The fastest way to get rid of fungus gnats is to attack every stage of their life cycle at once: dry out the top of your growing medium, treat the larvae in the root zone, trap and spray the adults and block new eggs being laid. Fungus gnat larvae feed on fine roots, so acting quickly protects seedlings, cuttings and young plants in hydroponic and indoor grows.
This guide covers what fungus gnats are, why they're a problem, their life cycle, the signs of an infestation, the products you need, a step-by-step treatment plan, extra steps for hydroponic systems, prevention and common problems.
What are fungus gnats?
Fungus gnats are small, dark flies that look like tiny mosquitoes. Adults are usually about 1.5–3mm long, with long legs and antennae and clear to light grey wings. You'll usually see them around your plants and on the surface of the growing medium.
Their larvae live in the top few centimetres of the growing medium. They're legless, clear to whitish and up to about 6mm long, with a shiny black head.
Why are fungus gnats a problem in hydroponics?
Adult fungus gnats don't damage plants or bite people, but their larvae do the damage below the surface:
- Larvae feed on fine roots and can tunnel into stems at or below the surface
- Seedlings, cuttings and young plants suffer the most, with wilting and stunted growth
- Larvae can spread root diseases such as Pythium, and their feeding wounds let other soil-borne pathogens into the roots
- Adults can carry Botrytis (grey mould) spores between plants
The fungus gnat life cycle
Fungus gnats go through four stages: egg, larva, pupa and adult. Each female lays around 100–200 eggs into cracks in the growing medium, and generations overlap, so every stage is usually present at once.
| Stage | Duration (at about 24°C) | Status | Notes |
|---|---|---|---|
| Egg | About 3 days | Hidden | Laid in cracks in moist growing media |
| Larva | About 10 days | Damaging | Feeds on roots, fungi and organic matter |
| Pupa | About 4 days | Hidden | Develops in the growing medium |
| Adult | About 10 days | Nuisance | Mates and lays the next generation of eggs |
At about 24°C a full generation takes around 17 days, and warmer temperatures speed it up. That's why a single treatment rarely works: you need to keep going for at least 3–4 weeks to catch every stage.
⚠️ Signs of a fungus gnat infestation
- Small dark flies around your plants and on the surface of the growing medium
- Clear larvae with black heads in the top layer of the medium
- Wilting, slow growth or yellowing in young plants
- Weak or damaged roots on seedlings and cuttings
To check for larvae, place a slice of raw potato on the surface of the growing medium for 48 hours, then lift it and look underneath. Larvae are drawn to the potato and are easy to spot on the cut side.
What you need to get rid of fungus gnats
Treat each stage of the life cycle with the right tool:
- Yellow Sticky Traps small traps for pots, to monitor and catch adults
- Sticky Fly Traps (larger glue traps for bigger spaces)
- T-Drops to treat the root zone and nutrient solution
- Pyrethrum spray to knock down adults
- Gnat Nix! granules to stop adults laying eggs in the medium
💧 How to get rid of fungus gnats, step by step
1. Monitor with sticky traps
Hang yellow sticky traps about 2.5–5cm above the canopy or growing medium and check them weekly. Counting the gnats on the same traps each week tells you whether your treatment is working.
2. Dry out the top of the medium
Fungus gnats breed in moist media rich in organic matter. Let the surface dry between waterings, fix any overwatering, and clear away standing water, fallen leaves and old media.
3. Treat the larvae in the root zone
For soil or coco, add 2 drops of T-Drops per 4L of water and fully drench the growing medium. For reservoirs and recirculating systems, add 1mL (40 drops) per 100L of nutrient solution. Don't use T-Drops with hydrogen peroxide.
4. Knock down the adults
Mix Kendon Insect Destroyer at 20mL per litre of water and spray the adults, the surface of the growing medium and the undersides of the leaves. Tap the pots as you spray to flush adults out of the medium. Repeat at 7-day intervals as required, and check the label for withholding periods before harvest.
5. Block new egg laying
Cover the whole surface of the growing medium with a 2–3cm layer of Gnat Nix!, or thicker if you hand water. The granules act as a physical barrier that stops adults reaching the medium to lay eggs.
💡 A-Grade Tip: Add a layer of GnatNix! to the bottom of each pot as well as the top. The A-Grade team find that gnats can also get in and breed through the drainage holes.
6. Keep going for at least a month
Continue treating and monitoring for at least 3–4 weeks to cover a full life cycle. Stop when your sticky traps stay clear for a couple of weeks in a row.
⚙️ Fungus gnats in plumbed hydroponic systems
In recirculating systems, gnats can breed in the nutrient tank and spread through the irrigation lines. Follow the steps above, then:
- Add T-Drops to your nutrient tank once a week at 1mL per 100L as a maintenance dose
- Keep reservoirs covered so adults can't lay eggs in them
- Between grows, clean and sterilise your system and equipment with hydrogen peroxide at 1mL per 1L of water
💡 A-Grade Tip: Don't run hydrogen peroxide and T-Drops in your system at the same time. Finish sterilising and rinse the system before you start your T-Drops maintenance dose.
How to prevent fungus gnats
- Avoid overwatering and let the surface of the medium dry between waterings
- Keep air moving and humidity in check
- Keep your grow room clean and free of plant debris and standing water
- Hold new plants over a sticky trap for about 5 days before adding them to your grow room
- Check sticky traps weekly so you catch any new gnats early
Biological controls are another option. Bacillus thuringiensis subsp. israelensis (Bti), beneficial nematodes (Steinernema feltiae) and predatory mites that feed on gnat larvae are all used against fungus gnats.
Common fungus gnat problems and fixes
| Problem | Likely cause | Status | Fix |
|---|---|---|---|
| Gnats return after treatment | Treatment stopped before the life cycle was broken | Common | Keep treating for at least 3–4 weeks |
| Adults gone but larvae still in the medium | Only the adults were treated | Common | Drench the root zone with T-Drops |
| Gnats keep coming back in a plumbed system | Breeding in the reservoir or lines | Common | Cover the reservoir and add a weekly T-Drops dose |
| Large numbers despite treatment | Medium staying too wet | Common | Let the surface dry between waterings |
| Seedlings and cuttings wilting | Larvae feeding on young roots | Serious | Check with a potato slice and treat the root zone |
Summary
Fungus gnat larvae damage roots and can spread disease, so treat every stage of the life cycle at once. Dry out the surface of the medium, drench the root zone, trap and spray adults, block egg laying with a top dressing and keep going for at least 3–4 weeks.
Browse A-Grade Hydroponics' sticky traps and bug nets to get set up, or get in touch with the team if you've got questions about your own grow.
References
- University of California Statewide IPM Program, Pest Notes: Fungus Gnats (UC ANR Publication 7448), 2013. https://ipm.ucanr.edu/home-and-landscape/fungus-gnats/
- Purdue Extension Entomology, Fungus Gnats and Shore Flies (E-111), 2021. https://www.extension.entm.purdue.edu/publications/E-111/E-111.html
- University of Connecticut Integrated Pest Management Program, Biological Control of Fungus Gnats, 2019. https://ipm.cahnr.uconn.edu/wp-content/uploads/sites/3216/2022/12/2019biologicalcontrol-of-fungus-gnats3-2-1.pdf
