How to Get Rid of Fungus Gnats: Seven Simple Ways That Work

Fungus gnats are the bane of any indoor plant enthusiast! Those pesky little bugs fly around your houseplants, up your nose, in your eyes, and worst of all, they eat plant roots and cause damage to garden plants! 

Preventative Pest Control

I am a big proponent of using preventative pest control methods and organic pesticides to naturally and safely control unwanted bugs. Proper prevention means less work in the long run and less need for additional products to combat pest pressure, saving time and money.

Read these blog posts to learn effective pest prevention methods, not just control methods, for getting rid of wasps and hornets, soil mites, spider mites, squash vine borers, tomato hornworms, and snails and slugs in the garden.

What Are Fungus Gnats

Fungus gnats are small flies that are attracted to decomposing organic material in soggy soil. Pro-Tip: A fungus gnat is not a fruit fly! Fruit fly traps and vinegar will not catch them because fungus gnats are not attracted to the same things as fruit flies.

Life Cycle

Fungus gnats have a very short life span. The ones that you see flying around are adults. Down in the soil is where the real problem lies!

Eggs are laid in moist soil and hatch into larvae in less than a week. The larvae require adequate moisture to grow and develop into flies.

The larvae begin to eat plant roots, organic material, compost, and, as their name implies, fungus in the soil. Eventually, they pupate and hatch as adults.

Each female adult fungus gnat can lay up to 500 eggs in a single life cycle! As you can see, the population can quickly multiply out of control.

What NOT To Do If You Have Fungus Gnats

Fungus gnats can be quite the annoying little buggers. But what’s worse is the misinformation and myths out there about how to get rid of them! It’s important to know what not to use if you have fungus gnats:

• Diatomaceous Earth - Diatomaceous earth is very effective against ants, Japanese beetles, cucumber beetles, and other pests with a hard exoskeleton, but fungus gnats do not have a hard exoskeleton. Diatomaceous earth will not get rid of fungus gnats! (Read this blog to learn more about diatomaceous earth uses in the organic garden.)

• Cinnamon - While cinnamon does have antifungal and antimicrobial properties, it does not control fungus gnats when used on top of the soil! Applying cinnamon is not only expensive, but the cinnamon oil creates a thin barrier that prevents water from penetrating the soil. Most of the time, it will cause dry patches in the soil and unhealthy plants, and it won’t actually eliminate fungus gnats (or fungus) like many people claim it does.

• Only Adult Fly Traps - A common mistake that people make is only trying to kill the adult fungus gnats. Yes, the adults are part of the problem, but the eggs and larvae are the bigger issue. Target the adults with sticky traps, then focus your energy on the soil because that's where the problem lies!

Preventing Fungus Gnats

Sterilizing the soil is the best method for preventing fungus gnats because fungus gnat eggs could be lying dormant in the soil you bring indoors. When you bring it into your nice warm house and water it, those eggs will begin to hatch.

You should sterilize your soil, especially if you buy it from an outdoor source! Pro-Tip: Learn how to make homemade seed starting mix and DIY potting mix instead of buying soil from an outside source to prevent unwanted hitchhikers.

There are two ways to sterilize soil before planting:

1. Boiling Water - Use boiling water to pre-moisten your soil, mixing it in quickly to kill fungus gnat eggs and larvae.

2. Heat Treating - Place your soil on a baking sheet and throw it into the oven for 5 minutes at 350°F.

Your sterilized soil will be free of any unwanted fungus, mildew, bacteria, and pests! (Obviously, sterilization needs to be done before the plant is in the soil; otherwise, it will kill the plants.)

But contaminated soil is not the only way these pesky flies can get into your house. They may ride in on pets or dirty shoes or fly in through open windows and doors.

How to Get Rid of Fungus Gnats

The following methods can be used as additional prevention or as a control to help you get rid of larvae in the soil of an existing potted plant. A multi-prong approach is the best defense against fungus gnats!

• Sand - Apply a 1/4-inch layer of sand on top of the soil after sowing seeds or around existing plants. Seedlings will have no problem pushing up through the sand; however, the fungus gnats cannot deposit eggs more than 1/16-inch deep, so this will prevent them from laying eggs in the soil.

• Vermiculite - A fine grade of vermiculite will work much like sand. Fungus gnats have to lay eggs directly in organic material. They won’t lay eggs in inert material like sand or vermiculite. Mulching with a ¼-inch layer will provide a protective layer while conserving moisture in your soil.

• Neem Cake - You can also use neem cake by breaking it down and adding it to your growing medium. This has been shown to help against fungus gnats as well. Neem oil and neem cake come from the neem seed. The cake is made after the seeds are crushed and the oil is extracted.

• Neem Oil - Neem oil is a great all-purpose organic pesticide for controlling, not only fungus gnats, but aphids, thrips, white flies, spider mites, and other pests. To use, wait until your plants are ready to be watered, then add 2 to 3 tablespoons of neem oil and one drop of dish soap to one gallon of water in your watering can and water your plants with the neem oil solution. Repeat every time your indoor plants need watered.

• Reduce Watering - Overwatering encourages fungus gnats and leads to other problems like damping off and root rot. Letting your plants dry out between waterings will greatly reduce the risk of hosting a community of unwanted fungus gnats.

• Pyrethrum - Pyrethrum is from the daisy family and prohibits the fungus gnat's reproduction cycle.

• Sticky Traps - These sticky traps can be very effective against adult flies, but don’t use them exclusively. Yes, fewer adults means fewer eggs, but when one female can lay up to 500 eggs, you have to eliminate all of them to stop the cycle entirely! For that reason, don’t use sticky traps alone; instead, use them supplementally and focus on getting rid of the eggs and larvae.