Adult fungus gnats are tiny black flies which spend time running around on the surfaces and sucking sap from the leaves. After the adults lay their eggs on the soil (medium), the young worm-like larvae hatch out and begin to feed upon the roots. The larvae initially eat the fine root hairs before moving on to burrow throughout the larger roots and even up into the stems. After feasting on the roots the larvae will pupate before hatching out as adults and repeating the life cycle by laying their eggs. The larvae resemble little worms. They have clear or white bodies with black heads.
Life cycle: Adults live about 7 to 10 days and deposit eggs on the moist soil surface or in soil cracks. Females lay up to 100 to 300 eggs in batches of 2 to 30 each in decaying organic matter. Eggs hatch in 4 to 6 days; larvae feed for 12 to 14 days. The pupal stage is about 5 to 6 days.
Damage from the fungus gnat larvae feeding on the roots shows up in the leaves as overfertilization and nutrient deficiency symptoms: yellowing, browning, burning and curling leaves.
Three fungus gnat larvae biocontrol agents:
Bacillus thuringiensis var. israelensis (Bti) bacteria which can be purchased as mosquito dunks or gnatrol. Bti is the most appropriate anti gnat biological control agent for DWC systems.
Hypoaspis mites.
Steinernema feltiae nematodes.
Occasionally a dry layer of sand is recommended as a deterrent to fungus gnats. This technique should be discouraged as the close packing nature of the sand particles reduces the quantity of oxygen which penetrates down into the root zone.