If you're annoyed by tiny flying insects that seem to appear every time you water your houseplants, you're probably dealing with gnats—more specifically, fungus gnats. These pests are attracted to the damp soil of potted houseplants. They need the moist soil as a haven to lay their eggs, and the organic matter in the soil feeds their larvae. Besides being annoying, this feeding behavior can damage your plants.
Although they look a lot like tiny mosquitoes, fungus gnats are small flies of the Orfelia and Bradysia species. They can be identified by their narrow legs, light gray or clear wings, and segmented antennae that are larger than their heads. These are fairly tiny insects. The adults grow to about 1/16 to 1/8 inch long.
The good news is that fungus gnats do not bite people or pets. The adult gnats don’t do much damage to plants, either; rather, it’s the larvae that will munch on your plant's tiny feeder roots, limiting the plant's ability to take up nutrients and stunting its growth. This is more of a problem in nurseries, where susceptible young seedlings are grown in damp conditions. While you may not be growing your plants in a nursery or greenhouse setting, with a large enough population, they can pose a threat to common houseplants, too. If you notice these gnats flitting about and your plants seem to wilt for no reason, it could be root damage being caused by the feeding larvae.
3 Ways to Get Rid of Fungus Gnats
If you do find a fungus gnat problem, try to quarantine the infected pots away from other houseplants, so the problem does not get worse or spread. Since both the majority of the life of fungus gnats and the damage they do take place when they are in the soil, this is a good place to begin targeting them.
Let the Soil Dry
The easiest first step is to allow the soil to remain dry for several days before watering again. Both the eggs and the larvae will eventually die off in dry soil. Also, remove any standing water from runoff saucers. Another easy tip is to use a sterile potting mix because there is not as much organic matter for the young to feed on.
Existing fungus gnat larvae can be trapped using pieces of raw potato. Place the potato pieces flesh side down on the soil, and check under them every couple of days. Remove any feeding larvae, and replace the potato with fresh pieces. Do not let the potatoes dry out.
The adults have short lives, but reducing their population will also reduce the number of new eggs being laid. The best way to control adult fungus gnats is with yellow sticky traps. These are exactly what they sound like: sheets of yellow paper with adhesive on them. Yellow sticky traps can be found in the pesticide section of your garden center. There are also small versions that come with little stakes that you stab into the soil.
The gnats are attracted to the color yellow and get stuck on the traps. When the trap is full, throw it away and place another one. It’s not a pretty sight, but it is an easy, nontoxic way to eliminate large quantities of adult fungus gnats. It is also a great way to monitor for the presence of fungus gnats and to see if their population has become a problem.
Apply Biological Controls
There are a handful of biological controls for fungus gnats, but they are mainly used in greenhouses where plants are being propagated. If a gnat problem gets so out of hand with houseplants that you are considering biological controls, you are probably better off sacrificing your houseplants and starting over.
However, if you would like to give botanical controls a try, the best choice is Bacillus thuringiensis subspecies israelensis (Bti), bacteria that are used for mosquito control in ponds. These bacteria also control gnats but pose no risk to people or pets. Look for "Bt" in the form of mosquito dunks or granules (in the pest control section or near pond supplies in a home or garden center). You will probably need more than one application, but follow label instructions. Mosquito dunks are dissolved in water and the solution is used for watering the plants. Granules are sprinkled over the soil so some bacteria is introduced with each watering.
What Causes Fungus Gnats?
Fungus gnats tend to remain near their source of food—the organic matter and fungus in the soil. The adults lay their eggs in the soil, and the emerging larvae feed on the organic material in the soil as well as the plant’s root hairs. The larvae are 1/4 inch-long, translucent white or gray worms with shiny black heads. They may go unnoticed at this stage unless you have a crop of them, then you could see slime trails similar to those of snails and slugs.
Since it is the larvae doing the bulk of the damage, it helps to be aware of the fungus gnat’s life cycle. In a warm house, the tiny eggs can hatch into larvae in only three days. They remain in the larval stage for about another 10 days before developing into pupae. Approximately four days later, the adults emerge and start the cycle all over again. They can produce a new generation of fungus gnats in less than three weeks, so catching them early will make it easier to eliminate them.
Preventing Fungus Gnats in Houseplants
Fungus commonly gnats get into a house either because the plants have been outdoors, where adult gnats can lay their eggs into the soil, or the plants came from their source with contaminated soil. The only way to prevent this migration is to avoid bringing the plants inside. Of course, houseplants by definition live indoors, so this is not a permanent solution. The answer is to quarantine new plants or those that have been outdoors for extended periods. Keep the plants in their own room for at least 17 days (a full life cycle), and inspect the plant carefully for the presence of adult gnats. Once the plant has the all-clear, you can bring move it in with your other plants.
As an ongoing prevention method, be careful not to overwater your houseplants, since soggy soil and a full runoff saucer are just what the gnats are looking for. Let the soil dry between waterings, but only as much as the plant will tolerate; this is no reason to threaten your plant's vitality.
Do Worms Eat Fungus Gnats?
Steinernema feltiae is a nematode that feeds on fungus gnat larvae and release a bacterium that kills them. The nematodes typically must be mail-ordered from a plant pest control company, so this solution can be more of a hassle than other, quicker remedies.
Can I Spray Fungus Gnats?
Biological and synthetic insecticide sprays are effective for quick results against fungus gnats, but they are not a long-term solution and are seldom necessary. Common options include pyrethrins, permethrin, and bifenthrin. Sprays can harm beneficial insects, so use them carefully and judiciously. Typical application involves spraying the soil surface and the plant parts where adult gnats tend to land.
Do Fungus Gnats Fly Around the House?
Fungus gnats are relatively slow flyers that typically hang out on the soil of plants or nearby the plants, although they may venture out far enough to fly near your face if you're sitting close to a plant. Again, they are harmless to humans, and they do not bite. Think of them as fruit flies that hang around plants instead of fruit.