Larvae live in stagnant water & adult females need blood to drink.
Mosquitoes multiply fastest in tropical and sub-tropical habitats, but they have managed to adapt to a wide variety of habitats. The eggs or larvae of some kinds can survive frozen in ice, while others hibernate as adults so they can live where snow covers the ground for months and the temperature falls below zero. Thanks to their adaptability, mosquitoes thrive on every continent except Antarctica.
Mosquitoes need a habitat with stagnant or slow-moving water, since that's where their eggs hatch and where their larvae live until they're old enough to transform into adults. They pass through the larval stage quickly, though, sometimes going from eggs to adults in a week or less, so they don't necessarily need permanent water sources. They can survive even in places where temporary puddles appear after a hard rain and dry up soon after. Warm summer weather encourages mosquitoes to go from egg to adult the fastest. "The shortest life cycle on record for a mosquito is about 4.5 days," according to "Mosquito Breeding Habitats," by Dr. David N. Gaines, Public Health Entomologist.
Larvae have difficulty living in fast-moving water, though some kinds can survive in woodland creeks. "Stream breeders will find vegetation along banks with which to anchor themselves or attempt to remain away from the main flower of the stream by seeking isolated eddies," according to Larval Habitats of Mosquitoes, posted by the Rutgers Center for Vector Biology.
Besides water, the other thing that almost all female mosquitoes need in their habitat is a source of blood. The males survive on nectar and other non-living food resources, but the females of most species use the nutrition in other creatures' blood to develop their eggs, so they need a breeding site close enough to humans, livestock, wild animals or birds so they can get nourishment.
Though each species of mosquitoes may prefer a slightly different habitat, almost all kinds of still or slow-moving water will be attractive to at least some kind of mosquito, unless humans take action to discourage them.
The species Aedes sollicitans prefers stagnant salt marshes along the ocean for its eggs and larvae, but will fly 10 miles or more inland to find animals to bite. Culex salinarius will also breed in salt water marshes, or even roadside ditches filled with cattails.
Anopheles quadrimaculatus, a common mosquito in the United States which used to spread malaria, is more particular about where it lays its eggs. It prefers clear, shallow water, that's not too acid or stagnant, but it has found plenty of suitable habitat, wherever there are ponds, slow-flowing streams or the kind of clean, attractive bodies of water with shallow edges, that humans like to live near.
Aedes aegypti, the mosquito which used to spread yellow fever in the United States and is still common here, will breed in cities, needing only the standing water in old tires, barrels or gutters. Aedes albopictus, the invasive Asian tiger mosquito, can live in a similar habitat. Introduced to the United States in the 1980s, it has spread over most of the southeast and is taking over some of the yellow fever mosquito's habitat. Both can find blood nearby, from humans, birds and pets.
Some mosquitoes prefer flooded rice fields, Florida swamps, irrigation ditches out west or northern forests, but all their habitats contain some kind of water and some kind of food--usually the kind of food that doesn't want to be eaten, like us and our animals.
Changing Habitat to Get Rid of Mosquitoes
One way to have fewer mosquitoes is to get rid of standing water, so their larvae can't survive. Some forms of standing water are unattractive to humans, so removing discarded tires, old cans and other trash that holds rainwater beautifies the landscape as well as discouraging the little pests.
Other forms of standing water are a necessary part of human activity, such as drainage ditches and culverts for roads and fields, watering tanks for domestic livestock, or the puddles caused by farm irrigation. Natural wetlands are important habitats for useful and necessary species, and even dry woodland contains holes in trees or low spots that collect water or melting snow. Humans even create some kinds of standing water to beautify the landscape, by building ornamental ponds or installing birdbaths. Mosquitoes consider these perfect breeding grounds. In those cases, other ways of getting rid of mosquitoes may be necessary.
Sometimes slightly altering the habitat will discourage them. They don't like deep ponds or ponds that also have a flourishing population of predators that like to eat mosquito larvae, such as frogs, fish and insects and spiders. Other options are trapping and killing the adults, or using dunks or mosquito bits that are easy to add to water.
They spread a bacillus which is a natural enemy of the larvae but won't harm fish, birds, people or pets. When mosquito larvae swallow Bacillus thuringiensis israelensis, it damages their mid-gut cells, but is harmless to almost all other species except for a few closely related flies, according to the article "Mosquitoes" by Bruce Eldridge, former director of the UC Mosquito Research Program. The article also contains other useful information on controlling mosquitoes by altering their habitat.