Fungus Gnats: Where Do These Little Flying Bugs Come From

Updated on July 19, 2018
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Thoughthole has more than eight years of hands-on experience in the horticultural maintenance industry and shares many tricks of the trade.

What are those little flying black bugs?
What are those little flying black bugs?

What Are These Tiny and Annoying Black Flies?

Fungus gnats are tiny flying insects often mistaken for fruit flies. A fungus gnat is much smaller than a fruit fly and has a tiny black body (while fruit flies are commonly tan and have very visible bodies). Gnats are also attracted to decaying organic material, wheres fruit flies only feed on produce. These bugs lay eggs in soil where conditions are moist and there is decaying matter to feed upon. The eggs of the gnats hatch in the soil, and the larva live and feed on fungal material found there. They can eat away at the roots of your plants and cause them to yellow or die. When the larva grows into an adult, it flies out in search of new locations to procreate and feed.

Most people notice that they have fungus gnats in their home because these tiny bugs will often try to fly into a person's nose, mouth, or eyes, as they are attracted to moisture. This trait causes them to be commonly reported as "those little, annoying, flying bugs." They can be especially irritating to someone who sits at a desk near the origination of the infestation.

Where Do Gnats Come From and What Are They Attracted to?

  • Moisture: They love to live and breed in the moist soil of overwatered house plants, in containers or pot liners, or around areas with standing water, such as sinks, open bottles of beverages, etc. Tap on the pots of any houseplants in the area where they seem to swarm. If there are gnats, they will begin to fly up out of the plant.
  • Decaying organic matter: Fungus gnats feed on moist, decaying, organic material, such as dead leaves, leaf husks, fungus, or any other organic debris on top of the soil's surface or down inside the pots and liners. They are also attracted to soil that contains organic material, like wood chips, peat moss, or compost, which is also why you may find them in compost bins or trash cans with decaying material.
  • Light source: They typically swarm around windows, lamps, and other light fixtures.
  • Carbon dioxide: Like most flies, fungus gnats love CO2, which is why they like to hover around your mouth and nose.

What's the Difference Between a Fungus Gnat and a Fruit Fly?

Fungus Gnat
Fruit Fly
smaller than a fruit fly
small
tiny black body
brown or tan body
attracted to moisture and decaying organic material
attracted to fruits and vegetables
feed on moist, decaying organic material found in houseplants' soil
feed mainly on produce
Source

How to Get Rid of Fungus Gnats

Use the following methods to catch and kill fungus gnats that are present in your home. Use a combination of methods and do it repeatedly for at least 4 weeks.

Dry Out the Soil

For an over-watered plant, you must begin drying out the soil. Overwatered plants get root rot, and rotten roots are a perfect food source for fungus gnats. As a side note, overwatering will eventually kill most houseplants, so please don't do it.

Remove Dead Leaves

If there are dead leaves or any other kind of debris in the soil or inside the liners or containers, it must be removed. Dead, decaying material is a food source for gnats.

Massacre With Steel Wool

This method works like a charm. Place a thin layer of steel wool on top of the soil of your potted plants. Any larvae trying to fly out will literally get shred to pieces, and adults flying in to lay their eggs will also die. Leave the wool in for at least 4 weeks to ensure that all generations of larvae are exterminated as they try to fly out. It's a simple yet magical technique.

Remove the Top Layer of Soil

Once you believe all the larvae are dead, remove the top part of the soil. This is to ensure that any eggs remaining will be removed. Replace this top half with sand. Sand drains well and dries quickly.

Drown Them in Vinegar or Beer

Place jars filled halfway with apple cider vinegar or beer near the source of origin. Screw on the lids and poke holes in them large enough for the gnats to enter. If you don't want to ruin your lids, you can cover with plastic wrap and poke holes in it. The gnats will enter to drink the vinegar or beer and drown. Do this repeatedly until you see fewer and fewer gnats in the jars.

Use Potato Slices to Control Larvae

Cut slices of potato up and place them on the surface of your potting soil. Fungus gnat larvae will swarm onto these tater slices after about four hours, and you can then dispose of them. Do this repeatedly until all the larvae are gone.

Kill Larvae With 3% Hydrogen Peroxide

Allow your soil to dry before watering your plant with a mixture of 1 part hydrogen peroxide and 3 parts water. You must dilute the hydrogen peroxide and only use the 3% hydrogen peroxide you find at the drugstore and nothing higher.

Control Larvae With Food-Grade Diatomaceous Earth

DE is natural and non-toxic, but it contains microscopic shards of silica that can rip larvae apart as they crawl through it. Mix some into the top layer soil (this is where gnats lay their eggs). If your repotting or planting something new, mix DE into the potting soil for prevention.

Purchase Products Containing BTI

These products, such as BTI Mosquito Bits, contain the bacteria Bacillus thuringiensis var. israelensis, which kills the larvae of many insects, including fungus gnats. Mix into your soil and larvae will be wiped out instantly.

Catch Flying Gnats With Yellow Sticky Traps

Like flypaper, these yellow sticky tabs catch gnats so you can discard of them. Just place the stick traps horizontally on top of the plant soil, and they will catch the adults that fly up out of the pot. Discard the sticky trap once it is covered in gnat carcasses, and continue using new traps until you see fewer gnats.

Discard or Repot the Plant

Organic material inside the soil is the most difficult issue to correct. The easiest solution is to discard the plant. If you are fairly handy, you may try repotting the plant in proper indoor potting soil. But for this to be successful, most of the old soil must be shaken away from the plant's root system.

Get a New Liner

If a liner with built-up residue and fungus is discovered, wash it thoroughly or get a new one.

The Fungus Gnat Life Cycle

It takes about 17 days for gnats to mature from egg to full-grown flying adults. They then live a week before they die. Use a combination of methods and do it repeatedly for at least 4 weeks to ensure that at least two generations of gnat larvae cannot fully form into adults, therefore killing off their population.

How to Prevent a Fungus Gnat Infestation

  • Don't Overwater Your Plants: Give plants the proper amount of water and allow them to dry out between waterings.
  • Keep Your Plants Clean: Remove dead leaves, stems, debris, etc.
  • Use Sand: Fill the top of your pot with 1/2 inch of sand. Water drains quickly in sand, keeping the top layer dry, and thus preventing gnats from laying their eggs.
  • Use Indoor Potting Soil: Always use indoor potting soil for indoor plants. Do not add compost or wood chips to the soil.
  • Don't Use Compost, Peat Moss, or Mulch: These materials retain moisture and contain decaying material that fungus gnats are attracted to. Don't using potting mixes containing these materials and replace the potting soil with indoor soil if necessary.
  • Consider Using Perlite or Vermiculite: Both of these materials improve aeration while retaining moisture. Like diatomaceous earth, they are sharp, so they present an unwelcoming environment to fungus gnats. Ask your local nursery which material you should use for your plants as perlite dries out too quickly for water-loving plants and vermiculite holds too much water for plants that need well-drained soil.
  • Check Plants For Infestation Before Buying: Avoid purchasing any plants that have evidence of fungus gnats.

Goodbye Gnats

Following these simple tips and keeping your plants properly watered will work wonders for keeping your home or workspace free of obnoxious little fungus gnats. There is no need to throw away all your houseplants! All you need is a little know-how.

Questions & Answers

  • We're in an office with minimal plants and fungus gnats are all over. Where could they come from? Are they dangerous?

    Fungus Gnats are not dangerous; they are however incredibly irritating. It does not take much to produce a substantial Fungus Gnat population. If the plants in your office have been inspected for organic material like dead leaves and bark, standing water causing root rot, or any other moist rotting material in the soil surface or in the liner and the source cannot be located it would be good to check garbage disposals, look for leaks or spills in kitchens, garbage can, etc to see if they may be the cause of the issue.

  • Are these bugs also attracted to light? I thought we had fleas. Keep finding little black bugs appearing to hop however they also fly, we don't have any bites, and they are mostly near my house plants, and also I am finding them in the bathtub and bathroom sink. I want them gone.

    I have not noticed Fungus Gnats to be attracted to light. Moisture/humidity is the big draw for them, so they are commonly found around sink drains, and human faces in addition to houseplants.

  • Mine are larger than fruit flies and I can't find them in plants, are they really fungus gnats?

    If the insects in your home are larger than Fruit Flies, and there is no sign of plant infestation, then you must have an issue that is not due to Fungus Gnats. It may be best to consult a pest control specialist to identify the offending bug in your home, & treat it accordingly.

  • We are in an office/warehouse. There are no plants, but we have had a few leaks from the roof. could it be from there the fungus gnats are coming from?

    If there is a persistent condition of moist material like wood, soil, leaves, paper, etc. associated with the roof leak, that could very well be the place your gnats originated.

Got Gnats?

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    • thoughthole profile imageAUTHOR

      thoughthole 

      11 days ago from Utah

      Antonia, remove the dead leaves. You are correct they are living in and feeding on the dead leaves, if you are to remove the leaves the Gnats will not have anything to support their colony.

    • profile image

      Antonia 

      13 days ago

      I have these tiny, flying bugs near my dwarf sunflowers. They are extremely annoying, and they seem to be eating the dead leaves I put in the pot earlier to help a small sprout grow stronger. I want to get rid of them but do not want to kill my plant. What should I do?

    • profile image

      Petals 

      6 weeks ago

      Have loads if fruit flies/ knots in my house mostly the bedroom I live in a first floor flat where my downstairs neighbour is growing potatoes in bags under the window could this be a cause

    • profile image

      Scott 

      2 months ago

      There is an OMRI Organic product called gnatrol or gnatrol.com that is a miracle cure for fungus gnats. It has the highest concentration of BTI available. BTI is the ingredient that kills the larve. There is nothing better and it's harmless to people, pets and plants. I think I paid less than $10 for it and it will last up to 2 years.

    • profile image

      3 months ago

      White brown gnats in house. No house plants. Fly in eyes and nose. How do I get rid of them?

      Have hired 4 exterminating companies that could not exterminate the gnats.

    • profile image

      Susan 

      3 months ago

      We have no plants or food exposed. I often have dish water that stays overnight which they tend to stay by most but can’t find the source. My daughter raises snakes with shavings , no raising mice and just got rid of guinea pigs, after those removals, we still maintain about 15-20.

    • profile image

      Judy 

      6 months ago

      We had fungus gnats for weeks and have no house plants. After killing 20-30 adults per day, cleaning out drains, covering windows with plastic, we finally found the breeding grounds. It was my bag of used espresso coffee pods! Got rid of bag...no more gnats!

    • thoughthole profile imageAUTHOR

      thoughthole 

      7 months ago from Utah

      Dianna I did not get any follow up information from Karen B, so I do not know what the outcome was on that situation.

      You may also want to inquire about the issue with a pest control expert familiar with your location, & climate.

    • profile image

      Dinana 

      7 months ago

      I have the same problem as Karen B. was there ever a resolution to her problem of fruit fly infestation? It just keeps getting worse and no fruit or plants in my home.

    • profile image

      Brenda Adams 

      8 months ago

      My bugs are fruit flies according to the description above. Only thing is I do not have "any fresh fruits or vegetables" in my home. I quit keeping any of these things in my home several months ago when these creatures started to show up. I have gone thru almost 2 gal. of vinegar and have bombed my home 3 times. These flies are causing me to question my own sanity....PLEASE HELP!!!!!!

    • profile image

      Connie 

      8 months ago

      Hi, I put apple cider vinegar in a small container and dropped a couple of drops of dish detergent in it and they’re attracted to it and drown themselves in it. It works!

    • thoughthole profile imageAUTHOR

      thoughthole 

      9 months ago from Utah

      That sounds like quite an unpleasant discovery Karen B.

      The scope of my expertise in this area is limited to indoor situations, and I live on the other side of the country in a very different climate from that of NJ, so I would likely not be the best person to answer your questions. This may be a subject in which a general pest control expert could provide insightful information.

      I would imagine that the recent storm systems hitting the East coast have given rise to all manner of pests that flourish in moist conditions, fungus gnats included.

      Sorry I could not be of more help.

    • profile image

      karen b. 

      9 months ago

      I was quite disturbed upon finding a writhing slimy mass of small "worms" on my concrete driveway yesterday...grossed me out and have never seen anything like it before. I sprayed it with raid a few times until I saw no more signs of life. Went online and found out this was a fungus gnat larvae train. Much to my horror, there was another train this morning on my concrete porch not far from the train yesterday...sprayed it, also. They appear to be coming from my lawn. I'm in New Jersey and it was a rather soggy summer. Is this event unusual where I live? Should I be concerned?

    • thoughthole profile imageAUTHOR

      thoughthole 

      10 months ago from Utah

      Sandy & Casey,

      While my expertise with gnats is in respect to indoor plants, I am confident fungus gnats will only be present if they have some decomposing moist material to feed on. The key is to find the food source, and get rid of it. Fungus gnats are often more prevalent during seasons of greater precipitation, namely spring & fall for much of the US. It is possible they may be originating outside, if this is the case it may be a bit more difficult to isolate a specific source. Outside or in look for moist wood, bark, roots, dead leaves etc. If the source remains elusive it may be an indicator of something out of the ordinary going on in or around the home causing structural decay, perhaps a water leak.

      Good Luck!

    • profile image

      Casey 

      10 months ago

      I don't have indoor plants and when I got out of the shower the little bugs were all over the bathroom and in the window. I went to my kitchen and there all over the lights?? What do I do?

    • profile image

      Sandy 

      10 months ago

      I believe I have fungus gnats. Everything here sounds right except I don't have indoor plants. I do have a little aquarium. Could that be it and if it could how do I fix this?

    • profile image

      Gwen 

      10 months ago

      Someone gave me a house plant, instead of flower, for the passing of a loved one. This article was very helpful.

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