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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>Christina</span> Tue, 06/08/2021 - 10:58
Plain, drugstore 70% isopropyl rubbing alcohol sprayed on the surface soil has worked for me. Not sure if it would hurt the plant to water immediately after, but I've always waited a day to water after spritzing on alcohol. I don't get these gnats very often, but when I have, rubbing alcohol has always worked.
<span>Barbara Turner</span> Fri, 04/23/2021 - 10:00
Oh Lord I got them from re-potting a houseplant. They must have been in the new soil. I used the yellow sticky traps and made water from mosquito bits. I almost have them under control. Now I have another plant that needs to be re-potted. So I have potting soil baking in the oven at 400 degrees. After it cools I will feel safe to use it. These things are a hot mess!!!!!
<span>Jeannette. Lackmore</span> Sat, 11/12/2022 - 20:21
I had a terrible I mean terrible time with these things I used everything everyone on this sight is saying the d earth will kill your plant an your soil neem oil slow but helps I ordered everything but the only thing that worked was yellow sticky traps neem oil an the blue light sticky traps all brought on amazon .I left the light plugged in 24hrs changed yellow sticky paper once full an light as well an sprayed with the oil but you have to stay with it check traps every day (traps dynatrap)please buy extra sticky paper it will fill up fast an let plants completely dry out between watering an make house completely dark at night only traps glowing this totally saved my plants an I didn't lose any I did almost lose my mind though! But I Won
<span>Lnelson</span> Sat, 04/03/2021 - 11:29
Tried and true...did it many times.....DE foiliar spray slurry knocked the he'll out of aphids and white flies also......eliminates them quickly......my infestations started prior to flowering, ergo most of the "dirty leaves are gone before harvest.....I'd rather not spray buds with all that particulate (probably would wash up nice) . Be sure to cover soil when spaying so pests don't fall in substrate.
<span>Mart</span> Thu, 02/04/2021 - 13:00
I tried all those neem oil and stuff. The only way I got rid of them was to take out two inches of soil in the pots and replace it fresh soil. Worked, and haven’t had a problem since.
<span>Lois Olson</span> Wed, 02/03/2021 - 23:01
A surprisingly easy method I heard about a few years ago & have used successfully ever since is Mint Tea. Specifically, mint tea bags. I brew my usual pot of mint tea, using 3 tea bags, then place the (slightly) used tea bags on top of the soil in my indoor pots (& I enjoy drinking a pot of tea). Some recipes call for you to brew the tea & drench the soil with it, but I've found putting the tea bags on the soil works continuously & effortlessly. Just dampen them every time you water your plants & you have constant mint aromatics that the gnats do not like & they disappear. You can replace the tea bags (or just let them compost into the soil...) as often as you drink pots of mint tea.
<span>Ane</span> Wed, 02/03/2021 - 11:26
I found that a light sprinkling of Diatomaceous earth on the soil and a misting of apple cider vinegar kills adults and babies. It doesn't damage the plants and the smell quickly goes away. No traps just d. earth and the all trust-worthy a.c. vinegar.
<span>Shelly</span> Wed, 02/03/2021 - 08:53
I moved into a house that had been vacant for about 2 years. There were the usual spiders and other bugs, there were also interactive cat toys (aka mice). So I called pest control to come out and spray for the bugs and set up mouse traps that would be safe to use around my cats. Well the cats seem to have taken care of the mouse problem. When I went to go pick up the mouse traps, I noticed it was all sticky stuff and they were covered with gnats, flies, spiders and my arch nemesis - Stink bugs. Shame on me for not asking to see the traps before they were set because I would not have allowed them to be put out, I wanted traps that I could bring the mouse outside and release it. So if you know you don't have a mouse problem but want to get rid of bugs this might be the trap for you. The pest person said these traps have a scent that would attract mice. I also use the cider vinegar traps and they work well. I tried the tape traps for the gnats and that was a total fail. Someone suggested that I spray some flowery scent on the tape. I moved and the plants did not survive the trip.
<span>Debbie</span> Wed, 02/03/2021 - 08:01
I use grit or sand in top of pot which seems to help. Then letting it dry out from the top and water from below. I like the sticky cards idea
<span>Terese</span> Wed, 02/03/2021 - 07:08
While in college (MANY years ago) I was too broke to afford potting soil for my split leaf philodendron so used outdoor dirt. OOPS. Fungus gnats worked hard to eat the roots and take over my dorm room. In desperation I put the plant into a large plastic trash bag and sprayed regular aerosol Lysol (from the cleaning closet!) on the dirt, then pulled up the bag around and over the plant. I gave it another heavy spray onto the bag walls and finished very tightly closing the bag. I left it sealed for a couple days, to live or die. Upon opening it, I found a living, if still stressed plant, and a thick layer of larvae. I scooped off the nasty baby bug layer then gave my green pet a good drink. SUCCESS!!! I make no guarantees how it might work on more tender plants, but it worked on the philodendron!