Small black flies, known as fungus gnats, are a common problem when you raise houseplants. Though the flies themselves typically do not create problems for the plants, the flies' larva attacks the plants' shoots and roots and can cause leaf loss. Get rid of the flies to protect the beauty and health of your houseplants.
Reduce watering as much as possible for your specific houseplant species. Use the plants' foliage as a moisture indicator, only watering when wilting is evident. Dry soil environments immediately kill the flies' larva, breaking the pest's life cycle.
Minimize the organic material in the soil, since the gnats and their larva are attracted to soils that are rich in organic compounds. Repot your houseplant using a synthetic potting mix based on vermiculite or perlite.
Spray the houseplant's foliage with a standard insecticidal soap. This immediately kills the adult gnat flies. Mist the soap evenly on all exposed surfaces of your houseplant.
Treat the houseplant's soil with a chemical insecticide as a last resort. Use a standard diazinon-based dust mixed into the top inch of the potting mix according to the insecticide's label. Diazinon kills gnat larva on contact.