Houseplants are just as susceptible to pest infestations and diseases as plants growing outdoors. If not controlled in time, houseplant pests spread to other indoor plants, as well. Little black flies are among the likely pests of houseplants. Poorly growing plants are more likely to get infested with insects, as cited by the University of Minnesota Extension.
Fungus gnats (Bradysia species) are very small sized black flies that hover on houseplants. The flies breed in the growing medium and are most prevalent in early spring. All houseplants are potential hosts to black flies.
Adult gnats do not cause any damage to plants and lay their eggs at the plant's base. The emerging maggots or larvae feed on fungi in the soil and on the roots of plants. This leads to poor plant growth. Fungus gnats complete their entire generation in a month.
Reduce soil moisture to make growing mediums less habitable for pests. Allow soil surface to dry between watering. Use sticky cards to capture adult flies. Insecticidal control options include the use of pyrethroid and pyrethrin-based products.