How To Get Rid Of Fungus Gnats In Cannabis Plants

How To Get Rid Of Fungus Gnats In Cannabis Plants

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Fungus gnats are a common problem for cannabis growers, especially those using soil as a growing medium. These tiny insects can cause significant damage to your plants' root systems, leading to stunted growth, nutrient deficiencies, and in severe cases, plant death. In this comprehensive guide, we will discuss how to identify, prevent, and eliminate fungus gnats from your cannabis plants using various methods, ensuring a healthy and thriving crop.

What are Fungus Gnats?​

Fungus gnats, also known as sciarid flies or sciaridae, are small, dark-colored insects that belong to the diptera order. They measure between 3 to 5mm in length and are often mistaken for fruit flies or mosquitoes due to their similar appearance. These insects are primarily attracted to moist, warm conditions and decaying organic matter, making cannabis plants grown in soil an ideal environment for them to thrive.

Fungus gnats have a short and fast life cycle, lasting between 18 to 30 days in total. Female gnats lay around 50-200 eggs in the soil, which hatch into larvae after 48-72 hours under favorable conditions. The larvae feed on decaying plant matter, fungus, and the fine root hairs of cannabis plants, causing direct damage to the root system.

Identifying Fungus Gnats​

Knowing how to identify fungus gnats is crucial for implementing the appropriate pest control measures. Some common signs that you have a fungus gnat infestation in your cannabis plants include:

  • Tiny flies: Adult fungus gnats can be seen jumping and crawling around on the surface of your plants' soil. They are small, dark-colored insects with transparent wings, resembling mosquitoes.
  • White maggots: Fungus gnat larvae are white, with black heads and almost transparent bodies. They can be found in the soil, feeding on decaying organic matter and plant roots.
To differentiate between fungus gnats and fruit flies, note that fungus gnats are generally smaller and darker in color, while fruit flies are larger and have a brown, yellow, or orange body.

Symptoms and Damages Caused by Fungus Gnats​

Fungus gnats cause both direct and indirect damage to cannabis plants. Direct damage occurs when the larvae feed on the plant's root system, leading to:

  • Stunted growth
  • Yellowing of leaves
  • Wilting and drooping
  • Nutrient deficiencies
Indirect damage is caused by the waste products left by the larvae, which clog up the soil, affect drainage, and promote the growth of harmful fungi and pathogens.

How to Prevent Fungus Gnats in Cannabis Plants​

Prevention is always better than treatment when it comes to protecting your cannabis plants from pests like fungus gnats. Here are some steps you can take to prevent an infestation:

1. Control Soil Moisture​

Fungus gnats are attracted to wet, humid conditions. Therefore, it is essential to maintain proper moisture levels in your growing medium. Allow the top layer of your soil to dry out between waterings and ensure adequate drainage to discourage fungus gnat infestations.

2. Use Yellow Sticky Traps​

Yellow sticky traps are an effective method for monitoring and controlling adult fungus gnats. Place these traps around your plants to catch any adult gnats that may be present. Regularly inspect the traps to assess the severity of the infestation and gauge the effectiveness of your pest control measures.

3. Sterilize Your Soil​

Fungus gnat eggs and larvae may already be present in store-bought soil. To eliminate this risk, sterilize your soil by baking it in the oven at 82-93°C for at least 30 minutes. This will kill off any eggs, larvae, and other harmful organisms that may be hiding in the soil.

4. Use Soil Covers​

Covering the soil surface with a layer of sand, gravel, or perlite can help prevent fungus gnats from laying eggs and breeding in your cannabis plants. This barrier also makes it more difficult for adult gnats to access the soil, reducing their ability to lay eggs and reproduce.

5. Maintain Good Hygiene in Your Grow Area​

Keeping your grow area clean and free of decaying plant matter is essential for preventing fungus gnat infestations. Remove any dead leaves, stems, or other organic debris from your plants and growing area regularly. This reduces the amount of food available for the fungus gnats and their larvae, making your plants less attractive to these pests.

How to Eliminate Fungus Gnats from Cannabis Plants​

If you already have a fungus gnat infestation in your cannabis plants, don't worry - there are several ways to eliminate these pests and restore your plants to good health.

1. Water Less Frequently​

The first and most important step to getting rid of fungus gnats is to adjust your watering routine. Allowing the soil to dry out fully between waterings can help eliminate the moist conditions that these pests thrive in. In most cases, reducing the frequency of watering will be enough to resolve the fungus gnat problem.

2. Use Yellow Sticky Traps​

As mentioned earlier, yellow sticky traps are an effective method for monitoring and controlling adult fungus gnats. Place these traps around your plants to catch any adult gnats that may be present. Regularly inspect the traps to assess the severity of the infestation and gauge the effectiveness of your pest control measures.

3. Apply Neem Oil​

Neem oil is a natural insecticide that can help control fungus gnat infestations. Treat the top layer of your soil with neem oil to eliminate the larvae and prevent further reproduction. Neem oil can also be used as a preventative measure to keep fungus gnats at bay.

4. Use Diatomaceous Earth​

Diatomaceous earth is a natural, non-toxic substance that can effectively kill fungus gnats and their larvae. Sprinkle a layer of food-grade diatomaceous earth on the soil surface around your plants to control the infestation. This powdery substance works by puncturing the exoskeleton of the insects, causing them to dehydrate and die.

5. Introduce Beneficial Insects​

Beneficial insects, such as predatory mites (Hypoaspis Miles) or the beetle Atheta Coriaria, can help control fungus gnat infestations by feeding on the larvae. Introducing these natural predators into your growing environment can help keep fungus gnat populations in check and prevent future infestations.

6. Use Bacillus Thuringiensis Bacteria​

Bacillus thuringiensis (BT) is a naturally occurring soil bacterium that can be used to control fungus gnat larvae. There are several strains of BT available, with Bacillus thuringiensis israelensis (BTi) being the most effective against fungus gnats. Add the bacteria to your watering solution to target and kill the larvae in the soil.


Fungus gnats can be a significant problem for cannabis growers, but with the right prevention and control measures, you can keep these pests at bay and ensure a healthy, thriving crop. By maintaining proper soil moisture, using yellow sticky traps, and employing natural insecticides like neem oil and diatomaceous earth, you can protect your cannabis plants from these pesky insects and enjoy a bountiful harvest.
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