How to Control Fungus Gnats on Indoor Plants. Fungus gnats are those little hopping bugs you see when you water your indoor plants. They are 1/32 to 1/50 inches with black or gray bodies and clear wings. Plants grown under cover are most susceptible. The larvae feed on dead roots and leaves, but sometimes consume tender new roots.
Look for small insects on the surface of the soil. They will be active when you water or move the plant. The larvae are 1/4 inch white-bodied maggots with black heads and can be found just under the surface of the soil.
Keep plants clean of fallen leaves and debris. Fungus gnats feed on dead plant material.
Plant pots of wheat to lure fungus gnats, then compost the soil and the wheat once the female fungus gnats have laid their eggs in the pots. This technique works well in greenhouses or greenhouse windows.
Use yellow sticky traps placed near the infested plant to catch adult fungus gnats.
Water with a solution of Bacillus thuringiensis, an organic control for caterpillars, to kill insect larvae. Bacillus thuringiensis (or Bt) is a bacteria and is safe to use around children and pets.
Drench the soil with a solution of pyrethrin, an organic pesticide made from chrysanthemums.