The encephalitis vector surveillance (EVS) traps/Carbon Dioxide (CO2) Traps use dry ice (CO2) as a bait to attract female mosquitoes looking for an animal or human to bite. When searching for a source of a blood meal, mosquitoes sense where CO2 is being expelled by a living creature. Since dry ice evaporates into CO2 gas, it tricks the mosquito into thinking there is someone to bite; drawing them into the trap which uses a fan to suck them into netting collected the following day after placement. EVS/CO2 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 disease testing.