Do Bti dunks kill mosquito larvae safely?

What it does — and its limit

Bti — Bacillus thuringiensis subspecies israelensis — is the rare control tool that is both highly effective and remarkably specific. It’s a naturally occurring soil bacterium sold as floating "dunks" or granules. When mosquito larvae feed in treated water, they ingest the bacterium’s Cry and Cyt protein crystals; in the larva’s distinctive alkaline midgut, those proteins are activated and bind to specific receptors on the gut-cell membranes, punching pores that rupture the gut and kill the larva within a day or so.

Its safety comes directly from that mechanism. The toxin requires a high-pH gut and the matching receptors, which essentially only mosquito, black fly, and fungus-gnat larvae possess. Mammals, fish, birds, bees, dragonflies, and other aquatic invertebrates lack the activating gut chemistry and receptors, so Bti doesn’t harm them at label rates — which is why EPA and WHO treat it as a low-risk larvicide safe for use even in water sources. It targets the larva before it ever becomes a biting, flying adult.

The limit (the honest "but"): Bti only works in standing water you treat, and only against the larval stage. It does nothing to adult mosquitoes already flying, including those arriving from outside your yard, and it must be reapplied as it degrades (typically every ~30 days, sooner in sunlight or flushing rain). It’s also only as good as your coverage — untreated cryptic containers keep producing adults. So Bti is the ideal partner to source reduction for the water you can’t drain (rain barrels, ponds, drainage you must keep), but like dumping water it addresses the nursery, not the adults. The adult females already host-seeking your patio need an adult-targeting method.