BIONOMICS OF CULEX-TARSALIS (DIPTERA, CULICIDAE) IN RELATION TO ARBOVIRUS TRANSMISSION IN SOUTHEASTERN CALIFORNIA

Population dynamics and bionomics of host-seeking Culex tarsalis Coqui llett were studied in the imperial and Coachella valleys of California during periods in 1991 and 1992 when western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were transmitted to se ntinel chickens. Female abundance was greatest during the spring and f all, before and after most virus transmission occurred and was not cor related with temperature, humidity, or rainfall. Parity rates were hig hest during late summer when virus activity peaked and were lowest dur ing December when females may enter a short-term reproductive diapause . Although most likely underestimated, the proportion of order multipa rous females were collected at a consistent, but low level throughout the year. Changes in the parity rate seemed to be influenced primarily by the proportions of 1-parous females. Survivorship estimated from t he parity rate (adjusted to account for autogeny) was highest in winte r; however, the proportion of females surviving to potentially transmi t either WEE or SLE virus was highest in summer and early fall. Wing l ength decreased in summer as an inverse correlate of temperature and i ncreased as a function bf female age, implying that larger females liv ed longest. However, autogenous females were larger than anautogenous females at emergence and only parous autogenous females were collected host seeking, thereby confounding the relationship between size and a ge. The proportion of females testing positive for fructose was greate st during winter and lowest during summer, perhaps affecting survivors hip and blood-feeding avidity. The vector competence (infection, disse mination and transmission rates, and ID50) of females collected host s eeking or emerging from field-collected pupae for WEE or SLE viruses r emained similar over time, even though the wing length of females used in these experiments differed among samples. We conclude that in natu re virus transmission progressed efficiently during midsummer because elevated temperatures shortened the extrinsic incubation period withou t markedly decreasing survivorship resulting in an increased proportio n of females surviving extrinsic incubation to become infective.