Bti is a naturally occurring soil bacterium — Bacillus thuringiensis subspecies israelensis — that kills mosquito larvae in standing water while being essentially harmless to people, pets, fish, birds, and bees. It works because of a beautifully specific quirk of biology: Bti's toxin only activates in the gut of a few closely related fly larvae (mosquitoes, black flies, fungus gnats) and does nothing in the gut of basically everything else. You drop it in water you can't drain, mosquito larvae eat it, and they die within a day or two — while a bee, a goldfish, or a curious dog drinking from the same water is unaffected. It's the closest thing mosquito control has to a smart bomb.
Sold as Mosquito Dunks and Mosquito Bits, Bti is the one product we point people to even in the free source-reduction playbook, because it's the only thing that handles water you genuinely can't dump. Here's exactly how it pulls off "deadly to mosquitoes, safe for the food chain."
What is Bti, exactly?
Bti is a specific subspecies of a common bacterium found naturally in soils around the world. It was discovered in the late 1970s in a stagnant pond in Israel (hence israelensis), and it turned out to produce proteins that are remarkably lethal to mosquito and black fly larvae. Since then it's become one of the most widely used biological larvicides on the planet — deployed by public-health programs, including for community mosquito control, precisely because it's so effective and so selective.
The EPA registers Bti products for use in mosquito control, including in water sources near people, animals, and food crops. In its Bti fact sheet, the EPA notes that Bti has essentially no toxicity to people and other mammals and is used to control mosquito and black fly larvae in standing water.
How does Bti kill mosquito larvae?
This is the part that makes it safe, so it's worth understanding. When mosquito larvae ("wigglers") feed in water, they eat Bti's protein crystals along with everything else. Then two conditions have to be met for the toxin to activate:
- A highly alkaline (high-pH) gut. Mosquito, black fly, and fungus gnat larvae have very alkaline digestive systems. That specific chemistry dissolves the Bti crystal and switches the protein "on." Mammal stomachs are acidic, so the crystal never activates — it just passes through.
- Specific gut receptors. The activated toxin then has to bind to particular receptors lining the larva's gut. Those receptors exist in these fly larvae and not in mammals, birds, fish, bees, or the vast majority of other insects.
Both conditions met, the toxin punches holes in the larva's gut lining and it stops feeding and dies within hours to a day or two. Either condition missing — wrong pH, wrong receptors — and nothing happens. That two-key lock is why Bti can be so lethal to the target and so gentle on everything else.
Is Bti safe for bees, pets, and kids?
Yes — and this is Bti's whole reason for being famous. Because the toxin needs that alkaline gut and those specific receptors to work, it simply has no mechanism to harm animals that don't have them:
- Bees and pollinators: Bti is not toxic to bees. It targets aquatic fly larvae, not foraging adult insects, and the EPA and independent reviews find no meaningful risk to pollinators.
- Pets: Dogs and cats can be around treated water — a dog drinking from a Bti-dosed rain barrel isn't harmed, because the toxin can't activate in a mammal's acidic stomach.
- Kids: Bti is used in water sources near people. Follow the label like any product, but there's no toxic mechanism for mammals. (Keep the actual dunks and bits stored out of reach, as you would anything — they're food only to larvae, but they're not a snack.)
- Fish and birds: Considered safe; Bti is used in ponds with fish and in bird-frequented water.
How do I use Bti? Dunks vs Bits
Bti comes in two formats, and they're better together:
Mosquito Dunks are slow-dissolving "donuts." You drop one in standing water and it releases Bti for about 30 days, making it ideal for water that sits for a while and that you can't drain:
Mosquito Bits are granules. They release Bti faster for a quick knockdown of an active larval population, and they're easy to sprinkle into smaller or trickier spots — plant saucers, trays, bromeliads, tree holes:
Where to use them: rain barrels, ornamental ponds, birdbaths you want to keep filled, clogged catch basins and yard drains, corrugated drainpipe, tree holes, and any low spot that floods and won't drain. Anywhere you found water during your standing-water audit that you can't dump, Bti is the answer.
Where you don't need it: anything you can simply empty. Dumping is free and instant — save the Bti for the water that's stuck.
Does Bti replace dumping standing water?
No — it's the backup, not the main plan. The hierarchy still holds: first eliminate every water source you can (that's free and permanent), and then treat what's left with Bti. A rain barrel you use for the garden, a pond you love, a chronically flooding corner — those are the Bti jobs. Everything you can tip over, tip over. This is exactly how Bti fits into both the free playbook and the full gear stack: the small, cheap exception that covers the water you're stuck with.
Want to turn Bti into an offensive weapon? Our free DIY bucket-trap guide uses a Bti-dosed bucket as bait — it attracts egg-laying females and then kills their larvae, pulling breeding away from the rest of your yard. A little larvicide jiu-jitsu.
The bottom line
- Bti is a naturally occurring bacterium (Bacillus thuringiensis israelensis) that kills mosquito larvae in water.
- It's safe for people, pets, fish, birds, and bees because its toxin only activates in the alkaline gut and specific receptors of mosquito, black fly, and gnat larvae (EPA).
- Use Dunks for slow release in big/standing water (~30 days) and Bits for fast knockdown in small spots.
- It's the fix for water you can't drain — a backup to source reduction, not a replacement.
Targeted, proven, and gentle on the food chain. In the mosquito arsenal, Bti is the precision weapon.
Player questions
What is Bti?
Bti stands for Bacillus thuringiensis subspecies israelensis, a naturally occurring soil bacterium that kills mosquito, black fly, and fungus gnat larvae in water. It's sold as Mosquito Dunks and Mosquito Bits and is one of the most widely used biological larvicides in the world, registered by the EPA for use in water sources near people and animals.
How does Bti kill mosquito larvae?
When larvae eat Bti's protein crystals, the toxin only activates if two conditions are met: a highly alkaline gut and specific gut receptors, both of which mosquito, black fly, and gnat larvae have. The activated toxin damages the larva's gut lining and it dies within hours to a day or two. Animals without that alkaline gut and those receptors are unaffected.
Is Bti safe for bees, pets, and kids?
Yes. Because Bti's toxin needs an alkaline larval gut and specific receptors to work, it has no mechanism to harm mammals, birds, fish, or bees. The EPA reports essentially no toxicity to people and other mammals, and Bti is used in water near people and animals. Follow label directions and store the product out of reach as you would any product.
What's the difference between Mosquito Dunks and Mosquito Bits?
Both are Bti. Dunks are slow-dissolving donuts that release Bti for about 30 days, ideal for standing water like rain barrels and ponds. Bits are granules that release Bti faster for a quick knockdown of an active larval population and are easy to sprinkle into small containers, saucers, and tree holes. They work well used together.
Where should I use Bti?
Use Bti in standing water you can't drain: rain barrels, ornamental ponds, birdbaths you keep filled, clogged catch basins and yard drains, corrugated drainpipe, tree holes, and low spots that always flood. For any water you can simply empty, just dump it — dumping is free and permanent, so save Bti for water that's stuck.
Does Bti replace removing standing water?
No. Eliminating standing water is still the first and best step because it's free and permanent. Bti is the backup for water you genuinely can't remove, such as a rain barrel or pond. Dump everything you can, then treat what's left with Bti. It's the small, targeted exception that covers the water source reduction can't reach.