Some of the most effective biological enemies of mosquitoes that you can buy, interfere with the growth of mosquito larvae. Methoprene (sold commercially as Altosid) is one of the top two choices to purchase for adding to water. The other is a naturally found bacillus that targets mosquitoes, called Bti.
Methoprene is a growth regulator which prevents the larvae of mosquitoes and other insects from becoming adults, by imitating insects' natural juvenile hormone. One of its common uses is in the product Altosid, which you can buy to add to water where mosquito larvae live. Because methoprene affects other insects but is safe for humans and mammals, it's also formulated and sold for controlling a variety of other pests, from flies on cattle farms to fleas on dogs.
The World Health Organization even recommends adding methoprene to large storage containers of human drinking water, in countries plagued by mosquito-spread diseases. They consider it so safe that humans can actually drink water treated with it, based on a risk assessment in 2007.
The commercial formulation of it, Altosid, is an effective, natural control for mosquito larvae, and is useful for bird baths, animal watering troughs, cisterns and any area where mosquito live, but where fish, frogs, crayfish, and other crustaceans don't.
The name "Altosid" is a combination of "Palo Alto," the location of the company that created it, and "John Siddall," head of the chemistry group who developed it, according to Carl Djerassi in his book, The Pill, Pygmy Chimps, and Degas' Horse.
In 1975, the Environmental Protection Agency first registered methoprene as a regular chemical pesticide, but later changed its classification to a biochemical pesticide. The EPA originally found its toxicity to be low and reported that it "poses little risk to people and other nontarget species, with one exception."
That exception was an important one. The EPA stated it is "highly acutely toxic to estuarine invertebrates," such as crayfish, shrimp and crabs. Also, for freshwater fish, it is "moderately toxic" to warm water species and "slightly toxic" to cold water species. It is also poisonous to amphibians, such as frogs, salamanders and toads. Though it degrades rapidly in sunlight, the slow-release formulas, designed to control mosquito larvae, are also the most dangerous to other invertebrates and fish in the water, since they're active over a long period of time.
In 1996, after further studies, the EPA concluded that the actual use of methoprene as a mosquito control didn't spread enough of it to harm other species, and recommended that the restriction "do not use in fish-bearing waters" be removed from the label, and instead, the warning should read, "this product is toxic to aquatic dipteran (mosquitoes) and chironomid (midge) larvae"--though of course its toxicity to mosquito larvae is the whole point.
Methoprene remains controversial, as it has been accused of causing frog deformities and lobster die-offs. Groups have attempted to get court orders, to stop widespread spraying of marshes. 
Using Altosid to kill mosquito larvae
You can see a sample label of Altosid Pro-G here, in a pdf file. It contains 1.5 percent S-methoprene.
For killing mosquito larvae, 1/2 tsp will treat 25 square feet of surface area on water that's less than 2 feet deep, or about 100 gallons of water. It continues to release methoprene for up to 30 days after being applied, so treatment should be repeated at least once a month during mosquito season. It can be applied at any time mosquito larvae are present, or in the spring before eggs hatch. It's a gritty powder, which you can measure out with a teaspoon, tablespoon or cup measure (there's a chart showing how much to use for different areas) and sprinkle over the surface of the water.
Don't be disappointed if the larvae are still happily wriggling the next day! Altosid doesn't work by killing them directly, so they continue to live as larvae until they're eaten by predators or die naturally, but after their water is treated, they'll never turn into adult mosquitoes.
The EPA's current fact sheet on methoprene is available here. The Altosid label recommends the usual precautious of wearing long clothing and gloves and avoiding getting the product on your skin or in your eyes.
If the area you need to treat is free from fish, frogs or crustaceans, Altosid is an excellent choice. Just sprinkle it over the area once a month during mosquito season to prevent larvae from turning into biting adults.
If you want to treat a fish pond, natural wetlands, or similar areas where a variety of species live, and you want to avoid any chance of harming other wildlife, another natural biological treatment, Bti is a better choice, since it's harmless to fish, frogs or crustaceans, and is just as safe as methoprene for humans and other animals as well.
 "Mosquito Arrives, Its Enemies Divided" by John Rather, New York Times, June 17, 2007. "Court Battles Over Spraying and Runoff Return with Mosquito Season," by Aaron Boyd, www.Hamptons.com
Altosid, Ovitrol and Precor 2000 photos courtesy of Amazon Services LLC Associates Program. Pony photo from Stock.Xchng. Larvae photo from the CDC.