Stratiolaelaps scimitus

Little black flies rising off the soil when you water — that is fungus gnats, and the larvae doing the damage are down in the top inch of the pot. Stratiolaelaps scimitus, still widely sold as Hypoaspis miles, is the soil mite that patrols that layer and eats them, along with the thrips pupae that have dropped off the leaves.

What separates it from every other soil answer is that it does not need a pest to stay alive. It feeds on debris, algae and whatever else is in the substrate, so it holds for weeks with nothing to hunt — which makes it the one you put in and forget about. In a bioactive terrarium or a permanent bed with living soil it establishes and persists for months or years, quietly preventing the problem instead of responding to it.

And it takes about a minute to deploy. Nothing to mix, nothing to spray, no residue and no re-entry interval — you moisten the substrate, tumble the container to mix the mites through the carrier, top-dress about a teaspoon at each stem base and leave it. No drench schedule, no timing against a life cycle, and nothing a gnat larva can develop resistance to. Safe around people, cats, dogs, plants, and all reptiles and amphibians.

It is also the answer for snake mites in reptile enclosures, where it attacks them in the bedding they breed in rather than on the animal.

Best for: Preventing fungus gnats, holding ground after a knockdown, and snake mites in reptile bedding. 59–86°F, in substrate that is slightly moist — not bone dry, not saturated.
How many: Sizes go by plant count, at roughly a teaspoon of carrier per plant at the stem base. Top-dress and leave the carrier in place. In sterile mix, plan on re-applying every 6–8 weeks if you are holding ground; in living soil it will usually carry itself.
Arrives as: A bottle or tube of loose mites — adults, nymphs and eggs — through a sphagnum moss carrier. Tumble it for 20–30 seconds to mix them through. Deploy on arrival; there is no holding option.
Organic: Allowed in certified organic production — releasing predators is a first-line pest control practice under the USDA National Organic Program (7 CFR 205.206(b)(1)), with no input approval needed. Record the release in your Organic System Plan.
Safe around: People, cats, dogs, plants, and all reptiles and amphibians.
Keep separate from: Springtails — scimitus preys on them, so if a pot is running a springtail culture, keep the two apart. And diatomaceous earth, which slices beneficial mites as readily as pests.
Not for: Anything on the foliage. This is a ground predator, and carrier dusted on leaves is carrier wasted. Not a fast cure either — against a heavy active infestation, Sf nematodes knock the larvae down faster.