Adults are about 1/32 to 1/16”, long and slender with long legs, similar to mosquitoes. Their colour is usually black, brown or yellowish with smoke coloured wings. Mature larvae usually are slightly longer than respective adults.
Females lay their eggs singly on larval food material and eggs hatch within a few days. They usually pupate in the ground, and adults emerge in about 3 days. The larvae of some species are luminescent (glow). Adults are typically found on or near larval food materials. Larvae feed primarily on fungi growing in the soil and moist decaying organic matter.
Inside, fungus gnat infestations are almost always associated with the soil of overwatered potted plants and atriums. Secondarily, consider signs of water leaks or moisture problems such as water stains, peeling paint, swelling of walls or wall covering, to indicate where fungi may be growing which can support larval development. Consider flat roofs which are particularly prone to water leaks. Also, the feces in pet bird cages can be the source if the feces are not removed in a timely manner. Outside mulched areas and the soil associated with them is usually the source.
The key to control is finding and eliminating all the breeding sources, so don’t stop until all potential sources have been inspected. Look for moist/damp areas which will support fungal growth. Inside, check the soil of potted plants and atriums and then move on to the other areas mentioned above. Once the source has been found the key to solving the problem is to either remove or dry out the moist material so that it can no longer support fungal growth. Sometimes turning over the top 2-3” of soil and adding fans to blow over the moist surface will speed the drying process. Insect light traps work well to reduce the numbers of adults indoors. Once the source has been eliminated a non-residual space treatment can be used to quickly kill the remaining adults if necessary.