The encephalitis vector surveillance (EVS) traps use dry ice (CO2) as a bait to attract host-seeking female mosquitoes.  EVS traps are used mainly to determine whether WNV (West Nile virus),  SLE (St. Louis Encephalitis), WEE (Western Equine Encephalitis) or occurs within a localized area.  During mosquito season, traps are set overnight once a week at variable locations throughout the District to capture mosquitoes.  These live mosquitoes are identified, counted and then sent to the West Valley Mosquito and Vector Control District for testing.