# Fungus gnat

> Small dark flies of the genus Bradysia (family Sciaridae) whose larvae live in wet growing media, feeding on fungi, organic matter and young roots.

- Canonical: https://genetixbay.com/knowledge/glossary/fungus-gnat.html
- Also written: fungus gnats, Bradysia, sciarid fly, sciarid, soil gnats, dark-winged fungus gnat
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Fungus gnats are small, delicate, dark grey flies, about 2-4 mm long, mainly *Bradysia* species in the family Sciaridae. Adults are weak fliers with long legs and a Y-shaped wing vein. The larvae are clear to white legless maggots with a shiny black head, living in the top few centimetres of moist media.

## In practice

Adults are a nuisance; larvae do the damage. They chew [root hairs](https://genetixbay.com/knowledge/glossary/root-hair.md) and can spread root pathogens such as *Pythium*, which is why they matter most for seedlings and fresh clones. Constantly wet media and organic-rich mixes breed them. Growers monitor with [yellow sticky traps](https://genetixbay.com/knowledge/glossary/sticky-trap.md) near the medium surface and with potato slices pressed into the top layer, checked after a couple of days for larvae. Letting the top layer dry between waterings, [bottom watering](https://genetixbay.com/knowledge/glossary/bottom-watering.md) and top-dressing with coarse sand help. [Biological control](https://genetixbay.com/knowledge/glossary/biological-control.md) uses *[Steinernema feltiae](https://genetixbay.com/knowledge/glossary/steinernema-feltiae.md)* nematodes, the soil mite *[Stratiolaelaps scimitus](https://genetixbay.com/knowledge/glossary/stratiolaelaps-scimitus.md)*, and *[Bacillus thuringiensis](https://genetixbay.com/knowledge/glossary/bacillus-thuringiensis.md)* subsp. *israelensis* (Bti) drenches.

## Don't confuse with

- **Shore fly**: stockier, with dark wings marked by pale spots, feeding on algae.
- **Root aphid**: winged forms on traps look gnat-like but have [aphid](https://genetixbay.com/knowledge/glossary/aphid.md) bodies.
- **Whitefly**: white wings, lives on leaf undersides.

## Related terms

[Shore fly](https://genetixbay.com/knowledge/glossary/shore-fly.md), [Steinernema feltiae](https://genetixbay.com/knowledge/glossary/steinernema-feltiae.md), [Stratiolaelaps scimitus](https://genetixbay.com/knowledge/glossary/stratiolaelaps-scimitus.md), [Bti](https://genetixbay.com/knowledge/glossary/bti.md), [Sticky trap](https://genetixbay.com/knowledge/glossary/sticky-trap.md), [Pythium](https://genetixbay.com/knowledge/glossary/pythium.md)

## Read more

- [Fungus gnats: the watering problem with wings](https://genetixbay.com/knowledge/pests/fungus-gnats.md)

## 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/