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 haven to lay their eggs and the organic matter in the soil to feed their larvae. Besides being annoying, this feeding behavior can do serious damage to plants.
About 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. The 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 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.
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.
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 and they remain in the larval stage for about another 10 days before they develop into pupae. Then, 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.
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.
The easiest first step is to allow the soil to remain dry for several days before watering again. Then, 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.
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.
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.
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 would be 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" as mosquito dunks or granules in the pest control section or near pond supplies. You will probably need more than one application but follow label instructions.
Some nematodes 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.