Bti (mosquito dunks, Gnatrol) for fungus gnat
Bti kills larvae in the compost and nothing else — not the adults you are swatting, and not pupae, which do not feed. The Gnatrol label says larvae generally respond within 24 hours. It does not persist or build up indoors, UC IPM says, so one dose disappoints and repeated doses through a generation work. That makes it the short-term half of a plan whose long-term half is letting the compost dry between waterings.
Before you order
A bacterium sold two ways: as Gnatrol, watered into compost for fungus gnats, and as Mosquito Dunks and Bits for rain barrels, bird baths and ponds. Larvae that eat it stop feeding and die. It does nothing to adult gnats or mosquitoes, and UC IPM says it is toxic only to fly larvae. How it works, how to tell it has established and the supplier list are on the full Bacillus thuringiensis israelensis profile.
Other controls for fungus gnat
- Steinernema feltiaeNematode · 250k–500k /m² of compost surface; about 5 million per 25 houseplants
- Stratiolaelaps scimitusPredatory mite · 100–500 /m² of compost surface (Dragonfli: 100 preventive, 500 on a heavy infestation)
- Rove beetlesWild predator · Nothing to release — the greenhouse species Dalotia coriaria is the one sold
- Dalotia coriariaPredatory rove beetle · 0.1–2 /sq ft (about 1–22 /m²); suppliers differ
- Macrocheles robustulusPredatory mite · 100–500 /m² per release, repeated once or twice