Fungus Gnats- a DIY, Natural Way to Kill Gnats and Larvae

Fungus Gnats- a DIY, Natural Way to Kill Gnats and Larvae

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If you’re anything like me, you love having fresh basil, rosemary, and thyme year round. These herbs are good dried, but having these fresh will elevate homemade dishes. One unfortunate side effect of cultivating these (and plants in general) in the home is that their soil is conducive to fungus gnats.

What is a fungus gnat?

Fungus gnats look similar to mosquitoes, and adult gnats have an ⅛” long dark body with tan-colored legs. However, unlike mosquitoes, they do not sting or cause harm to people. Sometimes they can be mistaken for fruit flies, but checking that they are concentrated near a houseplant is a good indicator that you are dealing with fungus gnats, not fruit flies.

closeup of clear fungus gnat larvae
Fungus gnat larvae on soil

Fungus gnat larvae are clear with a tiny black head (sort of look like clear worms), and are often found chained together. The larvae love damp soil, and find it a perfect environment to thrive.

First Natural Method to Remove Fungus Gnats : Hydrogen Peroxide

While chemical methods work, I do not like to use them because I like to stay as organic as possible, especially when growing organic herbs. First, I put sticky fruit fly traps near the plant to catch the adult gnats. Then I make a mixture of hydrogen peroxide and water.

Recommended Mixture:

  • 4 parts hydrogen peroxide
  • 1 part water
bottle of hydrogen peroxide next to shot glass and large measuring cups
Basic ingredients for hydrogen peroxide method

Here’s how I mix them. Grab a 2-cup (or larger) measuring cup and a liquor measuring glass. I pour 4 oz of water into the large container, then put in 1oz from the liquor glass into the container. I keep doing that until I get the amount of mixture I need to thoroughly water the plant. Lastly, I give the solution a swirl to combine.

shot glass with 3 ounces of hydrogen peroxide and large measuring cup with 12 ounces of water
4:1 mixture of water to hydrogen peroxide

When watering, it is important to water evenly to cover every square inch of soil. Doing so ensures no larvae are missed. After the water has absorbed into the soil, if larvae are present, the hydrogen peroxide will foam (like the image below) and make a sizzling sound. This is a good sign, and means the larvae are being killed.

This process will need to be completed a few times, as the female fungus gnat lifecycle is very rapid. It is a good sign if you pour the hydrogen peroxide mixture on the soil, and no foaming or sizzling noise occurs. Continue using the hydrogen peroxide during regular watering until you can no longer hear the sizzling (the below image shows foaming after application). After a while, you will be gnat-free!

foaming on soil surface from hydrogen peroxide
Soil foaming after hydrogen peroxide application

Second Method to Remove Fungus Gnats: DIY Insecticidal Soap

This is my second favorite method- I always love when I have a multi-use regular household item. Dr.Bronner’s pure liquid castile soap has been around for a long time, and serves functions from body wash and shampoo to insecticidal soap. The method of mixing is very simple:

  • 1 teaspoon concentrated Dr.Bronner’s liquid castile soap
  • 1 quart hot water

Mix the two together and put in a spray bottle. Spray the soil every three days until the infestation is gone. Remember to reapply after watering also.

Bonus: If All Else Fails Method

Remove the top inch of soil from the potted plant and dispose of the soil. Replace the removed soil with new soil from a fresh bag of potting soil (not an old bag that could be the source of the infestation). Then use sticky bug catchers to gather any stray fungus gnats flying in the area.

yellow square sticky fungus gnat catcher mounted to Popsicle stick and placed in soil of plant

Have any other great natural DIY methods of controlling fungus gnats? Let me know in the comments below!