悟る · diagnose & fix
Cannabis Fungus Gnats
Pest · also: little black flies, soil gnats, gnats in soil
Small black flies hovering over wet soil — their larvae feed on roots and thrive in constantly damp medium.
Not sure it's fungus gnats?
Snap a photo and get a free read — no signup. Satoru weighs the visual against the conditions that cause look-alikes, so you treat the real problem.
What it looks like
Tiny (~2–3mm) dark flies drifting up from the medium when you disturb the pot, and crawling on the surface. The damage is underground: translucent larvae in the top inch chew fine root hairs, so seedlings stall and older plants show unexplained droop or nutrient-deficiency signs.
Symptoms
- Small black flies around the base and rising when disturbed
- Larvae (tiny clear worms) in the top layer of wet medium
- Stalled seedlings / weak, slow growth
- Droop or deficiency signs despite correct feeding
Causes
- Chronically wet topsoil / over-watering
- Rich, water-retentive medium that never dries
- Warm, humid tent
- Introduced in a bag of soil or a new plant
How to fix it
- 1Let the top 1–2 inches of medium dry fully between waterings — larvae need constant moisture
- 2Add a layer of sand, perlite, or a soil cover to break the surface
- 3Yellow sticky traps to knock down adult flies and monitor numbers
- 4Bti (mosquito-dunk/Gnatrol) drench or beneficial nematodes to kill larvae
Prevention
- Water less often, let the surface dry
- Bottom-water where practical to keep the top dry
- Sticky traps early as a warning system
悟る · how satoru tells it apart
Gnat-root damage mimics over-watering and deficiencies from above ground. Satoru cross-checks your watering log and stage so you look for larvae in a soggy pot instead of chasing the leaf color.
Commonly confused with: Overwatering, Root Rot, Nitrogen Deficiency.
Not sure it's fungus gnats?
Snap a photo and get a free read — no signup. Satoru weighs the visual against the conditions that cause look-alikes, so you treat the real problem.
More plant problems
Educational reference for cultivation troubleshooting. Symptoms overlap — when in doubt, match against your own conditions. Satoru fuses your photo with your logged environment for a grounded read.