The fungus gnat answer. Steinernema feltiae are microscopic roundworms that hunt in the water film between soil particles, tracking larvae by carbon dioxide and vibration, entering them through natural openings and killing them within a day or two. Where a predatory mite works the top layer of the pot, these go wherever water goes — which is everywhere the larvae are.
They are also the reason a thrips program works. Most thrips drop off the leaves into the substrate to pupate, and that stage is out of reach of anything on the foliage. Sf intercepts it. If you are treating thrips on the leaves and nothing in the soil, you are treating half the population.
Application is a watering. Nothing to spray, no residue on the leaves, no re-entry interval, nothing to keep children or pets away from — you mix them into a can of dechlorinated water, water them in and leave them to it. They reach the whole root ball rather than the surface a drench wets, and neither fungus gnats nor thrips can develop resistance to being parasitized. Safe around people, pets and anything you plan to eat.
Two things decide whether they work. The soil has to be moist when you apply and stay moist for about two weeks afterwards, because they move in water films and die in dry soil. And the water has to be dechlorinated — chlorine and chloramine kill them. Apply in the evening or under low light, since UV kills them on exposed surfaces, and use the mixture within two hours of mixing.
Field Notes
Steinernema feltiae
The beneficial
Species
Steinernema feltiae
Type
Entomopathogenic nematode — a microscopic soil roundworm
How it works
Infective juveniles move through the water film between soil particles, tracking larvae by carbon dioxide and vibration. They enter through natural openings and release symbiotic Xenorhabdus bacteria; the host dies within 24 to 48 hours, and a new generation emerges from the cadaver
Hunting style
Cruiser — it searches rather than waiting for a host to walk past, and it works the shallow layer rather than deep soil
What it eats
Hunts
Fungus gnat larvae (Bradysia spp.), thrips prepupae and pupae in the substrate, and shore fly larvae
Best against
An active fungus gnat problem in potted plants, and the soil half of a thrips program
Also kills
Fruit fly larvae and other small soil larvae it encounters
Won't touch
Aphids, adult thrips, whitefly, mites, mealybugs, scale — anything living on foliage. Beetle grubs deep in the root zone are Hb’s job; fleas and caterpillars crossing the surface are Sc’s
What arrives
Format
Live infective juveniles on a moist inert carrier
Sold by
Count — infective juveniles (IJs)
Shelf life
Up to 30 days refrigerated at 38–42°F. The most storage-stable of the three nematodes
Refrigerate
Yes — keep the package cold until use. Nematodes are the exception to the no-refrigeration rule that applies to everything else we sell
Where it works
Soil temp
50–80°F — ordinary indoor room temperature. This is the cool-season species and the one for growing indoors
Moisture
The soil must be moist at application and stay moist for about two weeks. They move in water films and die in dry soil
Light
Apply in the evening or low light. UV kills infective juveniles on exposed surfaces
Establishes
Short-term. Cycles while hosts are present, then dies out — not a standing population
Running it
Applied as
A soil drench. Suspend in dechlorinated water and water in thoroughly
Mix water
Dechlorinated. Chlorine and chloramine kill them
Mixed suspension
Apply within two hours of mixing. Agitate gently as you go — they are heavy and settle — and do not leave them standing in water
Screens and filters
Remove anything finer than 50 mesh from the watering equipment
Dose
There is no single correct dose, and anyone who gives you one is guessing. The honest version is in our article on dosing nematodes
Reapply
Every 7–14 days for two or three rounds against an active infestation
Compatible with
Predatory mites, Stratiolaelaps scimitus, springtails, lacewings, ladybugs, Aphidius, Cryptolaemus, Trichoderma T-22, mycorrhizae. Stratiolaelaps is the natural pairing — the nematodes work the depth, the mites work the surface
Not compatible with
Broad-spectrum soil insecticides. Fungicides are generally fine — nematodes are not fungi
Chemical interval
One day after neem oil or insecticidal soap. Sulfur is 21 days after any application, and never with a sulfur burner running in the same space
Time to effect
One to four weeks. The larvae die out of sight, so judge it on how many adults rise when you water
Safe around
People, pets, plants, pollinators and aquatic life
