Reproduced from Wing Beats of the AMCA, the official publication of the Florida Mosquito Control Association. Please use the following citation when referring to this article:
Reisen, W. 1993. The western encephalitis mosquito, Culex tarsalis. Wing Beats, Vol. 4(2):16.
DR. WILLIAM REISEN
Culex tarsalis is the most important mosquito vector of arboviruses in western North America, responsible for maintenance, amplification and epidemic transmission of St. Louis and western equine encephalitis viruses in irrigated and riparian habitats. This species is also a vector of Llano Seco, Turlock, Gay Lodge, and Hart Park viruses, and several species of avian malaria. Cx. tarsalis is an efficient experimental vector of Japanese and Venezuelan equine encephalitis viruses.
Range extends from northern Mexico and Baja California north to southern Canada, and from the Pacific east to the southern Atlantic coast. Although an abundant summer species in the mid and far west, Cx. tarsalis is relatively rare east of the Mississippi River. This species is active during winter in southern California.
Cx. tarsalis is separable from other North American Culex by its white median band on the proboscis, white bands overlapping the tarsal joints, white longitudinal stripes on the femoral and tibial segments of the middle legs, and dark 'chevrons' on the ventral aspects of the abdominal segments. Rubbed specimens may be recognized by white scaling at the antennal base.
Rafts averaging about 190 eggs are oviposited in newly-created sunlit surface water pools that are frequently surrounded by grasses and annual vegetation. Larvae tolerate a wide range of water conditions and may be abundant in agricultural tailwater, alkaline lake beds, fresh and saline wetlands, secondary treated sewage effluent and oil field run-off. Permanent water with fixed depth rarely supports abundant populations unless intermittently perturbated. Excessive organic pollution is not tolerated.
Larval habitats frequently are shared with Culiseta inornata, Culex quinquefasciatus, Cx. pipiens and Cx. stigmatosoma; other species include Culex erythrothorax, Cx. restuans and several species of Aedes and Anopheles.
Cx. tarsalis are among the first colonizers of newly-created surface pools and thus exploit microfloral blooms produced by the release of nutrients from decomposing vegetation. Larval development ranges from 7 days to <4 weeks and progresses as a curvilinear function of water temperature and food availability. Larval survivorship is typically <5%, with most losses attributable to predation.
Some females mature their initial egg batch without a blood meal and oviposit 4-5 days after emergence. The frequency of this trait is dependent upon temperature, photoperiod and nutrition and affects the vectorial capacity of a population. At northern latitudes, females overwinter in facultative diapause as inseminated nullipars (never developed eggs) that require a blood meal to produce their initial eggs in the spring.
In spring, when population abundance is low, most females feed on birds shortly after sunset. During late summer when abundance is high, bird mosquito-avoidance behavior diverts many females to feed on mammals including rabbits, horses, cattle alnd man. This host shift may be important in virus transmission to horses and man. Dispersal is primarily during host-seeking flights (up to 17 miles) which average about l00 yards a day from breeding sites in riparian and agricultural habitats.
Temporal abundance patterns vary from summer-active in the north to winter-active in the south. Northern populations overwinter in facultative diapause, whereas southern populations remain gonotrophically active throughout winter with intermittent inactivity during cold periods. At intermediate latitudes, populations remain vagile, imbibe sugar throughout winter, but undergo ovarian diapause.
Dr. Reisen is located at the Arborvirus Field Station in Bakersfield, California.