Bti is nature's sniper rifle

Mosquito dunks work because Bti produces a toxin that only activates in the alkaline gut of mosquito larvae, leaving everything else alone.

The short version: Bti (Bacillus thuringiensis israelensis) makes a protein that needs a very specific pH (alkaline) and certain enzymes to dissolve into an active poison. Mosquito larvae have that environment in their gut. Bees, dragonflies, fish, birds, and mammals don’t. So the toxin floats right through them harmless.

It’s also short-lived. The protein degrades in sunlight and water within a few days, so it doesn’t build up in the food chain. You drop a donut-shaped dunk in standing water, the Bti settles where the larvae feed, they eat it, and they die. Everything else? Totally fine.

I think people assume “biological control” means “vaguely safer,” but Bti is actually more selective than most chemical insecticides. It targets a very narrow window of gut chemistry that only certain fly larvae (mosquitoes, black flies, fungus gnats) have.

Worth noting: it won’t kill mosquito pupae or adults. You have to time it right. But for larvae? It’s about as surgical as it gets.