Fungus Gnats - Soil Gnats

Fungus Gnats

Issue: Fungus Gnats / Soil Gnats

From over watering, is what they say causes it. But I was only watering 1 cup, 1 time a week, per 10- 3 gal grow bags. Soil was dry, just under surface, in-between each watering. Top soil was barely slightly damp each week at watering time. Room humidity probably had a lot to do with this too, not drying.

How to kill them quickly

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This is same pic of the little Fungus Gnat, that I’m gonna Murder.



7-14-20, I made up Hydrogen Peroxide & water, Full spray bottle, then, foliar & Soil soaked, 10- 3 gal grow bags.
Hopefully, since this is supposed to kill Fungus Gnat larva on contact, this will take care of this issue.
I will repeat this process in 3 days.

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Do you have any soil bags outside the tent in a humid area?

My wife did a large vegetable grow in our tent before I got ahold of it and started the biggest fungus gnat colony in the world. I cleaned and bleached everything and then started a seedling tray in there and noticed they were back and couldn’t figure out why until I remembered she had 2 cheap bags of soil stored in the corner of the room. Once I removed those and put in a dehumidifier I sat every night for 3 nights crushing flying bugs with my fingers until the LED burned my retinas.

Haven’t seen em since.


Nope, I used all my soil in the 10- 3 gal grow bags. And no tent… Can’t afford one, nor the ac, vents, and other stuff to complete the tent. @CitrusCircus
This below is my grow area.
A closet.

It doesn’t look bad. It’s just gonna be even more impressive when you’re drying out a lb from your grow! Grab a cheap oscillating fan it can make a 5-8 degree difference in temp and like 10% humidity at least from what I’ve seen. Plus it makes landing in the soil difficult for the gnats I think their life cycle is only like a week or 2 anyway and if they can’t lay eggs in the soil then they’re the last generation.

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True, life cycle is only 1 week. And when I sprayed, the Queen came out… That joker was half inch long. I grabbed it up and smashed it with fingers…
Busted your Bitch ass!!! It was moving to fast to take pic, didn’t have phone cam ready anyway. Didn’t mind the guts, cause I was glad it was Dead. I believe I’m on FAST TRACK to recovery. @CitrusCircus


Best cure for fungus gnats is BTi. It’s a bacteria that kills the larvae. It’s in mosquito bits as well as available in a pure form through eBay.

H2o2 in your root zone can damage the fine hairlike roots as well as kill off beneficial bacteria/fungi.

It’s fine if you’re using fox farms nutes or other synthetic bottled nutes, but if you’re “organic” it’d be bad.

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Ya… that might would kill my plants, no thanks. @Drinkslinger This, what I’ve done, according to first posted message site link, is supposed to do the trick. Will do it again in 3 days just to be sure.

Ok, I don’t think that the Hydrogen Peroxide is going to work, because I still see the Fungus Gnats crawling all over the Peroxide soaked soil.
Sooooo, I ordered some Neem Oil. This will definitely work. GET OFF MY BABIES!!
This will be here on July 25th.


Fungus gnats are almost always an indication of moist conditions in your grow room, and usually occur when cannabis plants are being overwatered.

      **Life cycle**

Fungus gnats develop in four stages: egg, larva, pupa, and adult. In favourable warm and wet conditions, adult females deposit hundreds of eggs in the soil. Larvae feed for about two weeks and then pupate near the soil surface. After 3–7 days in the pupal stage, adult gnats emerge and live for about eight days. It takes about 3–4 weeks for adult gnats to develop from eggs.

As fungal spores are around us everywhere pretty much all the time, all gnats need are wet conditions and organic matter to appear.

Many times, their eggs are already included in unsterilised store-bought soil.


At first Fungus gnats don’t consider the roots of your cannabis as their favourite food. They’d rather devour fungus and decaying matter in the soil. But, they do become a problem when organic matter in the soil is depleted. In that case, fungus gnat larvae devour and damage root hairs and the tender cannabis roots themselves.

Symptoms of fungus gnat infestation:

• When roots are damaged, plants can show various signs of disease including yellow leaves, wilting, spotting, and drooping.

• Symptoms of nutrient deficiencies can occur (sickly growth, leaf discolouration, etc.) despite correct pH and proper nutrients.

• Seedlings weaken, fall over, and die (“damping off”).

• Stunted growth and low yields.


A fungus gnat infestation is relatively easy to diagnose. Fungus gnats are small, but you can see them if you get close to your plants. Here’s how to spot them:

• Tiny “flies” crawling and jumping on the soil around your plants.

• White maggots wiggling in the soil. Not as easy to spot as they are very tiny and may be hidden in the soil.

• Otherwise unexplainable plant issues including, but not limited to, pale leaves, spots, brown edges on leaves, and drooping plants. Signs can vary and may appear as other plant issues and diseases.


In a certain sense, fungus gnats can be a good thing, as they alert you to issues with your watering regimen. When they appear, it’s almost always because your plants are being overwatered. Fortunately, getting rid of them is relatively easy:

Water less frequently

The first, and most important, thing you should do is rethink your watering routine. Allow the soil to dry out fully between waterings—it will be fine. Most of the time, if you do that, the fungus gnat problem will go away on its own.

Yellow sticky traps

Get yellow sticky traps from the garden store and position them around your plants. These traps attract the fungus gnats, which will stick to them. Know that yellow traps alone will not entirely eradicate an infestation. But, they are a good indicator of whether you (still) have fungus gnats and can prevent many of them from harassing your soil.

Neem oil

If you have a pest infestation, you don’t need to get all hardcore with chemical insecticides. Neem oil can help get your infestation under control. It’s also good as a preventative measure. Treat the top layer of your soil with it to eliminate the larvae.

**Diatomaceous earth / DE **

Diatomaceous earth is another way to get rid of bugs naturally. Sprinkle the fine powder on the soil around your plants. This should take care of the nasties in no time.

Blow air over the soil

A simple standing fan gently blowing air across your soil has lots of advantages. Making life miserable for bugs is one of them. It will also help dry out the soil quicker to eliminate the gnats for good.

BT bacteria

Some growers know the Bacillus thuringiensis bacteria for getting rid of caterpillars. There is a variety of these bacteria, Bacillus thuringiensis israelensis, that works for fungus gnats. Just add water.


Prevention is always better than treatment—here’s what you can do to keep those nasties away.

Water less frequently!

More than anything else, pay attention to the amount of water you give to your plants! Wet, warm conditions are practically begging for fungus gnats. Allow the soil to dry out. If you are not sure when to water, you can do the “lifting test”. Simply lift up your plant with its pot and compare the weight when fully watered versus dry. Water only if the pot feels noticeably lighter.

Cover the soil

Since fungus gnats live and breed in soil, you can cover it with sand, gravel, or perlite to prevent them from getting cosy. Keep in mind, however, that while covering the soil may prevent the gnats from appearing, your plants may still be water-logged.

Sterilise your soil

Sometimes, you simply can’t prevent bugs from appearing, for instance if they were already present in the soil. To be on the safe side, sterilise your soil first. Do this by putting some soil in a flat, oven-safe container like a baking pan and cover with foil. Place a meat thermometer into the centre and “bake” for at least 30 mins at 82–93°C. While you’re at it, ensure your pots and growing area are clean as well. This way, you can minimise the risk for all manner of diseases, fungi, and pest infestations.

Consider alternative growing media

If you find your cannabis plants are frequently “bugged”, consider growing in an alternative medium. When you grow weed in coco, perlite, or hydroponically, pest infestations are less likely.


Try diatomaceous earth, it’s fairly cheap and works great. I used on my outdoor grow and haven’t had a problem since… :grin::grin::+1::+1::+1::+1:


Did you spray DE wet and let it dry or sprinkle it all over the soil?

DE needs to be dry to work.