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Cannabis Pests - Fungus Gnats

Created by
JoshuaHolt JoshuaHolt
Added 17 December 2020

Cannabis Pests - Fungus Gnats

They look harmless in the beginning, but as their numbers start to increase a cannabis plant can become severely weakened to the point of death. Fortunately, treating fungus gnats is easy to do and they can usually be dealt with quickly using a few simple pest control techniques. Read on to find out how to get rid of them once and for all.

What Are Fungus Gnats?

Fungus Gnats Run and Fly Around The Top Soil

Fungus gnats, sometimes referred to as soil gnats, are tiny flies that measure around 2mm in length. They have long legs which they use to 'run' or crawl around with when they are not flying. Fungus gnats range in colour but are usually either black or brown.  

Fungus gnats lay their eggs in soil and the larvae feed off fungus, as well as a cannabis plants' delicate roots. A female fungus gnat can lay around 200 eggs each week, so they can indeed multiply very quickly. The white larvae/maggots can sometimes be seen wriggling around in the top soil, but they are very small and hard to see.

Fungus Gnat Larvae Live In The Top Soil

Although they do not seem like the most problematic pest in the world, fungus gnats have an appetite so they can and will eventually destroy a cannabis crop if the cause is not corrected. The real problem starts when so many eggs have hatched in the soil and the roots become seriously affected, leading to an overall decline in plant heath.

Fungus gnats are also particularly effective disease transmitters and also create an easy ecosystem for further pests or fungal infestations to appear.

The Causes of Fungus Gnats

Fungus Gnats Like Moist Conditions

The primary cause of fungus gnats settling in is overwatering. They love damp, humid conditions where there is a presence of fungus. A constantly wet soil provides exactly right opportunity to attack. When the soil stays wet for too long (especially in warm conditions), fungus develops and organic matter starts to break down; the ideal feast for fungus gnats.

Fungus gnats actually aid the decomposition of organic matter in the soil.

First they feed off the tiny fungus spores developing in the soil, before proceeding to munch away at the roots. Until the problem is corrected and the top soil is allowed to dry up, the fungus gnats will be there to stay.

Fungus gnats could also be a sign of rotting plant matter not just in the soil. Pythium (root rot), for example, which is also caused by overwatering can attract fungus gnats, giving you indications that there could be a more serious underlying problem somewhere.

Identifying Fungus Gnats

Fungus Gnats On Cannabis

If you suspect a fungus gnat infestation, there a few ways to check to confirm your diagnosis. Firstly, they are quite distinct in the way they move. They move extremely quickly on their feet for their size, and are distinguishable through the way they jump and crawl around the top of the soil. You may also see them flying at the tops of your plants.

If you're having trouble seeing the fungus gnats, place some yellow adhesive strips around the edges of your pots where they would normally hang out. After an hour or two there will likely be a couple of fungus gnats stuck to the adhesive, allowing you to take a closer inspection.

It could be that you notice something wrong with your plants before realising there are fungus gnats affecting their health, although it's likely you see something buzzing around first. A cannabis plant suffering from an infestation will start to show symptoms mainly through the leaves. Plants in vegetation are more susceptible to heavy damage, but they can also affect flowering plants.

Fungus Gnat Plant Symptoms

Symptoms of fungus gnat infestation:

  • Small flies around top soil
  • Slow growth
  • Wilting and drooping
  • Leaf discolouration and deformation
  • Spots and burnt leaf edges
  • Nutrient deficiency symptoms - root problems

You may see a couple of fungus gnats buzzing around one day and not think too much of it. However, the earlier you identify and fix the problem, the less of a task it becomes.

Fungus Gnat Treatment

Yellow Adhesive Strips Work Well To Catch Fungus Gnats

The most important thing to do in the event of a fungus gnat infestation is let the top of your soil dry out. In a dry environment, the larvae cannot survive and will eventually die off. Readjust your watering schedule and do not water until at least 2-3 inches of the top soil feel almost bone dry.

How To Treat Fungus Gnats:

  • Yellow Adhesive Tape - These sticky strips can be picked up in most hardware or garden stores. Place a few around the tops of your pots. Very effective at reducing the number of females laying eggs. Helps you monitor the populations more clearly.
  • Hydrogen Peroxide - For dealing with the larvae, a diluted food grade H2O2 solution can do the trick. After the top soil dries, water with 3% H202 at a ratio of 1 part hydrogen peroxide to 4 parts of water.
  • Diatomaceous Earth - Made up of fossilized remains of miniscule sea shells and has a texture similar to chalk. Desiccates and ruptures the larvae and adults, stripping them of water. Not toxic for humans (organic) but avoid inhaling. U
  • Neem Oil - This effective, organic insecticide is used to control many pests associated with cannabis. Can be applied to the top soil using a pressurized misting sprayer once or twice a week until the problem has subsided. Avoid spraying on flowers in the last 3 weeks of flowering.
  • Oscillating Fans - Place fans pointing at the top soil to help dry it out faster. You also prevent the adults from landing so easily to lay more eggs, creating an unfavourable environment for them.
  • Biological Control - There are a few predators you can add to the soil which help to kill the larvae. The most effective is Bacillus Thuringiensis, but you can also look into Steinernema Feltiae (nematodes), Hypoaspis Aculeifer (mites), and Atheta Coriaria (beetle).

Keep monitoring your plants daily after you have spotted the problem and begin applying treatments. Always check the top soil for dryness by sticking your finger into it before adding more water or other liquid solutions. The period between waterings should be extended until the pest has gone.

Fungus gnat running around in grow by Ice_R from GrowDiaries.

Finally, be as hygienic as you can from now on. Clean your hands after you do work in the grow room to avoid spreading any fungal spores.

Tip: When the infestation has become out of control, it can take longer for the soil to dry because the excrement of the larvae keeps it moist. 

Tips For Preventing Fungus Gnats

Prevent Fungus Gnats By Cleaning Up Decaying Leaves

Preventing infestations is the best way to avoid any kind of pest, and with the proper management we can create a safe environment for our cannabis plants by minimizing the risk as much as possible.

  • Regularly clean up brown/dead leaves falling off the plant onto the top soil.
  • Never let dying plant matter sit on your top soil, especially if the soil is wet. 
  • Do not overwater and give your plants a proper wet/dry cycle.
  • Be careful where you get clones from as they can carry disease into your grow room.
  • Provide plenty of ventilation, especially low down on the plant where moisture gets trapped.


If you see fungus gnats in your cannabis garden, act quickly. This pest is easy to get rid of when you don't allow it to reproduce by simply cutting the watering schedule. However, if the problem is already ruining your plants, you may need to apply some more serious measures to get fungus gnats out for good.

If you found this article useful, we'd love to hear from you! Leave your thoughts about fungus gnats in the comments section below!

External References

Fungus Gnats as Houseplant and Indoor Pests - W.S. Cransaw and R. A. Cloyd. 2009.

Effect of Diatomaceous Earth and Trichoderma harzianum T-22 (Rifai Strain KRL-AG2) on the Fungus Gnat Bradysia sp. nr. coprophila (Diptera: Sciaridae). Journal of economic entomology. - Cloyd, Raymond & Dickinson, Amy & Kemp, Kenneth. (2007). 

Management of Fungus Gnats in Ornamentals 1. - Price, James & Osborne, Lance & Nagle, Curtis & McCord, Elzie. (2020). 

Effect of Bacillus thuringiensis subsp. israelensis and neonicotinoid insecticides on the fungus gnat Bradysia sp nr. coprophila (Lintner) (Diptera: Sciaridae). Pest management science. - Cloyd, Raymond & Dickinson, Amy. (2006).

This article was updated November 2020.



Sprinkle some cinnamon on the earth.
They hate this.


Have a lot of problems with these gnats. I always use neem cake and vaccinate my soil with nematodes before the grow starts and sprinkle a bit of neem on the top of the soil. Works.