The western encephalitis mosquito, Culex tarsalis, is the most important indigenous species in the valley and the primary vector of St. Louis encephalitis (SLEV), western equine encephalomyelitis (WEE) and West Nile virus (WNV). This mosquito breeds year-round with two distinct peaks: In March/April and in late September/October. Cx. tarsalis prefers to feed on birds, but readily attacks people, horses, and cattle.