Dealing with Fungus Gnats in a Hydroponic System

What Fungus Gnats Do

Fungus gnats are small flies whose larvae feed on root hairs and organic matter in growing media, leading to reduced root function, stunted growth, and increased susceptibility to root pathogens. [1]

Latin Name

Bradysia spp. (commonly Bradysia impatiens) [1]

Identification

Adults: slender black flies, 2-4 mm long, fluttering near media surface
Larvae: translucent, legless maggots (3-5 mm) in moist media [2]
Damage: reduced root hairs, wilting, yellowing of lower leaves

Alternative Names

Sciarid flies
Mosquito gnats
Moth gnats

Varieties (Common Species)

Bradysia impatiens [1]
Bradysia coprophila [1]
Bradysia paupera [1]
Lycoriella auripila [1]

Climates Where They Thrive

Warm, humid environments with moist media (20-25 °C; RH > 60 %) [2]
Decline in dry (< 40 % RH) or very hot (> 30 °C) conditions

Companion Planting as Deterrents

Cultivar	Hydroponic Methods	Effectiveness
Basil (Ocimum basilicum)	All (DWC, NFT, Ebb & Flow, Drip)	Moderate-aromatic oils deter egg-laying [5]
Peppermint (Mentha × piperita)	Wick, Kratky, Drip	Moderate-strong menthol scent masks roots [5]
Chrysanthemum (Tanacetum cinerariifolium)	All methods	High-pyrethrins in roots reduce larvae [6]
Marigold (Tagetes spp.)	All methods	Moderate-thiophenes suppress larval development [6]

Natural Predators

Beneficial nematodes (Steinernema feltiae) [3]
Predatory mites (Hypoaspis miles) [2]
Rove beetles (Staphylinidae) [1]

Remedies

Treatment	Hold Period	Natural?	Effectiveness
BTi (Bacillus thuringiensis var. israelensis)	0 days	Yes	High-larval gut toxin [4]
Sticky traps (yellow)	0 days	Yes	Moderate-captures adults [2]
Hydrogen peroxide drench (3%)	0 days	No	Moderate-oxidizes larvae in media [5]
Insecticidal soap drench	1 day	Yes	Low-contact kill of larvae [5]

Other Remedies

Sand or perlite top-dressing: Dry barrier prevents female oviposition [2]
Vacuum removal: Gentle suction to collect larvae from surface [5]
Essential oil aerosol: Diffuse rosemary or thyme oil near reservoir [5]
UV LED strip: Low-intensity UV above media to disrupt adults [6]

Tips and Tricks

Allow media surface to dry between irrigations
Maintain lower humidity (< 60 % RH) at canopy level [2]
Inspect roots and media weekly for larvae
Sanitize trays and replace media between crop cycles
Quarantine new cuttings/seeds before adding to system

Common Hosts

Leafy greens (lettuce, spinach) [1]
Herbs (basil, mint) [5]
Ornamentals in propagation trays [6]

Citations

[1] Fungus gnat - Wikipedia - https://en.wikipedia.org/wiki/Fungus_gnat
[2] Fungus Gnat Management in Greenhouses - UC IPM - https://ipm.ucanr.edu/PMG/PESTNOTES/pn7458.html
[3] Beneficial Nematodes for Fungus Gnat Control - University of Florida - https://edis.ifas.ufl.edu/publication/IN1075
[4] Use of Bacillus thuringiensis israelensis Against Fungus Gnats - UH Extension - https://www.ctahr.hawaii.edu/oc/freepubs/pdf/ip-3.pdf
[5] How to Get Rid of Fungus Gnats in Hydroponics - Hydrobuilder Learning Center - https://learn.hydrobuilder.com/get-rid-of-fungus-gnats/
[6] Companion Planting Principles - Wikipedia - https://en.wikipedia.org/wiki/Companion_planting