Fungus gnats are small, dark, short-lived flies, 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 that can help spread mushroom spores as well as plant pollen.
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. Limiting moist environments and allowing the soil to dry will reduce their numbers. Fungus gnats are typically harmless to healthy plants, but can inflict extensive damage to seedlings; their presence can be indicative of more serious problems.