Short answer. Small dark flies running over the medium and flitting up when you water are fungus gnats. Adults are harmless nuisances; the larvae eat roots and root hairs, which hurts seedlings and fresh clones and gives root-rot fungi a way in. The first fix is watering: let the top few centimetres dry between irrigations. Then use a biological larvicide (Bti), Steinernema feltiae nematodes or soil predatory mites, and yellow sticky cards to watch the numbers fall.
What's happening
Fungus gnats (Bradysia spp.) lay eggs in moist media rich in organic matter and fungi. Larvae feed on fungi, decaying matter and young roots; damage is worst on seedlings, cuttings and plugs [1][2]. Larval wounding can spread or aid root rots such as Pythium and Fusarium [2]. Punja et al. recovered Pythium and Fusarium from cannabis roots in commercial production, so anything that wounds roots in a wet medium is worth taking seriously [5].
Lifecycle. Egg → four larval instars → pupa → adult, all in or on the medium. A full cycle takes about 3–4 weeks at warm room temperatures (temperature-dependent)being reviewed. Incompletely composted organic matter in potting mix often carries gnats in [1].
How to recognise it
- Adults: ~2–3 mm, dark grey-black, long legs and antennae, weak fliers that walk on the medium surface.
- Larvae: up to ~6 mm, translucent white bodies with a shiny black head capsule, in the top few centimetres of medium [1].
- Potato test: a slice of raw potato pressed onto the medium for 2–4 days collects larvae underneath [4].
- Plant signs: seedlings or clones that wilt, stall or yellow in a wet medium. Mature plants rarely show damage unless numbers are huge.
Differentials: shore flies (stockier, shorter antennae, white wing spots, algae feeders, rarely harmful) and root aphids (winged adults look like small aphids on cards). See kb-pests-root-aphids.
Numbers that matter
| Parameter | Value | Notes |
|---|---|---|
| Adult size | ~2–3 mm | |
| Larva size | up to ~6 mm, black head | [1] |
| Lifecycle | ~3–4 weeks warm | being reviewed |
| S. feltiae persistence | ~10–14 days in media | Reapply every ~2 weeks [3] |
| Preventive threshold (supplier guidance) | ~20 adults/card/week before predatory mites; ~50–75 to switch to nematodes | Supplier figures, not research (Grower practice) |
| Dry-back | top 2–5 cm (1–2 in) dry between waterings in soil | Grower practice; coco runs wetter by design |
What to check first
- Watering frequency and drainage. Pots that never dry, saucers holding water, or no drainage are the root cause. See kb-diagnosis-drooping-overwatering-vs-underwatering.
- The medium. Unfinished compost, bark or moist bags stored open.
- Algae and spills on floors and slabs.
- Count. Yellow sticky cards laid flat or just above the medium; count weekly [4].
How to fix it
Registrations differ by jurisdiction; check local regulations and the label. No products or rates are given here.
Cultural
- Water less often and more completely; let the surface dry.
- Use finished, pasteurised media; store bags closed [1].
- Clean up standing water, algae and plant debris.
- In coco and rockwool where the medium stays wet, rely on biologicals and covers rather than drought.
Physical
- Yellow sticky cards for adults [4].
- A layer of coarse sand or a fitted cover on the medium surface reduces egg laying (Grower practice).
- Bottom watering keeps the top layer drier (Grower practice).
Biological
- Bti (Bacillus thuringiensis subsp. israelensis): a bacterial larvicide drenched into the medium; kills larvae, not adults [1]. Repeat because new eggs keep hatching.
- Steinernema feltiae (entomopathogenic nematodes): soil drench; larvae die within a day or two; active ~10–14 days; repeat every two weeks; apply in low light to moist media [3].
- Stratiolaelaps scimitus (soil predatory mite): best preventively at planting; also eats thrips pupae and root-aphid crawlersbeing reviewed [3].
- Dalotia coriaria (rove beetle): preventive; compatible with the two above [3].
Least-toxic options
- Bti is the least-toxic larvicide class.
- Entomopathogenic fungi (Beauveria bassiana) as a drench are used by some growers (Grower practice)being reviewed.
- Hydrogen peroxide drenches are popular but poorly supported against gnats and can damage roots and beneficial microbes (Grower practice / Contested).
What not to do in flower
- Do not drench residual insecticides into the root zone in flower; most are not registered on cannabis and residues can show in tests.
- Do not starve mature plants of water to "kill gnats"; drought stress costs more than gnats do.
- Biological drenches (Bti, nematodes, predatory mites) are generally fine through flower as they act in the medium, but check that what you use is allowed on cannabis where you live.
How to confirm it worked
- Sticky-card counts fall over 2–3 weeks, then stay low.
- Potato slices show no larvae.
- Seedlings and clones root and grow normally.
Common mistakes
- Killing adults with cards and ignoring larvae.
- One Bti or nematode application, then stopping.
- Not changing the watering habit that created the problem.
- Treating gnats while ignoring the root rot they signal.
Evidence
- Biology, damage to young roots, association with overwatering and organic media: Established [1][2].
- Bti, S. feltiae and Stratiolaelaps as controls: Established in greenhouse crops [3]; extrapolated to cannabis (Supported).
- Gnats as vectors of Pythium/Fusarium in cannabis specifically: Supported (strong in other crops; cannabis-specific evidence indirect)being reviewed.
