyway.
Click to expand...
Also a product named Tanlin which has the instructions however it’s a 3 week process at least & the larvae create the real damage eating the roots & can cause a lot of destruction. Good luck
 
#17
Someone suggested the tanlin to me once, looks pretty expensive.

As for my post which angered someone above lol:

spinosad is a natural, non systemic pesticide made by fermenting bacteria and is safe to use as root drench all the way through flower. Latest I’ve used it personally is week 5.

Neem extract, which is a debated topic, used as a SOIL DRENCH in veg is okay in my eyes if used in rotation to break a life cycle. Also naturally derived

Pyrethins are derived from flowers and considered non toxic to humans, they are also approved for use on organic farms.

Bti, again, bacteria.

Pretty much everything I listed is allowed in Canada and that says something lol, plus who’s spraying anything to kill fungus gnats?
 
#18
encladd said:
I had enough last night and used my shop vac to suck up probably 100 adults lol.

Top dressing with neem seed meal helps keeps the larvae under control, maybe some diatomaceous earth as well. I don't think there's a magic bullet.
Click to expand...
The magic bullet is T-Drops. Paralysis. It freezes the maggots from chomping away at your roots. If your salvaging, preventing, or controlling it T-Drops every feed will keep them from multiplying and doing further damage. Nothing else should be needed, just use it every time you water.
 
#19
cheaper alternatives will not work as well as T Drops. I have experience with multiple products and if you don’t drop the coin for Tanlin (T Drops) you won’t get the results your looking for.
 
#20
Lockebox said:
Someone suggested the tanlin to me once, looks pretty expensive.

As for my post which angered someone above lol:

spinosad is a natural, non systemic pesticide made by fermenting bacteria and is safe to use as root drench all the way through flower. Latest I’ve used it personally is week 5.

Neem extract, which is a debated topic, used as a SOIL DRENCH in veg is okay in my eyes if used in rotation to break a life cycle. Also naturally derived

Pyrethins are derived from flowers and considered non toxic to humans, they are also approved for use on organic farms.

Bti, again, bacteria.

Pretty much everything I listed is allowed in Canada and that says something lol, plus who’s spraying anything to kill fungus gnats?
Click to expand...
My last grow only yielded 30% of normal grow due to fungus gnats. This was my first experience with them & now I use De-gnat barrier on top, use yellow sticky insect traps to detect adults & use Tanlin to eradicate larvae. This is extremely cheap compared to losing 70% of grow. Tanlin is the closest product to organic I’ve found & doesn’t appear to negatively effect plants
 
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