Fungus Gnats
Definition: Fungus gnats are small, dark flying insects, about 1/8 inch long, that breed in constantly moist growing media. The adults are mostly a nuisance, but their larvae live in the top few inches of soil, feeding on organic matter and fungi and sometimes on tender roots, which can stunt seedlings and young plants.
How Fungus Gnats works
If small flies scatter from your pots every time you water, they’re almost certainly fungus gnats. They’re common, annoying, and usually a symptom of watering habits more than a crisis.
Life cycle
Females lay eggs in the top 1 to 2 inches of moist soil. Larvae, which are translucent with a shiny black head, feed for about 2 weeks before pupating. The full cycle takes roughly 3 to 4 weeks, depending on temperature.
Why they matter
Established plants usually shrug off a light population. Seedlings and fresh clones are another story: larvae can damage fine roots and open wounds that pathogens exploit, contributing to damping-off and stunted growth.
Breaking the cycle
- Let the top dry: allow the top inch of soil to dry between waterings. This alone often solves the problem, and it ties into avoiding overwatering.
- Yellow sticky traps: catch adults and show whether numbers are rising or falling.
- BTi drench: Bacillus thuringiensis subsp. israelensis, sold for mosquito control, targets gnat larvae in the soil.
- Beneficial nematodes: Steinernema feltiae hunt larvae in the root zone.
- Predatory soil mites: Stratiolaelaps scimitus provide ongoing control.
- Top layer: a half-inch of coarse sand or perlite makes the surface less appealing for egg-laying.
Organic and living soil grows attract gnats more often because they stay moist and full of decaying matter, so biological controls fit naturally there.