Fungus Gnats

Fungus gnats (family Sciaridae) are small dark flies that look like miniature mosquitoes, often the first pest indoor cannabis growers encounter. The visible adults flying around plants and grow lights are mostly harmless — the actual damage comes from the larvae living in the top inch of growing medium, where they feed on root hairs, decaying organic matter, and beneficial fungi (mycorrhizae). Symptoms of significant fungus gnat damage: stunted vegetative growth, yellowing of lower leaves despite proper feeding, wilting between waterings even with adequate soil moisture (caused by destroyed root hairs), increased susceptibility to root pathogens like Pythium because larvae create entry wounds, and slowed nutrient uptake. The lifecycle: adult females lay 100-300 eggs in moist soil, eggs hatch in 4-6 days, larvae feed for 10-14 days in the top 1-2cm of soil, larvae pupate for 4-7 days, adults emerge and live 7-10 days. Population can multiply 10x in 30 days under ideal moist conditions. Optimal conditions: constantly wet soil surface, organic matter near surface, temperatures 65-80°F. Treatment: yellow sticky traps to monitor and reduce adult populations; beneficial nematodes (Steinernema feltiae) applied as soil drench attack larvae in the soil; Mosquito Bits (Bacillus thuringiensis israelensis, BTi) sprinkled on soil surface kill larvae as water dissolves the bacteria; predatory mites (Stratiolaelaps scimitus / formerly Hypoaspis miles) attack larvae and other small soil pests; hydrogen peroxide soil drench (1:10 with water) kills larvae but disrupts beneficial microbiology. Prevention: allow soil surface to dry between waterings (fungus gnats cannot reproduce in dry surface conditions), bottom-watering when possible, sand or perlite top-dressing creates a dry barrier, and avoid excess organic mulch on soil surface during early grow stages.