Fungus Gnat & Cool-Season Soil Pest Control — Steinernema feltiae (Sf)
How to Say It: sty-ner-NEE-muh FEL-tee-eye — but "Sf" works too.
Biological soil pest control for outdoor and indoor growing spaces.
Introduction to Sf Beneficial Nematodes
Have you ever sprayed for fungus gnats or thrips only to still see them bouncing back after every treatment? Fungus gnat larvae and thrips pupae stay tucked into the top layer of soil or growing medium, right where most pest control efforts miss them. These soil-dwelling pests spend a critical part of their life cycle just below the surface, out of reach of anything that doesn't penetrate the soil.
Steinernema feltiae (Sf) operates as a "shallow cruiser," staying active in the upper few inches of soil or growing medium. As a biological soil treatment, it targets pest populations in the shallow band of soil where fungus gnat larvae and thrips pupae develop. This application goes straight to the source, giving you targeted, natural pest control for the specific pest stages that live where they're hardest to reach. It's a standard pick for indoor and greenhouse growing environments.
What Beneficial Nematodes Are
So, what are nematodes, exactly? Nematodes are microscopic, unsegmented roundworms — among the most common soil-dwelling organisms on the planet, found in nearly every growing environment. Not all nematodes work the same way: some feed on plant roots and cause real damage, while others — entomopathogenic, or "beneficial," nematodes — do the opposite.
What does a beneficial nematode do differently? Rather than feeding on plant tissue, it hunts down soil pests directly, infects them, and lets its symbiotic bacteria do the rest. That distinction is what separates a beneficial nematode from a harmful one, and it's the whole reason this is a soil treatment rather than a soil problem.
Because they're microscopic, there's no real answer to "what do beneficial nematodes look like" with the naked eye, but under a microscope they are mostly translucent, tapering slightly at the ends, and forming an S or J shape when they are at rest.
What Sf Nematodes Target
Fungus gnats (Sciarid), thrips pupae, cabbage maggot, gall midge, humpbacked (phorid) flies, leafminers, onion maggot, root maggot, lesser cornstalk borer, raspberry crown borer.
How Beneficial Nematodes Work
The life cycle follows the same pattern as other beneficial nematodes: eggs hatch into non-feeding juveniles, which molt into infective juveniles that search the shallow soil layer for hosts, infect them with bacteria, and feed on the cadaver before repeating the cycle. What sets Sf apart is timing tolerance — it's the most cold-tolerant of the three species we carry.
When to Use Sf Nematodes
Beneficial nematodes work best applied preventively at the first sign of pests, like fungus gnats hovering around soil or pots, or thrips pupae showing up in your pest monitoring. The earlier you treat the upper soil layer, the fewer larvae and pupae get the chance to mature into breeding adults.
Because fungus gnat (Sciarid) populations reproduce continuously in moist growing media, plan on reapplying Sf every 3 to 4 weeks rather than expecting a single treatment to fully break the cycle.
Soil Temperature Range
- Optimal soil temperature range: 59–79°F
- Beneficial nematodes need consistently moist soil. Avoid drying out.
- Keep soil moist for two weeks following application. Avoid runoff when watering.
Application Rates for Beneficial Nematodes
| Application | Rate |
| Preventative/Light Infestation | 5 million nematodes per 300 sq. ft. |
| Moderate/Heavy Infestation | 5 million nematodes per 50 sq. ft. |
These are general starting points. Actual application should be adjusted based on pest species, plant type, and infestation severity. Monitor pest and predator populations regularly to gauge how well the program is working.
How to Apply Beneficial Nematodes
Apply only in the early morning, late evening, or on a cloudy day. Direct sunlight and UV rays kill nematodes. Wet the soil before and after application. Keep the ground damp for at least two weeks so the microscopic worms can move.
- Prepare the Soil — Water the treatment area first so the soil is moist, not dry. Nematodes move through water, so dry soil will stop them from reaching the fungus gnat larvae and thrips pupae in the upper soil layer.
- Mix the Solution — Combine the Sf nematodes with water between 59°F and 69°F. How much water you use depends on the size of the area you're treating and how established the infestation is.
- Apply to the Soil — Pour or spray the mixed solution evenly across the soil surface using a watering can, pump sprayer, or hose-end sprayer — whichever you already have on hand.
- Keep the Area Moist — Water lightly right after application, then maintain consistent soil moisture for the next two weeks to support nematode survival and movement. Avoid heavy watering that causes runoff, since this can wash nematodes away from the treatment zone.
Reapply every 3 to 4 weeks during peak pest seasons to maintain protection.
Watering Can or Pump Sprayer
- Moisten the soil beforehand to avoid runoff and improve habitat.
- Mix million nematodes with 1–5 gallons of water (59–69°F) to cover the desired area, diluting as needed for coverage.
- Apply the solution to the desired area. Full, even coverage isn't necessary, since nematodes will move through the soil on their own..
- Water the treated area immediately after application, and keep soil moist for two weeks.
Hose-End Sprayer
Use room-temperature water and let the nematode powder fully dissolve before spraying. Remember to remove any screen filter in your hose-end sprayer first.
| 5 million | Add all nematodes to the reservoir, fill to the 5 gallon mark. Spray until empty. |
| 10 million | Add all nematodes to the reservoir, fill to the 10 gallon mark. Spray until empty. |
| 25 million | Divide in half. Fill reservoir to the 15-gallon mark, mix in one half, spray until empty. Repeat twice. |
| 50 million | Divide into thirds. Fill reservoir to the 15-gallon mark, mix in one third, spray until empty. Repeat three times. |
Other Hose-End Sprayers (dial-style)
Use room-temperature water and let the nematode powder fully dissolve before spraying. Remember to remove any screen filter in your hose-end sprayer first.
| Package Size | Preventative/Light | Moderate/Heavy |
| 5 million | 4 oz | 8oz |
| 10 million | 4 oz | 8 oz |
| 25 million | 1.5 oz | 8 oz |
| 50 million | 4 tsp | 5 oz |
Pairing for Broad-Spectrum Soil-Pest Control
Using beneficial nematodes with other predators closes the gap between immobile and mobile soil-dwelling pests, and those that have moved above ground.
- Heterorhabditis bacteriophora (Hb) nematodes — Works deeper in the soil profile targeting grubs and beetle larvae.
- Steinernema carpocapsae (Sc) nematodes — Works at the soil surface level, in mulch and leaf litter targeting fleas and small caterpillars.
- Triple Blend Mix — Contains all three nematode species, Sf+Sc+Hb.
- Hypoaspis miles (Stratiolaelaps scimitus) — Also works the soil surface, feeding on fungus gnat larvae directly. Combining the two adds an above-surface predator to the below-surface nematode activity for fuller fungus gnat coverage.
- Orius insidiosus (Minute Pirate Bug) and Amblyseius cucumeris — Target thrips once they've emerged from the soil onto on foliage, while Sf handles the pupal stage in the soil below. Together, this covers thrips at both the soil and plant level. The Thrips Control Garden Pack bundles the Minute Pirate Bug, A. cucumeris, and Sf nematodes.
Safe, Sustainable Pest Management
- 100% natural — no chemical residue
- Safe around people and pets
- Safe for use on edible crops
- Fits seamlessly into existing IPM programs
- Requires no protective equipment
Storage
Before Activation Your nematodes arrive in a bubble mailer with an ice pack. It's normal for the ice pack to have melted and feel warm on arrival. The product remains safe and effective to use. If you're not applying them right away, refrigerate the unopened package at 36–45°F until you're ready, for no more than 30 days. Do not freeze.
After Activation Once the nematodes are activated with water, it's best to apply the solution immediately. If you can't apply right away, beneficial nematodes can survive up to 2 hours in still water — but they sink to the bottom and will start losing viability from oxygen depletion. You can extend that window several more hours with manual or mechanical stirring, or up to about 24 hours with an aquarium bubbler or small pump.
Pro Tips
- If you're dealing with fleas or surface-active pests, Sc is a better fit; for deeper-dwelling grubs, Hb is the one to use. If you're not sure, the Triple Blend Mix will cover all the bases.
- Avoid flushing nematodes away by minimizing runoff when watering.
- Check for water retention and drainage in the areas to be treated to ensure watering is effectively penetrating the soil.
Where to Buy Beneficial Nematodes
Beneficial nematodes aren't something big-box stores usually carry, so if you've been searching for where to buy beneficial nematodes near you, an independent garden center or hydroponics shop is worth a call first.
If you can't find them locally, or you'd rather not chase down a shop that has them, you can order online directly from Gardening Zone. It's also where to go if you're looking for other beneficial insects at the same time, since nematodes pair naturally with other biological controls.
FAQs
Where do Sf nematodes live day to day?
Anywhere the soil stays consistently damp — garden beds, potted soil, greenhouse benches, and container mixes are all viable habitat for Sf specifically.
Can I use Sf nematodes alongside neem oil or insecticidal soap?
Yes, but timing matters. Apply nematodes a few days apart from any foliar spray, since some residue can affect nematode survival in the soil. Nematodes themselves pose no risk to your other treatments.
How often do I need to apply Sf nematodes?
Since fungus gnat populations reproduce continuously in moist soil, monthly reapplication is recommended for ongoing control rather than a single treatment.
Are Sf nematodes safe to use around edible plants?
Yes. Sf nematodes are safe for use on edible crops with no waiting period before harvest, and they're safe around people and pets when used as directed.
Are beneficial nematodes eco-friendly, and safe around pets, kids, and pollinators?
Yes. Sf nematodes are a 100% natural biological soil treatment with no synthetic chemicals involved. They're safe around people, pets, and kids when applied as directed, and because they work below the soil surface on soil-dwelling pest stages, they don't come into contact with pollinators the way a foliar spray would.
Will Sf nematodes harm earthworms or other beneficial soil organisms?
No. Sf nematodes are host-specific to insect pests like fungus gnat larvae and thrips pupae — they don't affect earthworms, beneficial soil microbes, or other non-target organisms in your soil.
How long do beneficial nematodes live in the soil once applied?
As long as pest levels stay high enough to sustain reproduction, the population can persist in the ground; once hosts become scarce, numbers decline fairly quickly. That natural decline is the main reason reapplication every 3 to 4 weeks keeps control consistent rather than relying on one release to last all season.
How long until I see results?
Beneficial nematodes will begin to work immediately after application. It can take 5–7 days to see visible results through the reduction of pest populations.
Do Sf nematodes work in other types of growing medium than soil?
Yes. As long as the environment is adequate, beneficial nematodes are effective in coco coir, peat moss, perlite, and rockwool.
I've heard nematodes can damage plants — how do I get rid of nematodes if that's my problem instead?
That's a different category of nematode. Plant-parasitic species (like root-knot nematodes) feed on roots and can genuinely harm a garden, but they're biologically unrelated to the beneficial, insect-targeting nematodes. Beneficial nematodes don't feed on plant tissue at all. If you're dealing with root damage and suspect plant-parasitic nematodes, that's typically managed through crop rotation, resistant plant varieties, or a soil test to confirm what's actually present.
Where can I buy beneficial nematodes?
Right here from Gardening Zone! Beneficial nematodes ship directly to your door. You can also check a local garden center or nursery, though ordering directly from us online guarantees product freshness.