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
Reader Comments
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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 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
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
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.
Hydrogen peroxide & yellow sticky traps
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
From Cornell.edu Re: Fungus Gnats
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.
Kelp Remedy for Fungus Gnats
I sprinkle kelp on top of the soil to stop fungus gnats. Works every time.
*Almost eradicated!
*Almost eradicated!
Fungus Gnats
Peroxide and water absolutely worked for me! I had a terrible time with these pests and after 2-3 applications my plants are good. Still a few strays (found dead by door and window sills) but after a month seem to be also eradicated!
Fungus Gnats
I beg to differ about using peroxide to control fungus gnats. My tiny lemon trees ( I grew them from seed ) had fungus gnats and I mixed about 1/2 C of hydrogen peroxide to about 4 cups of water and watered the plants. The soil 'foamed' a bit, and after a 2nd application about a week later, the gnats were gone. Worked perfectly for me !
Best I've found
Mosquito dunks also come in "bits" that you sprinkle on the surface of the soil before watering. A container of bits lasts a long long time and works great! I have lots of houseplants and we go through bouts of fungus gnats. As long as I remember to put the bits out every few weeks, we see few.
Hydrogen Peroxide Does Not Work
Any suggestion that Hydrogen Peroxide kills larvae is an internet myth. Use targeted solutions that are proven.
The use of BTi is inaccurate
BTi is an effective treatment for larvae, however; it is not a contact killer as you refer to in the article. It must be ingested by the larvae to be effective which makes it not always a solution. Once larvae have a viable food source, especially inside of the roots or under the bark of a cutting they may not ingest the bacteria. Beneficial nematodes are a predatory round worm that seeks out the larvae to use as a host and are much more effective.
Getting rid of gnat eggs in plants
I am a terrible "overwaterer" and am trying to get that under control. When I do water now I use 1/4 hydrogen peroxide to 3/4 water for a couple of times. That is supposed to kill any eggs and doesn't harm the plant. Seems to be working.
Fungus gnat predators
I recommend Stratiolaelaps scimitus, which is a natural predator of fungus gnats. They're predatory soil mites and are what professional growers use at propagation to prevent fungus gnats, as well as root aphids and other soil pests. They're produced and sold all over the world. And much less labour to use than other options.