SEASONAL-VARIATION IN THE VECTOR COMPETENCE OF CULEX TARSALIS (DIPTERA, CULICIDAE) FROM THE COACHELLA VALLEY OF CALIFORNIA FOR WESTERN EQUINE ENCEPHALOMYELITIS AND ST-LOUIS ENCEPHALITIS VIRUSES

The vector competence of Culex tarsalis Coquillett from the Coachella Valley of California for western equine encephalomyelitis (WEE) and St . Louis encephalitis (SLE) viruses was monitored monthly from February to November 1993. The concentration of WEE virus required to infect 5 0% of the females increased during summer coincidentally with ambient temperature and was highest during July. Transmission rates of WEE vir us were high during March, low during May-June, and high again during July-September. Females expressed both mesenteronal escape and salivar y gland barriers limiting WEE virus dissemination and transmission rat es, respectively. SLE virus infection and dissemination rates did not vary among months, but transmission rates were highest during July-Sep tember Although infection rates with SLE virus were moderate, most inf ected females developed disseminated infections. Salivary gland infect ion or escape barriers prevented SLE virus transmission in 16-100% of infected females.