Fungus Gnats


Fungus gnats are very small, dark-colored flies with slender legs. Although some species are up to ½-inch long, most species commonly found in homes and businesses are about 1/16 to 1/8 inch-long. If you see small, gnat-like insects flying around or running over the soil surface of your plants, there is a good chance they are fungus gnats.


Fungus gnats are relatively weak fliers; thus, they tend to stay fairly close to their developmental habitats and don’t move around much indoors. However, their attraction to light may often cause the home or business owner to focus attention on where they are seen rather than where they are developing. If fungus gnats are identified, they most likely are developing in the wet, overwatered soil of individual indoor potted plants, atriums, and plantscapes within malls and other building. Fungus gnats can become very annoying, but are generally harmless unless their population is very large and the feeding larvae damage plants. Outdoors, fungus gnats are found in compost and mulch that are overly damp.


Fungus gnats feed on fungi that grow in the organic matter found in or on soil and may also feed upon the roots of plants.


Fungus gnat habitat is soil, both indoors and outdoors, that has a large percentage of organic matter, which encourages the growth of fungi. Also, fungus gnats can be found in piles of vegetative debris, compost, and mulch that is suitably damp.


Fungus gnats develop by complete metamorphosis and have four stages—egg, larva pupa, and adult. Their life cycle begins when a female fungus gnat deposits as many as 300 eggs on the surface of the soil. After about 5-6 days, the eggs hatch into larvae. The larval stage takes about 1.5 – 2 weeks to complete. The larvae molt into pupae that live for about one or two weeks, depending upon the environmental conditions. Once the pupae have matured, the adults will emerge, compete their maturation process, and fly away.


The annoyance of flying fungus gnats is the most obvious sign of an infestation. In addition, heavy populations of fungus gnat larvae can result in damage to the root systems of infested plants as they feed upon the roots. Larval infestations of established plants may show symptoms of little or no new growth, foliage that appears to be unhealthy, and foliage falling from the plant.


Since fungus gnats thrive in soil with excessive moisture, avoid overwatering plants and make sure the pot has good drainage. A good rule of thumb is to allow the surface soil to dry before re-watering the soil. Clean the overflow saucers under plants to ensure good drainage. In the winter months, plan to scale back the watering of houseplants. If possible, discard infested plants rather than allow them to be a source of infestation to other plants.

Periodically inspect open bags of potting soil to make sure that fungus gnats are not infesting the contents of the bags. Outside, repair any plumbing leaks, direct downspouts away from the house, ensure water doesn’t pool in the yard, and eliminate piles of grass clippings, leaves and mulch that can become overly wet and produce fungus gnats.


Since fungus gnats respond to light, use commercial insect light traps to help control fungus gnats. Also, using the bacterial insecticide known as BTI is effective. Always be sure to use this and any other product exactly as instructed by the product’s label. The use of BTI is particularly helpful when dealing with fungus gnats that are coming from plants that require soil that is very moist.