How to Get Rid Of & Kill Fungus Gnats

Fungus gnats are one of the more annoying pests you may potentially encounter when growing cannabis. While these small black flies are harmless to people, they can prove to be quite lethal to your cannabis crop if left unchecked.

What are Fungus Gnats

Fungus gnats are small flies that pretty much live for the sole purpose of reproducing. While adult flies are harmless to humans since they don’t bite or spread human pathogens, they can be a big problem for your plants.

Adult fungus gnats can be vectors for plant diseases like Botrytis, Fusarium, and Pythium. But even worse for your plants are the larvae of fungus gnats which can kill or stunt the growth of seedlings and young plants as well as severely weaken a more mature cannabis plant by feeding on roots, root hairs, and the organic matter in your growing medium.

Cannabis plant with stunted growth due to fungus gnat infestation at seedling age
(This is an autoflower cannabis plant at about 30 days old. You can see how small it is – growth was primarily stunted due to it dealing with a fungus gnat infestation when it was a seedling.)

What Causes Fungus Gnats

If you start noticing fungus gnats around your plants, it’s almost a surefire indicator that you’ve been overwatering your plants.

Fungus Gnat Infestation Indicators

A few indicators that can signal that you have a fungus gnat infestation:

  • Slow growth
  • Sudden yellowing and wilting of plant leaves
  • Multiple black flies around soil level

Still unsure whether you’re dealing with a shit ton of fungus gnats? You can try the following:

  • Put some yellow sticky traps in your growing medium. If you see a bunch of flies within a day or two then you have a problem.
  • Take a raw potato and slice it into 1-inch by 1-inch by 1/4-inch pieces. Place a bunch of the slices around the surface of your potting medium and leave it for about a day. When you check the slices, if you see If you notice next to each other on the surface of your potting media to attract fungus gnat larvae.
Yellow sticky trap with multiple fungus gnats.
Houston… I think we have a fungus gnat problem

Getting Rid of Fungus Gnats

If you want to get rid of fungus gnats and minimize your chances of having to deal with a future infestation then you’ll want to take measures to kill the current fungus gnat population as well as take steps for prevention.

To kill your fungus gnat population, you’ll want to take a two-prong approach that deals with both the adults as well as the larvae.

Killing Adult Fungus Gnats

Yellow sticky traps are a great first step in helping to control the problem. The gnats are attracted to the yellow color allowing you to easily capture the flying mature ones that will end up stuck to the trap.

I would suggest having at least one trap for each plant. Even though fungus gnats don’t typically fly very far, they can still easily end up in a neighboring plant.

Killing Fungus Gnat Larvae

There are several products that you can try to make use of to help with reducing and eventually killing off the larvae. As far as research shows, these products are nontoxic to people.

  1. Use a predatory nematode, specifically steinernema feltiae, which releases a symbiotic bacterium after entering its host. This symbiotic bacterium causes the fungus gnat larvae to die due to blood poisoning. Because this nematode is effective when conditions are moist, I would recommend using it as an initial measure before using something else. Of course, if you’re the type of person that continues to overwater their plants, then you can keep applying this nematode.
  2. Use any product that contains bacillus thuringiensis subspecies israelensis (BTI). This product is toxic only to the larvae of certain flies, one of those being the larvae of fungus gnats. Typically, you’ll need to do several applications for long-term control. You can use this in combination with steinernema feltiae.

Preventing Fungus Gnats From Invading Your Plants

The best way to avoid a fungus gnat problem is to ensure that you don’t provide a hospitable environment for them in the first place. As we mentioned earlier, fungus gnats are attracted to moist soil near the surface so let your soil dry between watering as much as your plants will tolerate. Furthermore, look to bottom-water your plants as this will make it easier to keep your surface layer dry.

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