Often considered only a minor houseplant pest, fungus gnats can quickly become a major issue (and annoyance) 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’ 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, using consistent management and prevention techniques is the 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.
Size: Adult fungus gnats are tiny. Their size ranges from about 1/16 to 1/8 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 1/8 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 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 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 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, 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 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 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.
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 in conjunction with other prevention methods listed above, especially covering drainage holes.
Do you have any tips for preventing or stopping fungus gnat infestations? Tell us in the comments below!