The Female Culex Pipiens are the primary source for West Nile being spread.  The Culex is a light brown colour and it feeds primarily on birds.  The Culex will also feed on other mammals which makes other hosts.  The Culex feed times and flight activity is mostly in darkness.  High Levels occur at dawn and dusk.  During the day they reside in dark, damp areas (ie: storm sewers and culverts).  They breed in polluted water which are high in organic content.  Their main breeding areas are sewage treatment plants. Controlling the Culex with Malathion is a solution of last resort.  In Las Angeles Malathion was being used and the mosquitoes built up a resistance to it.  It was documented that Malathion increased the mosquitoes aggressiveness and so they bit more people.