Mosquito Dunks are floating briquettes of Bacillus thuringiensis israelensis (Bti) at 10.31% active ingredient plus 89.69% inert carriers, and the bacterial toxins kill mosquito larvae that eat them — not adult mosquitoes.
One Dunk in a rain barrel stops larvae from reaching the biting stage. Below is what each part does, how the two Dunk products differ, and the label limits on where a briquette can go.
What Ingredients Are In Mosquito Dunks?
Every Dunk is built from a single active ingredient — Bti — suspended in inert material that holds the briquette together and lets it float and release slowly.
The EPA label (registration No. 6218-47) lists Bti at 10.31% and other ingredients at 89.69%. The label does not name those other ingredients individually, so any blog claiming to list the full carrier recipe is guessing.
Bti is a soil bacterium, not a synthetic chemical. While feeding, larvae swallow the bacterial solids, spores, and insecticidal toxins, which break down the larval gut; the larva dies before it can pupate into a biting adult.
One caution from the label: the percentage of active ingredient does not measure performance. Potency testing for these products is not federally standardized, so 10.31% on one label is not automatically equivalent to 10.31% on another.
Are Dunks And Dunk Chunks The Same Product?
No — they are separately registered products with different EPA numbers, Bti strains, and listed potency.
| Feature | Mosquito Dunks | Mosquito Dunk Chunks |
|---|---|---|
| EPA registration | No. 6218-47 | No. 6218-87 |
| Bti strain listed | BMP 144 | SUM-6218 |
| Active ingredient | 10.31% | 10.31% |
| Other ingredients | 89.69% | 89.69% |
| Listed potency | Not stated on label | 722 Aedes aegypti units per mg |
| Release period | 30 days or more | Per product label |
| Typical use | Whole briquettes in larger water | Smaller containers, spot dosing |
The Summit Chemical product page points to separate labels and safety-data sheets for the 2-, 6-, and 20-count packs, so the count changes the paperwork, not the formulation. Because the products carry different registrations, the application rate on one label does not transfer to the other.
How Long Does A Dunk Keep Working?
That is a label performance period, not a promise for your puddle. Heavy debris, fast rain overflow, and unusual water chemistry can shorten how long Bti stays at a useful concentration. Follow the label’s re-treatment interval for your water body rather than assuming every container needs a fresh Dunk on the same date.
A Dunk will never kill adults already flying — Bti must be eaten by a larva. Adults, eggs on the surface, and mosquitoes drifting in from a neighbor’s yard are outside its reach. Source reduction beats any larvicide when possible: the CDC advises emptying, covering, or turning over containers weekly, reserving Bti for standing water that can’t be dumped — rain barrels, fountains, ornamental ponds, septic tanks, and unused pools.
Where Can You Legally Use A Dunk?
A Dunk may only go into the water types its EPA label authorizes, and that list is narrower than most people assume.
The CDC notes some Bti products are registered for professional use in certain drinking-water storage containers while others are not registered for that use at all. Do not assume a Dunk is approved for drinking water or every outdoor water feature just because Bti is natural. Treated water should not be used for drinking unless the label specifically allows it.
Those findings assume label-compliant use — exceeding the rate or reapplying early just adds product to water that didn’t need it.
Two more label mistakes: there’s no universal “one Dunk per X gallons” rule, because a briquette and a chunk are separate registrations, so the rate must come from the exact package in hand. And treating water you could simply dump out wastes product and leaves the breeding site in place.
If you’re stocking up for a barrel, pond, or a season of standing water, this roundup of the best mosquito dunks compares package sizes side by side.
Application direction comes from the label, not habit. The EPA-approved Dunks product label spells out permitted sites, rates, and re-treatment intervals, and it governs any use that differs from a blog post.
Common Questions
Is Bti the same thing as Bt used on garden caterpillars?
No. Bti is Bacillus thuringiensis subspecies israelensis, which targets mosquito and black fly larvae. Garden products use different subspecies, such as Bt kurstaki, which target caterpillars. They are not interchangeable; the strain on the label determines what the product controls.
Do Mosquito Dunks hurt dogs or cats that drink from a treated bowl?
According to the CDC, Bti is not toxic to people, pets, or other animals when products are used as directed. Still, a treated container should not double as a drinking source for pets unless the label explicitly permits that use.
Can I crumble a Dunk for a small container?
Only if the label for your specific product allows it. Mosquito Dunk Chunks exist for smaller water volumes and carry their own EPA registration, strain, and potency listing. Splitting a standard Dunk is a dosing decision the label of that exact package has to support.
Sources
- U.S. Environmental Protection Agency. “Pesticide Product Label, DUNKS” Supplies the registered active ingredient percentage, strain, and permitted use sites.
- Centers for Disease Control and Prevention. “Mosquito Control: What You Need to Know About Bti” Supports the safety profile for people, pets, and aquatic life, plus recommended standing-water sites.
- Summit Chemical. “Mosquito Dunks Product Page” Confirms the Bti active ingredient and the separate labels for 2-, 6-, and 20-count packs.