Mosquito Dunks are a form of Bacillus thuringiensis v. israelensis in a solid form. As they float in the water where you put them they slowly release the BT on the water surface. The BT then gradually settles to the bottom where the mosquito larva eat it and die. Mosquito Dunks are 2 inches in diameter and 1/2 inch thick, with a small hole in the center. The Dunks float on top of the water and will be effective for approximately 30 days. While floating, they slowly release the larvicide that settles in the water where it is eaten by mosquito larvae. Alternate wetting and drying will not reduce their effectiveness. The center hole can be used to anchor the dunks in place with a string or stake. Recommended application rates are 1/4 dunk for 1-5 square feet of water surface area and one dunk for 25-100 square feet of water surface area regardless of the water depth.
The mosquito dunk treatment has no toxic effect to fish or waterfowl and are EPA registered.
http://www.epa.gov/pesticides/biopesticides/ingredients/factsheets/factsheet_006476.htm
