# Fungus gnats: the watering problem with wings

> Fungus gnats (Bradysia spp.) breed in constantly wet, organic-rich media; adults are a nuisance but larvae chew roots of seedlings and clones and can open the door to Pythium and Fusarium. Fix the watering first, then use Bti, Steinernema feltiae nematodes or soil predatory mites, and sticky cards to monitor.

- Canonical: https://genetixbay.com/knowledge/pests/fungus-gnats.html
- Category: Pests · Level: Beginner
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Flower · Systems: Soil, Coco, Living soil, Rockwool
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What are the little black flies in my grow?** Little dark flies running over the growing medium and flitting up when you water are almost always fungus gnats (Bradysia spp.). Adults are about 2 to 3 mm, grey-black, with long legs and antennae, and fly weakly. Their larvae, up to about 6 mm with a shiny black head, live in the top few centimetres of moist, organic-rich media. Shore flies are stockier and rarely harmful.

**Are fungus gnats harmful to cannabis?** Adult fungus gnats are harmless to cannabis, but their larvae chew roots and [root hairs](https://genetixbay.com/knowledge/glossary/root-hair.md). Damage is worst on [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md), fresh clones and plugs, which may wilt, stall or yellow in wet media. Mature plants rarely show damage unless numbers are huge. Larval wounds can also help [root rots](https://genetixbay.com/knowledge/glossary/root-rot.md) such as Pythium and Fusarium get in, so large populations deserve attention.

**How do I get rid of fungus gnats in soil or coco?** Get rid of fungus gnats by fixing the watering first: water less often and more completely, and in soil let the top 2 to 5 cm (1 to 2 in) dry. Coco and rockwool stay wetter by design, so rely on biological controls there: Bti drenches, Steinernema feltiae nematodes reapplied about every two weeks, or soil predatory mites. Count adults on yellow sticky cards weekly.

**Do fungus gnats cause root rot?** Fungus gnats do not cause root rot by themselves, but their larvae wound roots and can spread or aid root-rot organisms such as Pythium and Fusarium. Both pathogens have been recovered from commercially grown cannabis roots. Evidence that gnats vector them in cannabis specifically is indirect, though strong in other crops. A gnat outbreak also signals the wet conditions that favour rot.

**Do I need to let my pots dry out for fungus gnats?** Letting the top of soil pots dry between waterings is the main fungus gnat fix, but full drought is not needed. Grower practice is to let the top 2 to 5 cm (1 to 2 in) dry. Never starve mature plants of water to kill gnats, because drought stress costs more than the gnats do. In coco and rockwool, use biologicals and surface covers instead.

**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). Incompletely composted organic matter in [potting mix](https://genetixbay.com/knowledge/glossary/potting-mix.md) 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](https://genetixbay.com/knowledge/glossary/root-aphid.md) (winged adults look like small aphids on cards). See [kb-pests-root-aphids](https://genetixbay.com/knowledge/pests/root-aphids.md).

## 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 | |
| S. feltiae persistence | ~10–14 days in media | Reapply every ~2 weeks [3] |
| Preventive threshold (supplier guidance) | ~20 adults/card/week before [predatory mites](https://genetixbay.com/knowledge/glossary/predatory-mite.md); ~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

1. **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](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md).
2. **The medium.** Unfinished compost, bark or moist bags stored open.
3. **Algae and spills** on floors and slabs.
4. **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](https://genetixbay.com/knowledge/glossary/bottom-watering.md) keeps the top layer drier (Grower practice).

### Biological
- **Bti ([Bacillus thuringiensis](https://genetixbay.com/knowledge/glossary/bacillus-thuringiensis.md) 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 crawlers [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](https://genetixbay.com/knowledge/glossary/beauveria-bassiana.md)) as a drench are used by some growers (Grower practice).
- [Hydrogen peroxide](https://genetixbay.com/knowledge/glossary/hydrogen-peroxide.md) 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](https://genetixbay.com/knowledge/glossary/root-zone.md) 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](https://genetixbay.com/knowledge/glossary/overwatering.md) 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).

## Sources

1. UC IPM. Pest Notes: Fungus Gnats (Publication 7448). https://ipm.ucanr.edu/legacy_assets/pdf/pestnotes/pnfungusgnats.pdf
2. UConn IPM. Managing Fungus Gnats in the Greenhouse. https://ipm.cahnr.uconn.edu/managing-fungus-gnats-in-the-greenhouse/
3. UConn IPM. Biological Control of Fungus Gnats. https://ipm.cahnr.uconn.edu/ipm-biological-control-of-fungus-gnats/
4. Michigan State University Extension. Managing greenhouse insects. https://www.canr.msu.edu/news/managing_greenhouse_insects
5. Punja, Z.K. et al., 2019. Pathogens and molds affecting production and quality of Cannabis sativa L. Frontiers in Plant Science 10:1120. https://www.frontiersin.org/articles/10.3389/fpls.2019.01120/pdf