Fungus Gnats in Cannabis Soil
Fungus gnats are tiny flying insects whose larvae eat cannabis roots. Adults are annoying but harmless — the larvae in the soil do the real damage.
What it looks like
- ●Small black flies hovering around the soil surface
- ●Flies fly up when you touch the plant
- ●Seedlings wilting or stunted (larvae eating roots)
- ●Tiny translucent larvae in top inch of soil
Quick diagnostic checks
Measure before you treat
Plant diagnostic essentials
These tools help confirm the cause of fungus gnats before you change nutrients, watering, or the environment.
Carson MicroBrite Pocket Microscope
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Causes ranked by likelihood
1. Wet soil surface
Most commonSigns
- ●Soil stays moist on top
- ●Overwatering
- ●High humidity
Fix
- ✓Let top 2 inches of soil dry between waterings
- ✓Bottom water instead of top watering
- ✓Cover soil with sand or diatomaceous earth to block egg-laying
Recommended products
Gideal Dual-Sided Yellow Sticky Traps (20-Pack)
Catches adult gnats immediately — doubles as an early-warning monitor
Summit Mosquito Bits (BTI)
Sprinkle on the soil surface — BTI kills larvae in 2-3 days
2. Infested soil from bag
CommonSigns
- ●Just opened a new bag of soil
- ●Flies appeared within days of potting
Fix
- ✓Freeze soil before use or buy from reputable brands
⚠️When to worry
Gnats alone rarely kill an established plant but can destroy seedlings fast. Fix within a week.
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