Fungus gnats are small, dark, short-lived flies, of the families Sciaridae and Mycetophilidae (order Diptera), whose larvae feed on plant roots or fungi and aid in the decomposition of organic matter. The adults are 2-5 mm long, and are important pollinators, and can also help spread mushroom spores.
They can be controlled by Hypoaspis miles or biological larvicide that kills the gnats in their larval stage. Detergents and the nicotine from tobacco brewed into a toxic tea are used by some people to control fungus gnats.
In houseplants the presence of fungus gnats may indicate overwatering. They may be feeding on roots that have sat in drain water too long and are rotting or may be attracted to fungus growing in saturated top soil. Typically draining the excess water from the plants drain pan and allowing the soil to dry will eliminate them. Fungus gnats are typically harmless to healthy plants, while their presence can be indicative of more serious problems.