How to Control Gnats in Houseplants

Adult Fungus Gnat on Leaf

What Attracts Fungus Gnats to Houseplants

Have you ever been annoyed by tiny flying insects that seem to appear every time you water your houseplants? Those are gnats, more specifically, they are most likely "Fungus Gnats." These gnats are attracted to the damp soil in which your houseplants sit. They need the moist soil as a safe haven to lay their eggs, and the organic matter in the soil to feed their larvae. Besides being annoying, this feeding can do serious damage to plants.

What are Fungus Gnats?

Although they look a lot like tiny mosquitoes, fungus gnats are small flies in 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.  

Fungus gnats are not strong fliers, so they tend to remain near their source of food, which is the organic matter and fungus in the soil of your potted houseplants.

Fungus gnats 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 ¼ 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.

What Type of Damage Do Fungus Gnats Do?

The good news is that fungus gnats do not bite people or pets. The adults don’t do much damage to plants, either. It’s the larvae that will munch on tiny feeder roots, limiting the plant's ability to take up nutrients and stunting the growth of the plant. 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. 

Controlling Fungus Gnats Indoors

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 3 days. They remain in the larval stage for about another 10 days before they develop into pupae and then approximately 4 days later, the adults emerge and start the cycle all over again. They can produce a new generation of fungus gnats in less than 3 weeks, so catching them early will make it easier to eliminate them.

Controlling the Fungus Gnat Population - Since both the majority of a fungus gnat’s life and the damage it does take place when they are in the soil, this is a good place to begin targeting them. 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 less organic matter for the young to feed on.

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.

Trapping Fungus Gnat Larvae - 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.

Trapping Adult Fungus Gnats - The adults have short lives, but reducing their population will also reduce the number of new eggs being laid. The best way to kill off 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.

The gnats are attracted to the color yellow and get stuck on the traps. When the trap is full, throw it away and put up another one. It’s not a pretty sight, but it is an easy, non-toxic 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.

Biological Controls - There are a handful of biological controls for fungus gnats, but they are mainly used for control in greenhouses where plants are being propagated. If the problem gets that out of hand with houseplants, 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 would be Bacillus thuringiensis subspecies israelensis (Bti), a bacteria that is used for mosquito control in ponds. It also controls gnats but poses no risk to people or pets. You will probably need more than one application but follow label instructions. Look for "Bt" as mosquito dunks or granules in the pest control section or near pond supplies.

There are also nematodes that feed on fungus gnat larvae. These would need to be mail ordered from a plant pest control company, which can be more of a hassle than other, quicker remedies.