How I Finally Got Rid of Fungus Flies (and What Didn’t Work)

Fungus gnats are one of the most common problems for indoor gardeners—and one of the most difficult to deal with. When these little flying insects attack, you want them gone fast—but it took me several failed attempts before I was able to get rid of the problem for good.

There are a lot of fungus gnat treatments out there, and I’ve tried most of them in my quest to eradicate these pesky bugs from my 60-plus houseplants. Here’s what I learned about how to get rid of fungus gnats—and how to prevent them from affecting your plants in the first place.

What is a fungus gnat?

Outdoors, fungus flies feed on fungi and organic matter in the soil as it decomposes. Indoors, they are unwanted pests, laying eggs in the soil of your houseplants.

Once fungus gnats appear, they spread easily. You will see them buzzing around, which can be a nuisance.

They don’t bite or pose a health risk to people or pets, but they can harm your plants. Fungus gnat larvae can feed on the roots of your plants. An infestation can cause plants to wilt, grow slowly, or even die.

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Where do fungus gnats come from?

Fungus flies typically enter the indoor environment in one of two ways: through growing media such as compost or potting soil or through new houseplants.

In my case, the source was a cheap pothos plant purchased from the grocery store or bagged compost from my weekly kitchen waste collection company. The plant was likely infected at the commercial greenhouse where it was grown—it is not uncommon for greenhouses and outdoor nurseries to manage baseline levels of fungus gnats.

But since I noticed this problem so soon after a major repotting with that compost, and since it affected so many plants so quickly, I suspect that it was the culprit.

What didn’t work

I researched my options, which ranged from watering adjustments to household items like dryer sheets to commercial organic pesticides. Here are some common treatments that didn’t solve my fungus gnat problem.

Bottom watering

Fungus gnats seem to like moist soil, so by watering from the bottom up, you keep the top layer dry while still providing the plant with the moisture it needs. You can also let the plant dry out completely between waterings, which works well for drought tolerant plants like succulents and snake plants, but not for others like ferns and pothos. Fungus gnats will still survive.

Hydrogen Peroxide

I’ve read that watering plants with hydrogen peroxide—straight or diluted—can kill fungus gnat larvae, but it can also slow growth by killing soil microorganisms. I tried this on my problem pothos, which seemed fine, if still infected, but I’m too nervous to use it on my entire collection.

Sand on the ground surface

I came across a suggestion that spreading a shallow layer of coarse sand over the surface of houseplants can eliminate fungus gnats. By turning the top layer of soil into a fast-draining desert with no organic matter, the insects lose their habitat and die.

The sand doesn’t help, but it stains every time I water, and I don’t like how my plants look when there’s sand instead of regular soil around the trunk.

Beneficial nematodes

I’m bullish on beneficial nematodes, those microscopic creatures you stir into the water you water your plants. They invade fungus gnat larvae in the soil and secrete a bacteria that kills them.

I have applied these treatments to all my plants according to the package instructions, but the infestation continues.

What worked in the end

After trying and failing to eradicate fungus gnats, I finally had success with a combination of these strategies.

Sticky trap

These bright yellow, glue-coated plastic strips alone won’t cure a fungus gnat infestation, but they are an important tool for monitoring and reducing adult fly populations. Use them to assess fungus gnat levels and see if other treatments are working.

Bti (Bacillus thuringiensis israelensis)

Bti—abbreviation for Bacillus thuringiensis israelensis—is a naturally occurring soil bacterium that kills pests such as fungus gnats and mosquitoes. Non-toxic to humans, birds, mammals, fish and beneficial insects.

It is also approved for use in organic farming, but it is an EPA-registered pesticide, so it is important to follow all package directions when using products containing Bti.

You can buy Bti in powder form, mix it with water and apply it to your plants’ soil, which is what I did – three applications over a few months and my fungus gnat problem was gone.

Some people like to mix mosquito repellents, small pellets containing Bti, into the topsoil of houseplants, but these pellets must be replaced periodically after they wear off.

Fungus gnat prevention

Now that my plants and I have survived a fungus gnat infestation, I never want to experience another one. Instead of amending with compost, I use a potting mix that contains both a biofungicide and beneficial mycorrhizal fungi to support soil health while reducing the risk of fungus gnats.

I also carefully inspect new plants and quarantine them from my main collection for a few weeks after bringing them home.

I use sticky traps to assess whether they have cleared enough to get in. But now that I know how to kill fungus gnats and keep them away for good, I am much less worried about getting another infestation.