Scoop up some of your pond's water into a white cup or bowl. If you see any wriggling objects against the white surface, you have problems with mosquitoes.
Look at your pond. If there is any areas that has still water, your pond is a breeding ground for mosquitoes. This includes the surface of aquatic plants and water in the top of pots.
Agitating the water's surface with a fountain will prevent female mosquitoes from being able to lay their eggs.
Vegetation that hands over the edge of the pond can provide dark still water. Trim any vegetation that hangs over the edge and thin out any vegetation along the sides to allow the water to move freely through it.
Using Mosquito Eaters
One of the best methods for controlling mosquitoes in ponds is mosquito fish. Mosquito fish are native to the southern United States. Add between 35 to 100 fish to an ornamental pond. For larger ponds, add about 1000 per acre.
Mosquito fish can be aggressive toward other fish. In these cases, add minnows, guppies or small goldfish.
Keep in mind that an alternative to fish are tadpoles. Tadpoles eat mosquito larvae and grow up to become mosquito-eating toads or frogs.
Try to encourage a few toads into your garden. One toad will eat about 100 mosquitoes each night.
Periodically raise and lower the level of your ponds water to ensure that the fish or tadpoles have access to all areas of your pond.
Try adding Bti (Bacillus thuringiensis), a naturally occurring type of bacteria that kills mosquito larvae and some fly larvae. Bti is not harmful to fish, pets, wildlife or humans.
Methroprene can control late-stage mosquito larvae, but may be damaging to frogs and other wildlife. Don't use this in your pond.
Oil is an older remedy for controlling mosquitoes because it puts a film on the water that prevents the larvae from breathing. However, it also damages plants and beneficial aquatic insects. Some mosquito control agencies have access to approved oil that is used for this purpose. Contact them if none of the above steps have helped. Cooking and vegetable oil do not work.