Fungus Gnat

Bradysia spp. / Lycoriella spp.

Small dark flies that breed in moist potting soil and organic matter. Fungus gnats are a common indoor pest in Arizona, particularly around houseplants and overwatered landscaping.

Insect Low Risk

About Fungus Gnat

Fungus gnats are small, dark-colored flies typically measuring 1/16 to 1/8 inch long. Adults resemble tiny mosquitoes with long legs and antennae. They are weak fliers and are commonly seen hovering close to the soil surface of potted plants, walking on soil, or flying sluggishly around windowsills and indoor lighting. The larvae are the damaging stage — they feed on fungus, decaying organic matter, and the root hairs of plants in moist soil. In Arizona, fungus gnats thrive indoors year-round because of the warm climate and the tendency to overwater houseplants in an attempt to combat the dry desert air. They can also enter from outdoor potted plants and organic mulch near the foundation. Heavy infestations can cause plant decline, wilting, and yellowing leaves due to root damage.

Identification

Adults are slender, dark gray to black flies about 1/16 to 1/8 inch (2-3mm) long with long, bead-like antennae and long dangly legs. Their wings are clear with a distinctive Y-shaped vein pattern. They are often mistaken for fruit flies, but fungus gnats are darker, have longer legs, and are found near soil and plants rather than food. Larvae are tiny (up to 1/4 inch), translucent white or clear with a shiny black head capsule, and found in the top inch of moist soil.

Signs of Infestation

Small dark flies hovering near houseplants, soil surfaces, or windowsills. Flies walking slowly across the soil or potting mix rather than flying. Plants showing unexplained wilting, yellowing, or stunted growth despite adequate watering. Tiny larvae visible when the top layer of potting soil is disturbed. Increased fly activity near drains that are connected to plant watering runoff. Flying near windows or light sources indoors.

Health & Safety Risks

Fungus gnats pose minimal direct health risk to humans. Adults do not bite or sting and are primarily a nuisance. However, they can cause significant damage to houseplants and seedlings through larval root feeding. In rare cases, fungus gnat larvae have been associated with opportunistic infections in immunocompromised individuals if soil is handled without proper hygiene. The primary concern in homes is plant damage and the annoyance of persistent adult flies indoors.

Prevention Tips

Allow the top 1 to 2 inches of potting soil to dry out completely between waterings — this is the single most effective prevention method, as larvae cannot survive in dry soil. Use well-draining potting mix and avoid overwatering, which is a common habit in Arizona due to the dry climate. Place a layer of sand or fine gravel on top of potting soil to discourage egg-laying. Avoid using potting mixes that contain high amounts of peat or wood-based compost, which retain moisture and harbor fungal growth. Inspect all new plants and potting soil before bringing them indoors. Use yellow sticky traps near plants to monitor and reduce adult populations. Keep organic mulch away from the home's foundation. Ensure good drainage in all planting areas near the house.