Fungus Gnats

How to Identify and Get Rid of Fungus Gnats

Fungus Gnat
D. Kucharski & K. Kucharska/Shutterstock

Often considered only a minor houseplant pest, fungus gnats can quickly become a major issue if an infestation gets out of hand. Here’s how to identify, get rid of, 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 three days, the eggs hatch into larvae, which burrow into the soil to feed on fungi and decaying plant material. Two weeks after that, adult gnats emerge from the soil to repeat the process. Adults live for about one week.

Fungus gnats are completely harmless to humans, since they can’t bite and don’t spread diseases. They can be a problem for houseplants, however, when their population explodes and their larvae starts to feed on plants’ 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, using consistent management and prevention techniques is key to ending it. Further down on this page, we’ve listed a few of the best ways to both get rid of adult gnats and prevent new gnats from emerging.

Table of Contents

1. How to Identify a Fungus Gnat 
2. Fungus Gnat Damage 
3. How to Get Rid of Fungus Gnats 
4. How to Prevent Fungus Gnats

Identification

How to Identify a Fungus Gnat

  • Size: Adult fungus gnats are tiny. Their size ranges from about 1/16 to ⅛ of an inch in length (1.5 to 3mm), which is about the same size as a fruit fly. Fungus gnat larvae may be up to ⅛ of an inch in length.

  • Appearance: Adult fungus gnats are a grayish-black color and have gray or see-through wings. Their long legs and long antennae give them a mosquito-like appearance, though they are much, much smaller in size. 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 have a tendency to walk along the soil and fly only in short bursts. Their flight is erratic and they are much slower than fruit flies, acting more like mosquitoes while flying.
    Annoyingly, fungus gnats have a tendency 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, the adult gnats don’t actively harm plants nor people. If their population gets out of hand, however, the larvae may start feeding on plant roots, causing notable damage. This is especially bad for young plants, such as seedlings, which have only a few delicate roots. Fungus gnats are also capable of spreading the plant pathogen that causes damping off and the eventual death of seedlings.

Fungus gnat damage will appear similar to that of 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 completely. In particularly bad cases, wilting of the entire plant could occur, followed by the death of the plant if 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 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 cards traps: These traps consist of a yellow note card covered in 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 the soil. Adult gnats will fly or crawl onto the card and become trapped. 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, as well as 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 ¼-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 to refresh 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 two 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 isrealensis. This beneficial bacteria infects and kills the larvae of flying insects, including mosquitoes, fruit flies, and fungus gnats.

    • To use mosquito dunks: Fill up 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 this water for fungus gnat–infested plants. The bacteria will have leeched 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, but to also let water pass through freely. Attach with tape or rubber bands.

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

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Reader Comments

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Fungus gnats

Many years ago when " My Big Fat Greek Wedding" was new, i used Windex to kill scale on my aloe. I used it again on the soil when my plants got fungus gnats. It works. Spray the soil & any bugs who are disturbed every couple of days. The movie was right- it works for everything!!

Fungus Gnats

This is a terrible article. Sticky paper? Really? Apple cider vinegar? Really? I was hoping for something a lot more insightful coming from Farmers Almanac, but you gave me sticky paper.

How about this… For fungus gnat prevention, cover your topsoil with sand, or small pebbles. This prevents the adults from being able to burrow into the organic soil to lay their stupid eggs. Boom! Problem solved!
I’ve also heard that pepper sprinkled on the topsoil works, but I can’t verify that. I’ve tried it, but without knowing how much or how often, I’m not sure I did it right.

Now… my problem is that the gnats have infested the soil I blended for my succulents. I thought it would be safe due to the high ratio of sand that went into it. Unfortunately, it was not. Fungus gnats have infested my flat of baby succulents & I need to know how to get rid of them.

This article was a lot less than I expected. Disappointed.

Fungus Gnat Solutions

The Editors's picture

A layer of sand, small gravel, or even perlite can help to prevent adult gnats from accessing the soil from above, but it needs to be a relatively thick layer (at least ½ inch) and doesn’t prevent them from getting in through the bottom of the pot. 

We’ve had the most success using a combination of sticky fly traps to capture the adults and water treated with mosquito dunks to kill the larvae in the soil. 

Gnat larvae

I heard one part peroxide to four parts water kills larvae on contact. Don’t water too often with this though I do it every other time I water my plant.

Rubbing Alcohol

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.

FUNGUS GNATS

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!!!!!

Fungus knats

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.

Fungus Gnats

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.

Getting Rid of Fungus Gnats

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.

how to simply rid your plants from these pests

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.

Gnats

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.

Prevent curing knats

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

Fungus gnats in houseplants

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!

fungus gnats

I have powdered the soil as well as the saucers and trays upon which the containers sit, with diatomaceous earth. I also removed older plants from my greenhouse and repotted them with fresh soil with a smidge of d-earth. I then placed domes on the plants in trays to isolate the critters. What was an out of control greenhouse pandemic became a manageable small infestation quickly, easily handled with a spray bottle of 70% isopropyl alcohol. I knocked them out of the air when they took flight upon removing the dome.

Stinkers love bottom soil as well

Thank you! Everyone usually talk about how fn lay their eggs in the top two inches of soil, but no one really talks about them laying their eggs in the bottom of the pot as well, which is a problem im currently having. I have added a layer of sand and rock to the top of my potted plants and cut down on watering but to my suprise they seem to have just migrated to the bottom of the pot, thanks for the tips!

fungus gnats

Beneficial nematodes work great for eliminating fungus gnats. Had an an out of control infestation in my greenhouse, and after just two weeks of applying the nematodes, they were gone.

Hydrogen peroxide

I read on a forum, and then had success with, that using a solution of 1:4 hydrogen peroxide to water and watering the effected plant helps get rid of fungus gnats when used with other methods. It provides oxygen which kills the food source and the fizzy actions causes them to take flight and get stuck on those sticky traps. I usually treat potting soil with this before using because the bags in the garden center yard are exposed to all kinds of critters before being purchased. Don’t get carried away using this because too strong of a solution will harm plants.

fungus gnats

Can you not spray neem oil on the dirt and it kill these? I've used this in the past with houseplants that I normally set on my back porch during the warmer weather and I seem to recall it working well? I have not brought my plants in this year yet but will be doing so in the next week.

FUNGUS GNATS

A layer of playbox sand on top of the potting soil works every time for me. It is abrasive to the flies coming and going

Fungus Gnats

I read somewhere that cinnamon sprinkled on the soil worked. I tried liberally covering the soil around my peace lilies with cinnamon powder after a good watering. Following this, I then let the top dry out well and the gnats are gone. Plants are still healthy :)

Fungus Gnats

Either bake (solarize), or pour boiling water through any potting (at least the inert kind that contains no living compost in it's list of ingredients) mix that you intend to bring indoors. Also, buying potting mix well ahead of time and letting it sit in it's sealed bags for months before use, is often enough to disrupt the cycle. Most commercial potting mixes that contain active compost, will likely have fungus gnats.

Fungus Gnats

I'd like to ask a question regarding fungus gnats. It is often said that you should remove the top inch of soil regularly to remove eggs etc, so does this mean the adult gnats are capable of actually burrowing right down under the soil (up to an inch) to lay their eggs? The adults look so fragile you wouldn't think they would be able to burrow that far down.....

Fungus Gnats in Soil

Are there any suggestions for Fungus Gnats that are in the lawn? I live near a creek and the ground never gets super dry. Would beneficial nematodes help?

Thanks!

What concentration of hydrogen peroxide?

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

Hydrogen Peroxide

The Editors's picture

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

Rooting cuttings in water

You can't attract fungus gnats to cuttings rooting in water can you? Also where would I go to get verification of the answer.

Cuttings in Water

The Editors's picture

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.

Preventative Care

Hello, if two of my thrift indoor plants have fungus gnats, should I treat them all in case? Thank you.

Gnat eggs and larvae in outdoor pots

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?

Gnats

The Editors's picture

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.

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