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What Biological Controls are Available for Fungus Gnats?

Smokin Moose

Fallen Cannabis Warrior
Adult fungus gnats are tiny black flies which spend time running around on the surfaces and sucking sap from the leaves. After the adults lay their eggs on the soil (medium), the young worm-like larvae hatch out and begin to feed upon the roots. The larvae initially eat the fine root hairs before moving on to burrow throughout the larger roots and even up into the stems. After feasting on the roots the larvae will pupate before hatching out as adults and repeating the life cycle by laying their eggs. The larvae resemble little worms. They have clear or white bodies with black heads.

Life cycle: Adults live about 7 to 10 days and deposit eggs on the moist soil surface or in soil cracks. Females lay up to 100 to 300 eggs in batches of 2 to 30 each in decaying organic matter. Eggs hatch in 4 to 6 days; larvae feed for 12 to 14 days. The pupal stage is about 5 to 6 days.

Damage from the fungus gnat larvae feeding on the roots shows up in the leaves as overfertilization and nutrient deficiency symptoms: yellowing, browning, burning and curling leaves.

Three fungus gnat larvae biocontrol agents:

Bacillus thuringiensis var. israelensis (Bti) bacteria which can be purchased as mosquito dunks or gnatrol. Bti is the most appropriate anti gnat biological control agent for DWC systems.

Hypoaspis mites.

Steinernema feltiae nematodes.

Occasionally a dry layer of sand is recommended as a deterrent to fungus gnats. This technique should be discouraged as the close packing nature of the sand particles reduces the quantity of oxygen which penetrates down into the root zone.

Harry Red

Nug of the Month: Nov & Dec 2008, Mar 2009 - Plant of the Month: Oct 2008, Feb & May 2009
Yes, dunks can be used with soil by diluting and watering in. No need to foliar feed with it. You can grind up a little and sprinkle it on top of the grow medium as well. It needs to be applied every 3 weeks or so, until the life cycle is broken.

You have to break the cycle of adults laying eggs in soil -- to hatched larvae -- to new emerging gnats, which requires a multi-pronged attack.

For killing larvae check out Mosquito Dunks . They contain the same active ingredient as gnatrol (Bacillus thuringiensis subspecies israaelenses). They are cheap and available just about everywhere. One dunk can treat 100 feet of water surface so a little piece of a dunk will go a long way. I drop about a 1/8 of chunk down in a gallon of water and let it sit for 48 hours, then mix that at a ratio of 25% to 75% fresh H2O. You can remove the chunk, let it dry and use it again. If using soil, water with the 25%/75% solution every 3 weeks to kill the larvae, until cycle is broken. I've also read that predator nematodes are also great for killing off the gnat larvae... if you can find and afford them, that is...

For adults, just use victor fly ribbons: Victor Fly Ribbon 4 pack [197056]
They work well on lots of other flying insets as well.

You can also form a barrier over the soil surface to keep the adults from landing, tunneling and laying eggs. Some people use a layer of sterile aquarium sand but that may cut off some oxygen to the roots. I've found a layer of perlite works fairly well and provides more oxygen to the soil, but they can still dig in there and lay eggs. Some people stretch women's stockings over the buckets to keep the adults out so that may be another option.

Another organic approach is to apply Food Grade Diatomaceous Earth ( DE ) Powder - 6 oz. to the surface of the soil. The shell particles of the DE are sharp enough to cut the adult and emerging gnats to pieces and kill them. You really don't want to inhale this stuff or get it in your eyes so be careful when applying. It's non-toxic though and can be ingested by humans and animals to kill worms and other parasites.

That's about the end of my experience and research on the topic. Hope it helps and good luck!!




New Member
i had a real bad go of them in my last grow location. i tried the bacillus stuff, and gnatrol without success. the gnatrol made my resevoir smell like a toilet bowl!

what i ended up doing was a total extermination and cleanup. between flowering runs, i tore down the whole grow box, and used some nasty pesticides, and a lot of bleach and sterilized everything. i then brought in new coco, and covered the tops of the pots with 2 inches of perilite.

dont know if it was the perilite that did the trick, but the problem went away. I also really kept an eye on sanitation and made sure there was no standing water, and good drainage everywhere.

hope that helps - those bugers are nasty!



New Member
I composted my pot now i have gnats if i just stop composting will they go away o ya it is also a indoor plant used bread and orange peals


New Member
I had a bad case of gnats...on my first grow ever. I grow all organic and it took a while to understand that I actually had a problem and these are the different methods I used:

1) Diatomaceous earth - low to moderate impact on the fungus gnats
2) Neem oil + pyrethrum 5-10% in water, sprayed everything - moderate impact, better than diatomaceous earth
3) Nutrilife SM90 - high impact - it killed pretty much all the flies (not sure about the larvae)
4) Nematodes - insane impact....this is like hiring an assassin....seriously. I have 3 spare packs in the fridge...just in case.

So, go for nematodes if you can...


Well-Known Member
This is not biological but cheap and effective. Sand the tops of your pots and sand your catch trays. The chance of a mature adult Fungus Gnat penetrating the sand is unlikely. In the case that they do their offspring won't be able to get out. Their bodies are too soft and will get torn up in the sand. Just make sure it the washed sand. You can pick them up from most home depots in the concrete/drywall section. Where ever there is a point of entry into your medium/soil cover it with sand and you're good to go. My rooms have been gnat free ever since I did this. I grow organically. So this method works great!

Shadow Cat

New Member
First-timer here. I brought fungus gnats into my grow. It was my fault, and I let the little bastards kill my spinach and lettuce. I cleaned my space thoroughly and started two auto seeds from soil. They cracked, got their second pair of leaves, and BAM! The little bastards were back.

This is what I did:

First, I purchased a 14-pack of two-sided yellow sticky traps (about $8). I took two and cut them in half. I lay one half flat on the soil of each pot. I attached the remaining halves to clothes pins and stuck one in each pot. So, each pot had one horizontal and one vertical trap. This is to catch the adults, which are supposed to be attracted to yellow. If you have gnats on your traps, you know you have larvae in the soil to deal with.

Second, I purchased mosquito dunks/BTI ($10, available at the hardware store, big box, etc. I respectfully suggest buying from local businesses anytime you can. Keep your money in your neighborhood, and if you spend it elsewhere then support the sponsors of 420 Magazine:thumb:). I also purchased ceylon cinnamon sticks ($7, but make sure it's CEYLON. I've read that anything else won't work!). If you take one cinnamon stick and about 1/5 of a mosquito dunk and work them in a coffee bean grinder (about $25) you will have a nice fine powder. I transfer the powder to a small plastic container. Use your fingers to sprinkle the mixture on the surface of your soil. I mix mine in about two inches deep. This should kill the larvae. I reapply the mixture once or twice a month as a precaution, and change my sticky traps with the same frequency. Once your plant starts to grow up and out discontinue the vertical trap.

While you're at it, take any bagged medium you store indoors, dump it in a plastic bin, and treat that soil with your BTI mixture. Your fungus gnat problem should be solved, and for less than $50 you have a solution that can be used over and over. Other important preventative measures are found all over this site. DON'T OVERWATER is crucial for many reasons, but fungus gnats love wet environments and so do their eggs and larvae. There are many ways to lose a crop, but fungus gnats shouldn't be one of them!
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