All mosquitoes have four stages of development-egg, larva, pupa, and adult. Mosquitoes spend their larval and pupal stages in water. The females of some mosquito species deposit eggs on moist surfaces, such as mud or fallen leaves that may be near water but dry. Later, rain re-floods these surfaces and stimulates the eggs to hatch into larvae. The females of other species deposit their eggs directly on the surface of still water in such places as ditches, street catch basins, tire tracks, streams that are drying up, and fields or excavations that hold water for some time. This water is often stagnant and may be close to homes in discarded tires, ornamental pools, unused wading and swimming pools, tin cans, bird baths, plant saucers, and even gutters and flat roofs.
A few important spring species have only one generation per year. However, most species have many generations per year, and their rapid increase in number becomes a problem.