Small Black Flying Bugs in the House

An outbreak of flying insects indoors is troubling. When the intruder is small and black, suspect fungus gnats. These tiny, mosquitolike insects are a common indoor pest often seen flying around houseplants and near windows and doors. Understanding their breeding habits helps you eradicate them from the indoor environment.

green bottle fly
credit: MJL-Foto/iStock/Getty Images
An image of a fly.

Pest Identification

Fungus gnats (Bradysia spp.) are delicate, 1/8-inch-long, black or gray flying insects belonging to the Mycetophilidae and Sciaridae families. Fungus gnats breed in moist locations with a high organic content, such as damp potting soil. The insect flies around houseplants and scurries across the soil when you water. The fungus gnat's 1/4-inch larvae develop in the soil and are transparent, with black, shiny heads.

Life Cycle

Each female fungus gnat lays up to 300 eggs on the houseplant's soil surface, which hatch into larvae in five to six days. The larvae feed on organic matter in the soil for a couple of weeks, eventually pupating and transforming into adult fungus gnats. The adults live for a week, depositing eggs and continuing the cycle, which repeats indefinitely in the warm indoor environment.

Damage and Pathogens

Adult fungus gnats are not only annoying, but they carry on their bodies and in their feces soil-borne pathogens that can harm and even kill plants such as Fusarium, Verticillium and Thielaviopsis. Fungus gnat larvae feed on organic matter in the soil, such as fungi, as well as consume plant crowns and roots. Young plants affected by fungus gnat larvae often die. Mature plants exhibit weak growth, poor coloring and drop leaves.

Treatment and Control Measures

Control fungus gnats by letting the top 3 to 4 inches of the houseplant's soil dry out between waterings, if possible. This causes the larvae to die and discontinues the cycle. For plants unable to tolerate drought, eradicate fungus gnats by applying a soil drench that consists of water and the bio-pesticide Bacillus thuringiensis, israelensis. Trap adult fungus gnats and prevent them from breeding by placing yellow sticky traps on the soil surface.

Gnat Prevention

Prevent fungus gnat infestations by not overwatering houseplants. Wait until the top 2 inches of soil has dried, and never leave plant pots sitting in wet saucers. Avoid creating a soil surface breeding ground by removing leaf litter and plant debris. Always thoroughly inspect new plants for fungus gnats before introducing them to your indoor garden. Prevent fungus gnats from making their way indoors by ensuring that all doors and windows are properly sealed.