Source control
Mosquito dunks & BtiMosquito dunks and Bti: the cheapest, safest way to cut the mosquito population
A cheap, naturally occurring bacterium kills mosquito larvae in standing water before they can bite, and it is safe for fish, pets, birds, and bees.
Updated July 22, 2026 · Reviewed against our scoring methodology
Most mosquito products fight the adults that are already looking for you. Bti fights them a step earlier, in the water where they hatch. It is the one intervention that is both highly effective and safe for nearly everything else, which is why the CDC and EPA recommend it and mosquito-control districts use it by the ton.
What Bti is
Bti is short for Bacillus thuringiensis israelensis, a naturally occurring soil bacterium. When mosquito larvae eat it, its proteins rupture their gut and they die before they can grow into biting adults. The key word is target-specific: those proteins only affect the larvae of mosquitoes, black flies, and fungus gnats. Fish, frogs, birds, bees, pets, and people are not harmed, which is why it is approved even for water that animals drink from.
Why it is the standout
It is the rare mosquito product that scores high on both effectiveness and safety. Foggers work briefly and kill pollinators; gadgets do nothing. A dunk quietly removes the next generation of biters for a couple of dollars a month, without collateral damage.
How to use it
Find the water
Mosquitoes breed in still water that sits for more than a few days: birdbaths, rain barrels, clogged gutters, plant saucers, ponds, low spots, old tires, tarps. Walk your yard after it rains.
Drain what you can
The best larvicide is a tipped-over bucket. Empty anything you do not need to keep.
Treat what you can't drain
Drop a dunk (or a scatter of Bits for faster or smaller jobs) into the water you have to keep. One dunk treats about 100 square feet of surface for roughly 30 days.
Reapply monthly
Bti breaks down, so replace dunks about once a month through the season, and after heavy rain refills a source.
The honest caveat
Bti lowers the future population; it does not stop the mosquito biting you right now. Treat your standing water and your neighbors' if you can, but still keep a skin repellent for the bites that come from beyond your yard.
Common questions
- Are mosquito dunks safe for fish and pets?
- Yes. Bti is target-specific to the larvae of mosquitoes, black flies, and fungus gnats. It is documented safe for fish, amphibians, birds, bees, pets, and people by the EPA, WHO, and CDC, and is approved for use in water sources such as birdbaths and rain barrels.
- How long does a mosquito dunk last?
- About 30 days per dunk, and one dunk treats roughly 100 square feet of surface water. Replace them monthly through mosquito season and after heavy rain.
- Do mosquito dunks actually reduce bites?
- They reduce the mosquito population by killing larvae before they mature, which lowers biting over time in and around a treated area. They are source control, not on-the-spot protection, so pair them with a skin repellent for immediate bites.
- Dunks or Bits, which should I use?
- Dunks are slow-release briquettes for standing water you keep for weeks. Bits are granular, act faster, and are easier to scatter across a large or shallow area or to dose a small container. Both use the same Bti active.