Mosquito Dunks kill mosquito larvae before they can become disease-carrying adults. The active ingredient in Mosquito Dunks is B.t.i. (Bacillus thuringiensis israelensis) a bacterium that is toxic specifically to mosquito larvae but not harmful to fish, birds, pets, wildlife, people or beneficial insects. Mosquito Dunks have been approved by the EPA for use in fish habitats, and the dunks are approved for organic use.