Sf Nematodes

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 programme 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 parasitised. 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.

Best for: Fungus gnat larvae in potted plants, and thrips pupae in the substrate. Best between 50°F and 80°F, which is ordinary indoor room temperature — this is the cool-season nematode and the one for growing indoors.
How many: Sizes go by plant count. There is no single correct dose, and anyone who gives you one is guessing — the honest version is in our article on dosing nematodes.
Arrives as: Live infective juveniles on a moist carrier. Refrigerate at 38–42°F until you use them.
How you'll know: Fewer adults rising when you water, over one to four weeks. The larvae die out of sight, so trap counts are the measure — not anything you can see in the pot.
Safe around: People, pets, plants and food crops.
Not for: Anything living on the foliage. Not lawn grubs, which sit deeper than Sf hunts — that is Hb. Not fleas or caterpillars crossing the surface — that is Sc.