Mosquito Dunks - 20 ea. Bacillus thuringiensis v. israelensis is a bacteria which is specific to mosquito and blackfly larvae. Mosquito Dunks contain B.t.i. in briquette form which floats and slow releases for 30 days in a water environment. Use 1 dunk per 25 to 100 sq. ft. of surface area of standing water. Can be broken in pieces for smaller areas. Product = 20 briquettes.