The extrinsic incubation rate (inverse of the time in days from infection to median transmission) of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses by laboratory strains of Culex tarsalis Coquillett increased as a linear function of incubation temperatures from 10 to 30 degrees C. The estimated temperatures for zero transmission thresholds (intercept of the X axis) were 10.9 and 14.9 degrees C, and the number of degree days above these thresholds required for median transmission (inverse of the slope) was 67.6 and 115.2, respectively. Although the bodies of most Cx. tarsalis females remained infected and the WEE viral titer did not decrease significantly throughout the incubation periods at all temperatures, transmission rates by females incubated at 20 to 30 degrees C decreased markedly after peaking at 7-10 d after infection. In contrast, midgut escape and salivary gland infection barriers limited the transmission rates of SLE virus at all temperatures, but these rates did not decrease markedly as a function of incubation time, indicating that virus modulation did not occur. Degree-day models were used to calculate monthly changes in the duration of the extrinsic incubation period for WEE and SLE viruses in the San Joaquin and Coachella valleys based on mosquito temperatures estimated by combining nocturnal air and diurnal resting site temperatures. Temperatures in the San Joaquin Valley averaged 5 degrees C cooler than in the Coachella Valley, proportionately shortening the duration of the potential transmission season for WEE virus from 10 to 8 mo and for SLE virus from 8 to 5 mo, respectively.

The temporal and spatial abundance, dispersal, survivorship, and density of Culex tarsalis Coquillett and Cx. quinquefasciatus Say populations were studied in riparian, agricultural, and residential habitats along the Kern River bed during the drought year of 1990. The temporal abundance of both species was related to cotton agricultural practices and peaked during intensive irrigation after cultivation was terminated in July. Cx. tarsalis peaked in abundance 2 wk earlier than Cx. quinquefasciatus, perhaps because of the advantage of autogenous oviposition, which shortened generation time. Although host-seeking females of both species were most abundant in the riparian habitat, more Cx. quinquefasciatus than Cx. tarsalis were collected in the residential habitat. Marked females released within the riparian habitat were recaptured most frequently within 1 km of the release point; however, flights as far as 12.6 km were documented within the 180-km2 study area. On average, marked Cx. quinquefasciatus dispersed farther and more rapidly than Cx. tarsalis and were recaptured more frequently within the residential habitat. Survivorship, estimated horizontally from the female recapture rate, ranged from 0.60 in May to 0.79 in July for Cx. tarsalis and from 0.74 in September to 0.84 in July for Cx. quinquefasciatus. The estimation of survivorship vertically from the parity rate was complicated by elevated autogeny rates in Cx. tarsalis and by the low parity rate in Cx. quinquefasciatus. Cx. tarsalis population density ranged from 125 females per km2 in May to 65,500 per km2 in August, and was well correlated with relative abundance. Collectively, these ecological data indicated that Cx. tarsalis may be important in disseminating arboviruses within the riparian habitat, but that Cx. quinquefasciatus may be important secondarily by disseminating virus from the riparian habitat to the adjacent residential habitat.

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Cohorts of Culex adults were marked uniquely with date- and site-specific fluorescent dust colors and were released at centrally located residences and at peripheral breeding sources to study population dispersal, size, additions, and deletions. The recapture rate of Cx. quinquefasciatus females was higher and the mean distance dispersed was lower in residential than in agricultural or park habitats. Dispersal was associated with host-seeking activity and ranged from 0.6 to 1.0 km/d. Survivorship ranged from 0.65 to 0.84 per day, and population density ranged from 36,612 to 671,634 females per km2. The sampling efficiency of CO2-baited traps in residential habitats increased coincidentally with increasing population density. Gravid traps were most effective in residential habitats where there were few competitive oviposition sites. Teneral Cx. stigmatosoma were extremely dispersive, and few marked females were recaptured. Unmarked females were more abundant at CO2-baited traps in residential habitats than at traps near productive peripheral breeding sources. Few Cx. tarsalis were released, and the recapture rate in residential habitats was low when compared with rural sites.

Mosquito abundance and bionomics were studied intensively during summer and spring at two residential communities of contrasting economic status. Culex quinquefasciatus was the most abundant adult and immature mosquito collected in both communities, followed by Culiseta incidens, Culex stigmatosoma, and Culex tarsalis. Cx. stigmatosoma and Cx. tarsalis were more abundant in CO2 traps hung in tree canopy than at ground level and fed most frequently on birds. Cx. quinquefasciatus was abundant in both ground level and tree canopy CO2 traps and fed on both mammals and birds. Cs. incidens was collected most frequently by ground level CO2 traps and fed primarily on dogs. Cx. quinquefasciatus and Cs. incidens readily exploited peridomestic breeding sources, and resting adults were aggregated at houses with positive breeding sources. Although Cx. stigmatosoma and Cx. tarsalis larvae were collected primarily at peripheral breeding sources, the dispersion of resting adults was still clumped at houses within both communities. Mosquitoes were most abundant in the more affluent community due to an increased number of breeding sites created by automatic watering devices and poorly managed peripheral drainage channels. Resident opinion of recent mosquito annoyance was not related to the presence of mosquito breeding sources or the abundance of either resting or host-seeking mosquitoes.

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Estrogen treatment in symptomatic postmenopausal women appears to improve cognitive performance including memory, an effect which may involve enhanced nitric oxide formation in hippocampal neurons. To study whether 17beta-estradiol (E2) affects NO synthase activity in the hippocampus, we investigated the influence of E2 on hippocampal NO synthase expression and activity in female rats. Ovariectomy, which significantly decreased E2 serum levels, reduced neuronal (nNOS) and endothelial NO synthase (eNOS) expression and Ca(2+)-dependent NOS activity. E2 substitution reversed these effects. It is concluded that E2 increases nNOS and eNOS expression and activity in female hippocampus and thus improves hippocampal function.