Bti (Bacillus thuringiensis israelensis)

Dunks are a backup to dumping water

In summer mosquitoes can go from egg to adult in a week or less, so draining anything that holds water comes first; Bti is for water you can't drain, such as ponds, rain barrels and tree holes.^[4][1]

Where it is used

Dunks float and release Bti for up to 30 days; break them into pieces for small containers.^[1]

Dunks can be placed on dry low spots that flood after rain, so they start working when water collects.^[1]

Larvae that hatch after treatment may partly develop before dying; allow at least 48 hours to see control.^[1]

Indoors, the label allows use where water collects from time to time, such as flooding basements and sump pumps.^[1]

Products and registrations

Label rates

How to apply it, step by step

Steps for Summit Mosquito Dunks (DUNKS, Bacillus thuringiensis subsp. israelensis), EPA Reg. No. 6218-87, taken from its label^[1]. Other products with this active ingredient have their own labels; follow yours.

1. Protective gear

• The label lists no protective gear: it carries a CAUTION because it is harmful if inhaled and causes moderate eye irritation, so avoid breathing the dust, keep it out of your eyes and off your clothing, and wash with soap and water after handling.^[1]

2. Before you spray

• Look for standing water near the house where mosquitoes breed: bird baths, rain barrels, roof gutters, flower pots, tree holes, old tires, ponds and water gardens, animal troughs, ditches and unused pools.^[1]
• Don't put it in treated, finished drinking water meant for people; the label says it can be used where fish live.^[1]
• In New York State, the label limits use to containerized water with no outflow to the natural environment.^[1]

3. Mixing

• No mixing: a dunk floats on the water and releases the Bti slowly; break it into pieces for small containers.^[1]

4. Applying

• Use ¼ dunk for up to 25 sq ft of water surface, ½ dunk for over 25 to 50 sq ft, ¾ dunk for over 50 to 75 sq ft and 1 dunk for over 75 to 100 sq ft; above that, 1 dunk per 100 sq ft.^[1]
• Where water could wash it away, tie a string through the center hole or stake it in place.^[1]
• You can also place it ahead of time on dry spots that flood after rain (an empty pool, a low spot); it floats up and starts working when water arrives, and wetting and drying don't reduce its effect.^[1]
• Indoors it can go in places that collect water from time to time, such as flooding basements, sump pumps and building drainage areas.^[1]

5. After you spray

• A dunk releases effective levels of Bti for about 30 days under typical conditions.^[1]
• Larvae that hatch after you put it out may partly develop before dying; allow at least 48 hours for control.^[1]

First aid

• If inhaled, move the person to fresh air and call a poison control center or doctor for advice; if they are not breathing, call 911.^[1]
• In the eyes, hold them open and rinse slowly and gently with water for 15 to 20 minutes, removing contact lenses after the first 5 minutes, and call a poison control center or doctor; the label lists 1-800-222-1222.^[1]

Storage, spills and disposal

• Store dunks in their closed, labeled container in a cool, dry, well-ventilated place children and pets can't reach.^[1]
• Put the empty package in the trash or recycling; for leftover product, call your local solid waste agency, and never put it down a drain.^[1]

From the safety data sheet

From the product's safety data sheet^[5]. Where it and the label differ, the label is the law.

• Dunks are light-brown solid briquets with a mild odor; the only listed hazard is eye irritation.^[5]
• Keep them in the original labeled container in a cool, well-ventilated place away from children, strong acids and bases, and sources of ignition.^[5]
• Keep spilled product out of sewers and public waters.^[5]
• If dust gets in the eyes, rinse with plenty of water and get medical attention if pain, blinking or redness persists.^[5]

Need the amount for your sprayer? Use the mixing calculator, which works from the same label rates.

Safety and environmental hazards

People

Signal word CAUTION: avoid breathing the dust; it causes moderate eye irritation. Wash with soap and water after handling.^[1]

Pets

The label allows use in animal watering troughs and bird baths.^[1]

Bees and pollinators

Bt toxins target specific insects; NPIC lists Bt israelensis as controlling immature mosquitoes, flies and gnats.^[3]

Fish and water

The label says it can be used in fish habitats, but not in treated, finished drinking water meant for people.^[1]

Common questions

How long does a Mosquito Dunk last?

The current label says each dunk releases effective levels of Bti for about 30 days under typical conditions.^[1]

Are Mosquito Dunks safe for fish and birds?

The label states they can be used in fish habitats, bird baths and animal watering troughs; just don't put them in drinking water meant for people.^[1]

How much of a dunk do I need for a bird bath?

For up to 25 square feet of water surface, the current label calls for a quarter of a dunk.^[1]

Sources

1. U.S. EPA Pesticide Product Label System, “Summit Mosquito Dunks (DUNKS) label, EPA Reg. No. 6218-87 (EPA-stamped, Feb. 11, 2026).” www3.epa.gov/pesticides/chem_search/ppls/006218-00087-20260211.pdf (accessed 2026-10-16)
2. Insecticide Resistance Action Committee, “IRAC Mode of Action Classification Scheme, Version 11.5 (Feb. 2026).” irac-online.org/documents/moa-classification/ (accessed 2026-10-06)
3. National Pesticide Information Center, “Bacillus thuringiensis General Fact Sheet.” npic.orst.edu/factsheets/btgen.html (accessed 2026-10-06)
4. University of California Statewide IPM Program, “Pest Notes: Mosquitoes, Publication 7451 (revised April 2020).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnmosquitoes.pdf (accessed 2026-10-06)
5. Summit Chemical Company, “Dunks / Summit B.t.i. Mosquito Briquets / Mosquito Dunk Chunks Safety Data Sheet (Summit Chemical Co., posted October 2026).” summitchemical.com/wp-content/uploads/2026/10/6218-87-SDS-Mosquito-Dunks_Dunk-Chunks-1.pdf (accessed 2026-10-16)