Fungus Gnats in Hydroponics

What Are Fungus Gnats (Sciarid Flies)?

Fungus gnats—also known as sciarid flies—are among the most frustrating pests for hydroponic growers, houseplant lovers, and commercial horticulturists. Scientifically classified under the Sciaridae family, the most common UK species is Bradysia deformis. These tiny black flies are more than just an annoyance—they can significantly damage your root zone and even disrupt beneficial microbial life such as mycorrhizal fungi.

Why Fungus Gnats Are a Problem in Hydroponics

Despite feeding on fungi, fungus gnats—especially in their larval (maggot) stage—also attack young roots, hampering plant development and increasing susceptibility to disease. In hydroponics, even inert media like coco or perlite can support fungal growth, creating a perfect breeding ground.

✅ Common Signs of Infestation:

• Small black flies hovering near media
• Sudden stunted plant growth
• Wilting despite good watering
• Yellowing leaves in seedlings

How to Prevent Fungus Gnats

1. Control Moisture

Fungus gnats thrive in damp, decaying media. Avoid overwatering and keep your grow media just moist enough to meet plant needs. Over-saturated compost or coco can become a gnat magnet.

Top Tip: Avoid potting seedlings directly into their final containers. Use progressive potting to reduce excess wet substrate.

2. Increase Airflow

Fungus gnats are weak flyers. Increased air circulation confuses their navigation and reduces egg-laying. Proper airflow also reduces fungal growth, which is their main food source.

3. Use Sticky Traps

Yellow sticky traps are highly effective for monitoring and trapping adult gnats. Opt for large rectangular traps and cut them into smaller pieces. Place them directly on pot rims with sticky adhesive for best results.

4. Optimize Fan Placement

Position your outflow fan near the grow media surface. This helps suck out gnats before they can reproduce—especially when combined with filtered inlet fans.

Biological and Physical Control Methods

✔️ Diatomaceous Earth or Zeolite

Apply a thin layer to the media surface. These abrasive minerals deter gnats by disrupting their ability to sense moist compost and physically blocking larval entry.

✔️ Beneficial Nematodes

Use Steinernema feltiae, a biological control that parasitizes larvae. They work best in warm, moist media and are non-toxic and safe for hydroponics.

✔️ Predatory Mites

Milesia hypoaspis (formerly Hypoaspis miles) is a soil-dwelling mite that preys on fungus gnat larvae. These can be cultured at home, although it’s more practical to purchase them from biological control suppliers.

Chemical Options (Use with Caution)

• Pyrethrum/Pyrethroids: Fast-acting insecticides derived from chrysanthemums (natural) or synthetic alternatives.
• Tanlin (Chitin-Based): A commercial product believed to act by stimulating microbial activity that breaks down larval exoskeletons.

Note: The efficacy of chitin and chitosan for gnat control is debated. Soil sterilants are faster, more reliable results—especially in commercial hydro systems.

Houseplant Solutions

If you’re battling gnats in indoor potted plants:

• Use biological controls like nematodes or predatory mites.
• Avoid hazardous soil sterilant’s unless in a professional horticultural context.
• Maintain proper watering and use sticky traps for monitoring.

Final Thoughts: Integrated Pest Management is Key

Fungus gnats are a persistent pest, but with a combination of prevention, biological control, and smart physical traps, you can keep your hydroponic or houseplant environment free from infestations.

Top Recommendations:

• Keep media moist but not wet
• Use sticky traps and airflow to disrupt their lifecycle
• Try biological control before resorting to chemicals

Article by Dr Russell Sharp

If you would like to keep up to date with subjects just like this, you can listen to both our podcasts! Links can be found bellow:

Hydroponics Daily Podcast: https://open.spotify.com/show/207T7p7fw9sPjINfSjVXW2

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