Does Variation in Culex (Diptera: Culicidae) Vector Competence Enable Outbreaks of West Nile Virus in California

Abstract Since the invasion of California by West Nile virus (family Flaviviridae, genus Flavivirus, WNV) in 2003, we have annually monitored vector competence for the NY99 strain in Culex tarsalis Coquillett, Culex pipiens quinquefasciatus Say, Culex p. pipiens L., and Culex stigmatosoma Dyar populations from four areas: deserts of Coachella Valley, densely urbanized maritime Los Angeles, southern San Joaquin Valley in Kern County, and southern Sacramento Valley near Davis in Sacramento County. Overall, Cx. stigmatosoma was the most competent vector species, followed by Cx. tarsalis and the Cx. pipiens complex. The median infectious dose (ID50) of WNV required to infect 50% of the F1 female progeny reared from wild-caught females, a measure of mesenteronal susceptibility, ranged between 5 and 8 log10 plaque forming units/ml and was not correlated with annual human case incidence or summer maximum likelihood mosquito infection estimates. Odds ratios comparing nonoutbreak years with referent outbreak years...