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How to Identify and Get Rid of Fungus Gnats

fungus gnat on a plant leaf
Image Credit:
D. Kucharski & K. Kucharska/Shutterstock
Written By: Christopher Burnett Garden Writer

Do you have little flies on your houseplants? They may be fungus gnats! Often considered only a minor houseplant pest, fungus gnats can quickly become a significant issue (and annoyance) if an infestation gets out of hand. Here’s how to identify, eliminate, and prevent fungus gnat infestations in your plants.

What are Fungus Gnats?

Fungus gnats are a fruit fly–sized insect pest that primarily affects indoor houseplants. Attracted to the moisture of potting soil, adult gnats lay their eggs (up to about 200) on organic matter near the soil surface. After about 3 days, the eggs hatch into larvae, which burrow into the soil to feed on fungi and decaying plant material. Adult gnats emerge from the soil 2 weeks later to repeat the process. Adults live for about 1 week.

Fungus gnats are utterly harmless to humans since they can’t bite and don’t spread diseases. However, they can be a problem for houseplants; when the gnats’ population explodes, the larvae feed on plants’ thin roots. Fungus gnats may also spread Pythium, a group of plant pathogens that causes ”damping off” in seedlings.

Once you have a fungus gnat infestation, consistent management and prevention techniques are the key to ending it. Further down on this page, we’ve listed a few of the best ways to get rid of adult gnats and prevent new gnats from emerging.

Identification

How to Identify a Fungus Gnat

  • Size: Adult fungus gnats are tiny. Their size ranges from about 1/16 to 1/8 inch in length (1.5 to 3mm), which is about the same size as a fruit fly. Fungus gnat larvae may be up to 1/8 inch in length.
  • Appearance: Adult fungus gnats are grayish-black and have gray or see-through wings. Their long legs and long antennae give them a mosquito-like appearance, though they are much smaller. Compared to a fruit fly, fungus gnats have a thinner body with longer legs and antennae. Larvae have a small, black head and a thin, white, or see-through body.
  • Activity: Fungus gnats tend to spend most of their time on the soil surface of potted plants, but they may be seen flying around the outer edge of the pot or near drainage holes as well. They are not strong fliers, so they walk along the soil and fly only in short bursts. Their flight is erratic and much slower than that of fruit flies, acting more like mosquitoes while flying.
    Annoyingly, fungus gnats tend to fly into people’s faces and drinks, though they are completely harmless, and a few well-placed swats will show them what’s what.

Fungus Gnat Damage

In small numbers, fungus gnats are more of an annoyance than anything. In fact, adult gnats don’t actively harm plants or people. However, if their population gets out of hand, the larvae may start feeding on plant roots, causing notable damage. This is especially bad for young plants, such as seedlings with only a few delicate roots. Fungus gnats can also spread the plant pathogen that causes damping off and the eventual death of seedlings.

Fungus gnat damage will appear similar to any other root-related issue, such as root rot. Lower leaves may turn yellow and drop, and the plant’s growth may slow down or stop altogether. In particularly bad cases, wilting of the entire plant could occur, followed by the death of the plant if the roots are extremely damaged. 

Control and Prevention

How to Get Rid of Fungus Gnats

Getting rid of fungus gnats is all about consistency. Catching the adults in gnat traps is fairly easy, but because the adult population comes in cycles, you need to make sure that your traps are refreshed regularly. For the best results, use a combination of the traps listed here as well as the additional preventative methods listed in the subsequent section.

  • Sticky card traps: These traps consist of a yellow note card covered with a sticky adhesive. They are most effective when cut into small squares and placed directly on top of the soil or attached to skewers just above it. Adult gnats will fly or crawl onto the card and become trapped in the glue. Fungus gnats are attracted to the color yellow, so use the yellow sticky cards rather than the blue ones. Both can be bought at most hardware or garden stores and online.
  • Cider-vinegar traps: Simple and effective, cider-vinegar traps consist of a shallow container with a small amount of apple cider vinegar, water, and liquid dish soap.
    • To make a cider-vinegar trap: Find a shallow container—a tuna can is perfect—and fill it with equal parts water and apple cider vinegar. (The liquid should be at least 1/4 inch deep.) Put a few drops of liquid dish soap into the mixture and stir gently. Place the trap near the base of the affected plant or, ideally, inside the pot on top of the soil. Check it every few days and refresh it with new vinegar and water.
  • Flypaper: Flypaper ribbons, such as those hung in horse barns to catch outdoor flies, can also be used to catch fungus gnats. However, these traps are usually overkill for gnats and can easily get stuck to things you don’t want them to stick to, such as furniture, hair, plants, and so on.

How to Prevent Fungus Gnats

Use these prevention techniques in tandem with the traps listed above for the best results.

  • Keep soil dry: Fungus gnats seek out moist soil, so allowing your houseplants to dry out a bit between waterings can slow down or stop an infestation. Let the top inch or 2 of soil dry out before watering again, and try to go as long as possible between waterings. Gnats may be deterred from laying their eggs if the soil is dry on the surface.
  • Mosquito dunks (with beneficial bacteria): Mosquito dunks are used to keep mosquito larvae from populating fountains, animal troughs, fish ponds, and other small bodies of water. The product consists of a dry pellet containing a type of bacteria called Bacillus thuringiensis subspecies israelensis. These beneficial bacteria infect and kill the larvae of flying insects, including mosquitoes, fruit flies, and fungus gnats.

To use mosquito dunks: Fill a gallon jug (or watering can) with clean water and toss in a mosquito dunk. It’s a good idea to break up the dunk a bit before placing it in the water, or you can wait for it to soften before breaking it apart. Let the dunk soak in the water for as long as possible (at least overnight), then remove it from the water (the dunk can be reused) and use it for fungus gnat–infested plants. The bacteria will have leached into the water and will now infect and kill any larvae that come into contact with it in the soil. Repeat this process every time you water your plants for at least a few months.

  • Cover drainage holes: Though gnats typically remain near the tops of pots, they may find their way to the drainage holes on the underside of a pot and start laying eggs there, too. If this happens, cover the drainage holes with a piece of synthetic fabric to prevent the gnats from getting in or out of the hole and let water pass through freely. Attach with tape or rubber bands.
  • Cover exposed soil with sand: Some folks report that covering their houseplant’s soil with a layer (at least 3/4 inch thick) of sand prevents fungus gnats from accessing the soil and laying eggs. This can be an effective deterrent if used with other prevention methods, especially covering drainage holes.

Do you have any tips for preventing or stopping fungus gnat infestations? Tell us in the comments below!

Garden Pests

About The Author

Christopher Burnett

Christopher Burnett

Garden Writer

Chris is an avid gardener, maintaining a small vegetable garden for himself and his family, a variety of ornamental flowers and shrubs, and a diverse collection of houseplants.

Read more from Christopher Burnett

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Comments

<span>Kate</span> Tue, 05/12/2020 - 05:17
Having read all the positive comments regarding watering with a mixture of hydrogen peroxide and water, I thought I would try it to solve our gnats problem; but what percentage of peroxide is safe for this? I've seen various for sale from 3% all the way up to 90%? I obviously don't want to use anything that will be toxic to either myself or my plants? Thank you
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 05/12/2020 - 15:23

The typical "drugstore variety" 3% peroxide is enough for dealing with gnats. We wouldn't recommend anything higher than that for this purpose.

 <span>Jo Allen</span> Fri, 04/03/2020 - 19:27
You can't attract fungus gnats to cuttings rooting in water can you? Also where would I go to get verification of the answer.
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 04/06/2020 - 11:19

Fungus gnat larvae live in soil, so cuttings rooting in water will not attract them. A curious gnat may stop by to check out the cutting, but it will have nowhere to lay eggs and for the larvae to live.

 <span>Tiana</span> Tue, 03/24/2020 - 17:10
Hello, if two of my thrift indoor plants have fungus gnats, should I treat them all in case? Thank you.
<span>Christine Mueller</span> Mon, 03/09/2020 - 15:32
I'm looking for a solution for gnats in outdoor pots. At our Northeaster Wisconsin cottage we have many outdoor pots that stay outside all winter. In spring (Memorial weekend) my sister plants the pots up and we are immediately inundated with what we think are some sort of biting gnats, could be no-see-ems, but they are not mosquitos. They bite at the back of your neck at hairline or on scalp. These bugs seem to love us. It seems disturbing the soil is what sets them off. Does anyone have a protocol we can follow?
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 03/10/2020 - 10:11

They're not black flies, are they? Fungus gnats do not bite.

In any case, you could try watering the containers (starting a couple weeks prior to Memorial Day weekend) using water that has been treated with mosquito dunks. This may kill the larvae of the flies, if they are in fact living in the container soil.

 <span>Jennifer Rose</span> Thu, 02/27/2020 - 01:38
I did the combo of watering with diluted hydrogen peroxide to kill the eggs and larvae, and use yellow sticky traps to catch the adults. The traps must have caught over 100, way more than I thought we had. Too late for my touchy basil, but my cilantro, chives, and parsley bounced back
<span>Jackye Stephens</span> Sun, 02/23/2020 - 01:41
Nematodes applied to the soil will feed on fungus gnat larvae.Research support: Growth chamber studies showed that populations of fungus gnats were reduced 40-75% at 75°F and 70-90% at 55°F with weekly applications of Steinernema feltiae nematodes as a drench. Research results referenced in this report are based on a series of greenhouse/growth chamber experiments done at Cornell University and in on-farm trials.What you can do: Apply nematodes as a drench or a sprench at a rate and frequency based on sticky card counts of fungus gnat adults.
<span>Jackye Stephens</span> Sun, 02/23/2020 - 01:37
I sprinkle kelp on top of the soil to stop fungus gnats. Works every time.