Biological control · Fungus Gnats

Fungus gnat treatment that reaches the larvae in the soil

Bradysia and other dark-winged fungus gnats (Sciaridae)

Fungus gnats are the small dark flies around the soil, and the adults are harmless. The problem is their larvae, which live in the top inch of potting mix and eat fine roots. Biological control works right there: Sf nematodes watered in clear the larvae, Stratiolaelaps soil mites stay in the pot to stop the next wave, and rove beetles hunt across whole rooms. Let the surface dry between waterings, and count gnats on a yellow card to see it working.

Build my plan Not sure it's fungus gnats?

4.7 out of 5 · 108 reviews of our fungus gnat predators

Adult fungus gnats standing on damp potting mix in a terracotta pot, one hovering above, with a translucent white larva with a black head in the soil
Egg to adult
about 17 days at 75°F
In the soil
larvae live in the top inch
How you'll know
fewer gnats on the card each week
Safe around
people, pets, plants and food crops
Build my fungus gnat plan

1Confirm it

How do I know it's fungus gnats?

Small, leggy flies around the soil mean fungus gnats. The larvae in the top inch are the real problem.

Getting the pest wrong is the most common reason treatment fails. Check the signs, or answer a few quick questions.

  • Small flies around the soil

    Dark, mosquito-like flies that lift off when you water or move the pot

  • Gnats on yellow cards

    Cards at soil level catch more each week

  • Larvae in the top layer

    Clear or white, legless, with a shiny black head, in the top inch of soil

  • Seedlings and cuttings stall

    Young roots get eaten first, so new plants wilt or stop growing

Looks like fungus gnats, but isn't

Still not sure? Work backwards from the damage or send us a photo.

2Pick a plan

Which fungus gnat treatment do I need?

Pick by what you see: a few gnats, gnats every time you water, a whole room, or an unknown soil pest.

Find what you're seeing, then add that plan to your cart.

If you see

Just a few gnats

A gnat or two when you water, and only a pot or two

Stratiolaelaps scimitus is a soil mite that lives in the top inch of the pot and eats fungus gnat larvae. It lives on debris between outbreaks, so it stays put and keeps working. Top-dress about a teaspoon at each stem base and leave it.

$50.00

Arrives alive or we reship it

Start here

If you see

Gnats every time you water

Several pots, and cards filling up between checks

Sf nematodes (Steinernema feltiae) are microscopic roundworms you water in. They find larvae wherever the water goes, from the surface to the bottom of the pot, and work in soil, coco, leca and hydro. Three rounds, 7–10 days apart, cover the eggs that hatch behind the first.

$15.00

Arrives alive or we reship it

If you see

A whole room or greenhouse

Lots of pots across shelves, benches or a grow tent

Dalotia coriaria rove beetles hunt larvae and pupae in the soil, and both adults and larvae are predatory. The adults fly, so they spread across a room on their own and find the pots you didn't know had a problem. They need moist media.

$20.00

Arrives alive or we reship it

If you see

Not sure what's in the soil

Something's at the roots and you can't tell what

Three nematode species in one pack, for when you can't tell fungus gnat larvae from other soil pests. If you know it's fungus gnats, Sf nematodes on their own are the better match.

$40.00

Arrives alive or we reship it

Good to know: Fungus gnats in a terrarium or bioactive enclosure? Stratiolaelaps is the one for that. It lives on debris between outbreaks and is safe around reptiles and amphibians.

Prefer to build it yourself? Shop each predator. Just want to keep fungus gnats out? See prevention. Not sure what you're seeing? Take the quick check.

Or build it yourself

Every fungus gnat treatment we sell

Every predator and parasite we ship for fungus gnats, with bundles first when we have them.

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Sf Nematodes
From $15.00

51 reviews
Triple Blend Nematodes for Broad-Spectrum Soil Pest Control
From $40.00

24 reviews
Stratiolaelaps scimitus
From $50.00

30 reviews
Dalotia coriaria - Rove Beetle
From $20.00

3 reviews

How it works

What eats fungus gnats, and where?

Everything happens in the soil: nematodes clear the larvae, soil mites hold the top inch, rove beetles cover big rooms.

The flies you see are the last stage of a life spent in the soil. Eggs, larvae and pupae all sit in the top inch of potting mix, and by the time an adult is flying it has already laid the next batch. That's why traps alone never end it, and why the treatment goes into the soil.

Ink drawing of a potted pothos cut away to show the soil, with close-ups of fungus gnat eggs, larvae among the roots, a pupa, and an adult gnat on the soil surface
  1. Eggs are laid in the top layer of damp potting mix and hatch in about three days.
  2. The larvae live in the top inch, eating fungi, organic matter and fine roots. This is the stage that does the damage.
  3. They pupate in the same top layer and don't feed.
  4. Adults come up out of the soil, live about a week, and lay the next batch in damp soil. They don't harm plants.

What to release

Compare every fungus gnat predator in one table
PredatorWorks onEatsBest forFrom
Sf Nematodes Soil Larvae, wherever water reaches An active problem. Watered in, every 7–10 days, three rounds. Works in soil, coco, leca and hydro $15
Stratiolaelaps scimitus Soil Larvae in the top inch Prevention, and holding the soil after nematodes. Top-dress once; in sterile mix, again every 6–8 weeks $50
Dalotia coriaria - Rove Beetle Soil Larvae and pupae Big rooms and greenhouses. Adults fly and find the pots you missed. Needs moist media $20
Triple Blend Nematodes for Broad-Spectrum Soil Pest Control Soil Larvae of several soil pests When you're not sure what's in the soil $40

One generation, start to finish

  1. EggDays 0–3
  2. LarvaDays 3–13
  3. PupaDays 13–17
  4. AdultDay 17 on
What happens at each stage
Egg Days 0–3 · Soil
Laid in the top layer of damp potting mix, up to about a hundred per female
Larva Days 3–13 · Soil
Clear body, shiny black head. Eats fungi, organic matter and, in numbers, fine roots
Pupa Days 13–17 · Soil
Rests in the top layer and doesn't feed
Adult Day 17 on · Air
A small dark fly with long legs. Lives 7–10 days, doesn't harm plants, and lays the next batch in damp soil

3After it arrives

How do I release predators for fungus gnats?

Damp soil, dechlorinated water, nematodes at dusk, and three rounds 7–10 days apart.

  1. Before you order

    1. Confirm it's fungus gnats

      Flies around the soil, not on the leaves. Scrape back the top inch with a spoon and look for clear larvae with black heads

  2. While it ships

    1. Put a dated card in

      One yellow card at soil level per shelf, so you can count week to week. Using rove beetles? Leave cards out for 7–10 days, then hang them high

    2. Wait out soil drenches

      One day after neem oil or insecticidal soap. Systemic insecticides in the soil need weeks, so check the label. Never sulfur

  3. Release day

    1. Water the pots first

      Damp soil before anything goes in. Nematodes need it to move, and mites and beetles settle faster

    2. Nematodes at dusk

      Mix into dechlorinated water, use within two hours, swirl between pots, then keep the soil damp for two weeks

    3. Soil mites on top

      About a teaspoon of carrier at each stem base. Top-dress and leave it; don't water it in

    4. Rove beetles in piles

      A few small piles in the wettest pots, grow lights down, and no watering over them for a day

  4. After release

    1. Repeat nematodes

      Every 7–10 days, three rounds. Soil mites and rove beetles usually carry themselves

    2. Finish properly

      Keep going until the card count stays low two weeks in a row

Conditions they like best

These are the ranges they like best, not hard limits. Short spells outside them won't hurt; only conditions that last for days slow them down.

  • Temperature50–85°FSf nematodes work from about 50°F, the soil mites and rove beetles from about 60°F. Ordinary room temperature suits all of them
  • Soil moistureDamp, not soggyKeep pots evenly damp for two weeks after nematodes. After that, let the surface dry a little between waterings, but not to bone dry
  • Water for nematodesDechlorinatedTap water left out for 24 hours, or a dose of dechlorinator. Chlorine and chloramine kill them
  • Light at releaseEvening, or lights downUV kills nematodes on exposed soil, and rove beetles fly at bright grow lights

What you'll see

  1. 1Right after release

    Gnats keep flying for about a week. Those adults were already out of the soil

  2. 2As it takes hold

    Fewer gnats on each new card. The larvae that would have replaced them aren't coming up

  3. 3Plants pick up

    Seedlings and cuttings start growing again once their roots stop being eaten

  4. 4When it's working

    The card count stays low two weeks running. A stray gnat isn't a new infestation

Why fungus gnat treatment fails

Most failed treatments come down to judging too early, soil that dried out, or only treating some pots.

  • Judged by the air in week one

    Adults already flying live out their week. Count the cards instead

  • The soil dried out after nematodes

    They move through water films, so a pot that dries out early ends the treatment

  • Chlorinated water or bright light

    Both kill nematodes. Use dechlorinated water and apply at dusk

  • Only the pots with gnats

    Adults lay in any damp pot. Treat every pot in the room

  • One round of nematodes

    Eggs can't be infected, so the next batch hatches behind the first drench. Three rounds, 7–10 days apart

  • Soil that's always soggy

    Constantly wet mix is a nursery. Let the surface dry a little between waterings

4Afterwards

How do I keep fungus gnats from coming back?

Top-dress Stratiolaelaps when you repot, and let the surface dry between waterings.

Fungus gnats usually arrive in a bag of potting mix or on a new plant. Stratiolaelaps is the prevention: top-dress it when you repot and it lives on debris between outbreaks, so it's already in the pot when the next gnats show up. Let the surface dry a little between waterings, and keep a dated yellow card in the room so you spot the first ones.

Often shows up with fungus gnats

  • Thrips

    Sf nematodes, Stratiolaelaps and rove beetles take thrips pupae in the soil too, so one soil treatment covers both

    Thrips treatment →

  • Root aphids

    Winged root aphids land on the same yellow cards. If cards fill but you can't find larvae, check the roots for white wax

    Root aphids treatment →

  • Shore flies

    Same wet pots, often with algae on the surface. Rove beetles take shore fly larvae and eggs too

    Shore flies treatment →

Fungus Gnats questions

More about biological control in the Biocontrol FAQ, or ask us.

How do I know it's fungus gnats and not something else?
Fungus gnats are small, dark flies with long legs that hang around the soil and lift off when you water or move the pot. Their larvae live in the top inch of potting mix: clear or white, legless, with a shiny black head. Tiny insects on the leaves with silver streaks are thrips, and tan flies with red eyes around fruit or drains are fruit flies.

Are fungus gnats harmful to plants?
The adults aren't. They don't bite and they don't damage plants. The larvae eat fungi and organic matter in the soil and, in large numbers, fine roots. Seedlings, cuttings and young plants feel it most, and that's when a fungus gnat problem stops being just a nuisance.

Why are there still gnats after I treated the soil?
Soil treatments reach the larvae, not the adults already flying, and those adults live about 7–10 days. What changes first is that no new ones come up to replace them. Put a dated yellow card at soil level and replace it weekly: a count that falls card after card means it's working.

Should I use nematodes, Stratiolaelaps or rove beetles?
Sf nematodes for an active problem. You water them in, they reach larvae wherever the water goes, and three rounds 7–10 days apart cover the eggs that hatch behind the first. Stratiolaelaps soil mites for prevention and for holding the soil afterwards, since they live on debris between outbreaks. Rove beetles for big rooms and greenhouses, because the adults fly and find the pots you missed.

Can I use nematodes and Stratiolaelaps together?
Yes, and it's the usual pairing. The nematodes clear the larvae through the whole pot, and the soil mites stay in the top layer afterwards to keep the next wave down. They don't get in each other's way.

Do I need to treat every pot?
Yes. Adult gnats fly between plants and lay eggs in any damp pot, including ones that look fine. Size your order to cover every pot in the room, because a pot you skip is where the next generation comes from.

Does letting the soil dry out help?
Yes, and it's free. The larvae live in the top inch, which is also the part of the pot that dries first, so letting the surface dry a little between waterings takes away their nursery. Bottom watering helps too. The one exception: keep pots damp for two weeks after a nematode drench.

Do nematodes work in leca or semi-hydro?
Yes. Sf nematodes travel through the film of water between particles, so they reach larvae in leca, coco, deep pots and hydro setups. Stratiolaelaps needs a substrate to live in and does best in soil and coco.

Are they safe around kids and pets?
Yes. Sf nematodes, Stratiolaelaps and rove beetles are safe around people, pets, plants and food crops. There's nothing to spray, no residue and no wait before anyone can be near the plants.

Do I need yellow sticky traps?
Not to fix the problem, since an adult has usually laid its eggs by the time a card catches it. They're the best way to measure it, though. Keep a dated card at soil level and replace it weekly. If you're releasing rove beetles, take the cards down for 7–10 days, then hang them higher up, away from the soil, so they catch gnats and not your beetles.

How do I store nematodes until I use them?
In the fridge at 38–42°F, not the freezer, and use them within about four weeks. Nematodes are the one thing we sell that belongs in the fridge. Stratiolaelaps and rove beetles go out on the day they arrive.

What growers say about our fungus gnat predators

4.7 108 reviews across 4 products Read reviews

Every order arrives alive or we reship it. Live Delivery Guarantee

Field Notes

Fungus Gnats, at longer length

Reviewed by Karen Horn, FGMN Nursery, growing since 2005 · Updated October 10, 2026