Adult fungus gnats are greyish brown flies that are mostly 3-4mm long. They can often be seen running over the surface of seed trays and pots, or flying slowly around plants. The larvae are slender whitish maggots, up to 6mm long, with black heads. Their bodies are semi-transparent and it is often possible to see the dark coloured gut contents. They live in the soil or potting compost. This insect thrives in damp composts containing high levels of organic matter. Potting media formulated for houseplants is the best choice for indoor plants.
Adult fungus gnats do not damage plants but they can cause annoyance when they are flying around indoors. The larvae feed mainly on dead roots and other decaying plant material and associated fungal growth. Some species of fungus gnats may also feed on soft plant growth, such as seedling roots and the base of soft cuttings. Established plants are unlikely to be damaged by fungus gnat larvae.