Pest
Fungus Gnats
Tiny black flies on soil surface, larvae live in soil
Overview
Fungus gnats (Bradysia species) are the most persistently annoying flying pests in indoor potting. Adults are about 2mm long, thin, black, and delicate, flying around the pot surface. Adults themselves do not harm plants, but their larvae live in soil feeding on organic matter and fine roots. Heavy infestations can damage young roots and seedlings, causing damping-off and stunted growth. Constantly wet soil is the root cause.
Symptoms
Watering or touching the pot sends tiny black flies rising from the soil surface. They fly slowly and are easy to catch. The soil surface shows translucent white larvae (about 3-5mm) with distinct black head capsules wriggling in the top layer. Prolonged infestations cause weak growth and leaf yellowing.
Treatment
Cover the soil surface with 2-3cm of coarse sand or perlite — completely covering exposed soil to block adult emergence. Place yellow sticky traps near the pot. For heavy adult populations, spray with pyrethrin while also drenching the soil with BTi (Bacillus thuringiensis israelensis) to kill larvae. Allow the top 3-5cm of soil to dry completely between waterings.
Prevention
Keep the top 2-3cm of soil dry (gnat larvae need moisture to survive). Water only when the soil is dry. Use potting mix without excessive composted organic matter. Sterilize new potting soil by baking or microwaving before use.
