Dalotia coriaria - Rove Beetle

A fast, mobile hunter for fungus gnat larvae and pupae in the soil — and for the thrips pupae most people forget are down there. Dalotia coriaria, the greenhouse rove beetle, is 3–4mm and dark, with short wing covers that leave the abdomen exposed and a habit of curling its tail up when disturbed. Both the adults and the larvae are predatory, both hunt in the substrate, and both are voracious — 10–20 prey items a day each.

The adults fly, which is the whole point of them. Release into a few pots and they spread themselves across a room of disconnected shelves, finding the pots you did not know had a problem. And they are a coloniser rather than a one-shot: in moist substrate they build a standing population and persist, which is what separates them from nematodes — those cycle while there are hosts and then die out. Egg to adult is about three weeks at 70°F, and the females lay continuously.

There is almost nothing to running them. Nothing to mix, nothing to spray, no residue and no re-entry interval — you tip them into small piles on moist substrate and leave them to it. They hunt down in the pot where no drench reaches evenly and no spray reaches at all, and nothing they eat can develop resistance to being eaten. Safe around people, pets and anything you plan to eat.

Two details that pay off. Release into a few piles rather than spread thin — they find mates and establish faster from a few breeding stations — and leave those piles unwatered for a day so they can burrow in. Turn intense grow lights down while you do it, or they fly at the bulbs.

Best for: Fungus gnats and shore flies across a large grow tent, a greenhouse, or a room of disconnected shelves — and they take shore fly eggs as well as larvae, which most predators ignore. 60–85°F, in moist substrate.
How many: Sizes go by pot count. Make breeding stations in a few pots and let them spread.
Arrives as: Adults, sometimes with larvae, through a vermiculite or peat carrier. High-energy and short-lived in the container — release the day they arrive, in the evening, onto moist substrate.
Safe around: People, pets, plants and food crops.
Keep separate from: Springtails, which they take on encounter — do not run them in a pot carrying a springtail culture. They also compete with and predate Stratiolaelaps scimitus to some degree, though commercial programs run both anyway.
Not for: Dry media; they will not work in it. Nothing on the foliage — no aphids, thrips adults, spider mites, whitefly, mealybugs or scale. And not a small closed container where the ability to fly is wasted: Stratiolaelaps scimitus is the better fit for single pots and reptile tanks.