Mosquito dunks, also called larvicides, are tablets you drop in standing water - such as fish ponds, rain barrels, water fountains and bird baths - to kill mosquito larvae. The main source of mosquito-killing bacteria in mosquito dunks is called Bacillus Thuringienis (or BTI). Mosquito dunks with BTI contain no poisonous chemicals and are completely harmless to birds and fish. One mosquito dunk tablet can treat 100 square feet of standing water against mosquito larvae for approximately one month. A moderate to heavy dose of mosquito dunks with BTI will reduce the mosquito population by one half in 15 minutes, and the rest within an hour. Mosquito dunks can be purchased in packages of 12 for about $10 to $25 online.