Mosquito dunks and bits contain Bti (Bacillus thuringiensis israelensis), a larvicide that mosquito larvae eat in standing water, which kills them. According to the EPA, Bti is not harmful to people, pets, or beneficial insects. Drop them into any standing water you cannot drain.
The simplest way to think about mosquito dunks is the Larva-First framework: instead of chasing adult mosquitoes that already fly and bite, you treat the water where the next generation is still developing. Mosquito larvae and pupae live in standing water, so that water is where the whole life cycle can be interrupted. Mosquito dunks and bits are built for exactly that job. If you want the full picture first, see how to get rid of mosquitoes and what mosquito larvae actually look like.
What Mosquito Dunks Are (Bti)
Mosquito dunks and mosquito bits are larvicides. Their active ingredient is Bti, short for Bacillus thuringiensis israelensis — a naturally occurring bacterium. When Bti is placed in standing water and eaten by mosquito larvae, it kills them. According to the U.S. EPA, this Bti-based control targets the larvae specifically and is not harmful to people, pets, or beneficial insects, which is why it can be used in water features without harming other wildlife.
Because the mosquito life cycle runs egg → larva → pupa → adult, and both the larva and the pupa live in water, treating that water stops mosquitoes at the stage where they are easiest to reach. Dunks are solid rings that float and release Bti slowly; bits are smaller granules that scatter and act faster. Both rely on the same Bti, and both work only on the larvae in the water — not on the adults already flying.
Mosquito Dunks & Bits — The Key Facts
Feature
🟢 What the facts say
💧 Where it applies
Active ingredient
Bti — Bacillus thuringiensis israelensis, a larvicide.
Released into standing water.
What it kills
Mosquito larvae that eat it in the water.
The larval stage, before adults emerge.
Safety (per EPA)
Not harmful to people, pets, or beneficial insects.
Ponds, rain barrels, and other water features.
Life-cycle stage
Works on larvae; larva and pupa live in water.
Egg → larva → pupa → adult is interrupted.
Bti in mosquito dunks and bits kills mosquito larvae in standing water and, per the EPA, is not harmful to people, pets, or beneficial insects.
How to Use Mosquito Dunks & Bits
Using them is straightforward because they only need to reach the water. Drop a dunk into standing water and let it float; it releases Bti over time as the larvae feed. For faster action, or for shallow or smaller collections of water, sprinkle bits across the surface. In both cases the larvae ingest the Bti and are killed before they become pupae and then biting adults.
Simple Steps
Find standing water on your property that you cannot drain.
Float a mosquito dunk in it, or scatter mosquito bits across the surface.
Let the Bti do its work on the larvae living in the water.
For water you can drain, tip it out weekly instead — that removes the larvae directly.
Remember the Larva-First idea: dunks and bits are a larvicide, so they stop the next generation. They do not knock down adult mosquitoes already in the air.
Draining is always the first choice where it is possible, because eliminating standing water is the most effective mosquito control. Where the water has to stay — a pond, a rain barrel — that is where a Bti dunk earns its place. For the wider plan, revisit how to get rid of mosquitoes, and if you are also using catch devices, see mosquito traps.
Where to Use Them (Standing Water)
Mosquito larvae develop in standing water, so that is exactly where dunks and bits belong. The goal is to reach every pocket of water that holds still long enough for larvae to grow. Any container or low spot that collects rain and does not get emptied is a potential nursery.
Common places to check and treat include:
Rain barrels and other stored water you keep on purpose.
Ponds and ornamental water features.
Plant saucers under pots on patios and balconies.
Buckets, tubs, and other containers left outdoors.
Clogged gutters that hold water instead of draining.
Old tires and bird baths — empty these weekly where you can.
The rule of thumb is simple: if you can dump the water, dump it every week to break the larval cycle. If you cannot — because the pond, barrel, or feature needs to stay wet — put a Bti dunk or bits in it. To understand what you are targeting in that water, take a closer look at mosquito larvae, and return to the mosquitoes hub for the full topic.
Frequently Asked Questions
What are mosquito dunks made of?
Mosquito dunks and mosquito bits are larvicides that contain Bti, short for Bacillus thuringiensis israelensis. Bti is a naturally occurring soil bacterium. When it is placed in standing water and eaten by mosquito larvae, it kills them before they can develop into biting adults.
Are mosquito dunks safe for pets, fish, and people?
According to the EPA, Bti in mosquito dunks and bits kills mosquito larvae in water and is not harmful to people, pets, or beneficial insects. Because it targets the larvae specifically, it can be used in water features such as ponds and rain barrels without harming the surrounding wildlife.
How do mosquito dunks work?
Mosquito dunks release Bti into standing water. Mosquito larvae, which live and feed in the water, ingest the Bti and are killed. Because the treatment works on the larval stage, it stops the mosquito life cycle before the larvae become pupae and then adults that fly and bite.
Where should I put mosquito dunks and bits?
Place them in any standing water that cannot be drained, because that is where mosquito larvae develop. Common spots include rain barrels, ponds, plant saucers, buckets, clogged gutters, old tires, and bird baths. Water you can empty should simply be tipped out weekly instead.
What is the difference between mosquito dunks and mosquito bits?
Both contain the same active ingredient, Bti. Dunks are solid rings designed to float and release Bti slowly over time in larger standing water. Bits are smaller granules that scatter across the surface and act more quickly. Both work by killing mosquito larvae in the water.
Do mosquito dunks kill adult mosquitoes?
No. Mosquito dunks and bits act on the larval stage in the water, not on adult mosquitoes that are already flying. They are a larvicide, so their job is to stop larvae from ever becoming biting adults. To reduce adults you still need to eliminate standing water and use other measures.
Why is treating standing water so important?
Mosquito larvae and pupae develop in standing water, so removing or treating that water is the most effective way to control mosquitoes. Where the water cannot be drained, a Bti larvicide such as a mosquito dunk breaks the larval cycle before adults emerge.
Sources
U.S. EPA — Find the Repellent Right for You (Bti larvicides in mosquito dunks and bits kill larvae in water and are not harmful to people, pets, or beneficial insects).
U.S. EPA — Mosquito Life Cycle (egg → larva → pupa → adult; the larva and pupa live in water).
CDC — About Mosquitoes (larvae develop in standing water; eliminating standing water is the most effective control).