Spatial variation in host feeding patterns of Culex tarsalis and the Culex pipiens complex (Diptera: Culicidae) in California.

West Nile virus (family Flaviviridae, genus Flavivirus, WNV) is now endemic in California across a variety of ecological regions that support a wide diversity of potential avian and mammalian host species. Because different avian hosts have varying competence for WNV, determining the blood-feeding patterns of Culex (Diptera: Culicidae) vectors is a key component in understanding the maintenance and amplification of the virus as well as tangential transmission to humans and horses. We investigated the blood-feeding patterns of Culex tarsalis Coquillett and members of the Culex pipiens L. complex from southern to northern California. Nearly 100 different host species were identified from 1,487 bloodmeals, by using the mitochondrial gene cytochrome c oxidase I (COI). Cx. tarsalis fed on a higher diversity of hosts and more frequently on nonhuman mammals than did the Cx. pipiens complex. Several WNV-competent host species, including house finch and house sparrow, were common bloodmeal sources for both vector species across several biomes and could account for WNV maintenance and amplification in these areas. Highly competent American crow, western scrub-jay and yellow-billed magpie also were fed upon often when available and are likely important as amplifying hosts for WNV in some areas. Neither species fed frequently on humans (Cx. pipiens complex [0.4%], Cx. tarsalis [0.2%]), but with high abundance, both species could serve as both enzootic and bridge vectors for WNV.

Deposition of pyrethrins and piperonyl butoxide following aerial ultra-low volume applications in the Coachella Valley, California.

Data on adulticide deposition were collected during studies optimizing aerial ultra-low volume applications and droplet size in the desert environment of the Coachella Valley, Riverside County, California. Pyrenone 25-5 and BVA Spray 13 oil were applied by a single-engine, fixed wing aircraft equipped with 2 Micronair AU5000 atomizers. Data recorded by a portable weather station documented that weather conditions were suitable for application. Adulticide residue was collected using 24-cm-diameter filter papers positioned along 2-3 transects, with 3 positive controls held outside of the treated zone. The trace amounts of 2 major insecticidal components (pyrethrin I and II) and the synergist piperonyl butoxide (PBO) were detected from samples near the center of the spray zone by high-performance liquid chromatography (HPLC); pyrethrin deposition was highest at the center, 156 microg/m2, and it was not detectable 60 m beyond the center of the transect, whereas PBO deposition was 5,000 microg/m2 at the center but was not detectable beyond 150 m. Droplet diameters on spinning Teflon slides were larger than expected for the rated output of the atomizers. For these single swath trials, the lack of swath overlap due to drift resulted in low mortality in sentinel mosquitoes. Detection of residues was limited to the centroid of droplet densities on spinning glass slides and with mortality among sentinel mosquitoes, indicating HPLC may be useful in detecting postspray residues.

Persistence and amplification of St. Louis encephalitis virus in the Coachella Valley of California, 2000-2001.

The introduction of a St. Louis encephalitis virus (SLE) genotype new to southeastern California during 2000 was followed by focal enzootic amplification in the Coachella Valley that was detected by seroconversions of 29 sentinel chickens in five of nine flocks of 10 chickens each, isolations of virus from 30 of 538 pools of 50 Culex tarsalis Coquillett females, and collection of 30 positive sera from 2,205 wild birds. This SLE strain over wintered successfully and then amplified during the summer of 2001, with 47 sentinel seroconversions in eight of nine flocks, 70 virus isolations from 719 pools of Cx. tarsalis and Cx. p. quinquefasciatus Say, and 40 positive sera from 847 wild birds. Human illness was not detected by passive case surveillance, despite issuance of a health alert during 2001. Virus amplification during both years was associated with above average temperatures conducive for extrinsic incubation and below average precipitation during spring associated with below average vector abundance. Seroconversions by sentinel chickens provided the timely detection of virus activity, with initial conversions detected before positive mosquito pools or wild bird infections. Vertical infection was not detected among Cx. tarsalis adults reared from immatures collected during the fall-winter of 2000, even though SLE over wintered successfully in this area. Early seroconversions by a sentinel chicken during February 2001 and a recaptured Gambel's quail in April 2001 provided evidence for transmission during winter and spring when ambient temperatures averaged below 17 degrees C, the threshold for SLE replication.

Landscape affects the host-seeking patterns of Culex tarsalis (Diptera: Culicidae) in the Coachella Valley of California.

Effective arbovirus transmission requires that the principal vertebrate hosts and vectors have frequent contact. Vegetation and other landscape features used by roosting or nesting birds at night dictate their exposure to nocturnally active host-seeking Culex tarsalis Coquillett and therefore to western equine encephalomyelitis and St. Louis encephalitis viruses. Precipitin tests on 645 Cx. tarsalis that were collected resting and host-seeking near the Salton Sea in Coachella Valley, CA, indicated that passeriform birds (64%) and rabbits (25%) were the most frequent bloodmeal hosts and that the percentage of females feeding on birds varied temporally as an inverse fuction of mosquito abundance. Blood meals were not taken from communally roosting water birds. The spatial distribution of host-seeking females then was investigated by deploying dry ice baited traps within seven sites representative of habitats found along the Salton Sea. Mosquito catch was greatest at traps within elevated vegetation such as Tamarisk, mesquite, cattails, and orchards and lowest at traps positioned at snags over water, sand bars, open fields, and within housing in a small rural community. These data indicate that host-seeking Cx. tarsalis females congregated at specific landscape features that were not necessarily associated with large concentrations of potential bloodmeal hosts.

Patterns of avian seroprevalence to western equine encephalomyelitis and Saint Louis encephalitis viruses in California, USA.

Temporal and spatial changes in the enzootic activity of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were monitored at representative wetland study sites in the Coachella, San Joaquin, and Sacramento valleys of California from 1996 to 1998 using three methods: (1) virus isolation from pools of 50 host-seeking Culex tarsalis Coquillett females, (2) seroconversions in flocks of 10 sentinel chickens, and (3) seroprevalence in wild birds collected by mist nets and grain baited traps. Overall, 74 WEE and one SLE isolates were obtained from 222,455 Cx. tarsalis females tested in 4,988 pools. In addition, 133 and 40 seroconversions were detected in 28 chicken flocks, and 143 and 27 of 20,192 sera tested from 149 species of wild birds were positive for antibodies to WEE and SLE, respectively. WEE was active in all three valleys, whereas SLE only was detected in Coachella Valley. Seroconversions in sentinel chickens provided the most sensitive indication of enzootic activity and were correlated with seroprevalence rates in wild birds. Avian seroprevalence rates did not provide an early warning of pending enzootic activity in chickens, because positive sera from after hatching year birds collected during spring most probably were the result of infections acquired during the previous season. Few seroconversions were detected among banded recaptured birds collected during spring and early summer. Age and resident status, but not sex, were significant risk factors for wild bird infection, with the highest seroprevalence rates among after hatching year individuals of permanent resident species. Migrants (with the exception of mourning doves) and winter resident species rarely were positive. House finches, house sparrows, Gambel's quail, California quail, common ground doves, and mourning doves were most frequently positive for antibodies. The initial detection of enzootic activity each summer coincided closely with the appearance of hatching year birds of these species in our study areas, perhaps indicating their role in virus amplification. Bird species most frequently positive roosted or nested in elevated upland vegetation, sites where Cx. tarsalis host-seeking females hunt most frequently. These serosurveys provided important background information for planned host competence and chronic infection studies.

A geographical information system to manage mosquito and arbovirus surveillance and control data in the Coachella Valley of California.

A geographical information system was developed to monitor and analyze mosquito abundance and encephalitis virus activity in the Coachella Valley to facilitate mosquito control operations. Data layers include soil types, vegetation types, irrigation method, standpipes, larval occurrence, adult abundance, and viral transmission to sentinel chickens. Base maps are digitized aerial photographs, with data entry done through sections of the range/township system. The image resolution of a section is 100 ft2 (9.3 m2) per pixel. This system currently is operational and in use by the Coachella Valley Mosquito and Vector Control District for data management.

Effects of sampling design on the estimation of adult mosquito abundance.

During 1994-5, Culex tarsalis comprised 75% of the 902,643 adult female mosquitoes collected by 63 dry-ice-baited Centers for Disease Control (CDC)-style traps operated biweekly in a uniform sampling grid that covered the southern Coachella Valley, Riverside County, California. The ln(y + 1) transformation successfully controlled the variance and normalized the distribution of catch size among trap nights. When tested by analysis of variance, abundance varied significantly among months, years, and trap sites. Although the trap by months interaction was not significant, female distribution changed seasonally as larval habitats shifted from wetlands along the Salton Sea to agriculture to managed duck marshes. Conditional simulations utilized subsets of trap sites to compare sampling designs that required no (uniform, random, and transect designs) or prior (best-estimate and stratified random designs) knowledge of mosquito spatial distribution. All designs provided similar information on population seasonal trends, but a stratified random design provided the most accurate and precise simulation. A uniform trap grid that employed every 2nd trap site subsequently was adopted by the Coachella Valley Mosquito and Vector Control District to provide information on focal changes in abundance indicative of missed or newly created larval habitats or control failures.

Ecology of Aedes dorsalis (Diptera: Culicidae) in relation to western equine encephalomyelitis virus in the Coachella Valley of California.

The ecology of western equine encephalomyelitis virus (WEE) was studies during 1994-1996 along a portion of the north shore of the Salton Sea in Coachella Valley, California, known to support a focal Aedes dorsalis (Meigen) population. WEE was detected during 1995 by the seroconversion of sentinel chickens concurrently at sites within and outside of the area supporting Ae. dorsalis. WEE was not detected during 1994 or 1996; neither was WEE detected by seroconversion of sentinel rabbits nor by isolation from host-seeking females of either the primary vector, Culex tarsalis Coquillett (42,083 females tested in 913 pools), or Ae. dorsalis (10,804 females tested in 245 pools and 1,940 adults reared from field-collected immatures tested in 72 pools). Collectively, the results of this and previous investigations indicate that Ae. dorsalis may not be essential for the maintenance or amplification of WEE virus in southeastern California.

Time of host-seeking by Culex tarsalis (Diptera:Culicidae) in California.

Factors altering the pattern of Culex tarsalis Coquillett host-seeking activity were studied in Kern and Riverside counties of California using an automatic time-segregated sampler baited with bottled CO2 gas released at 0.5 or 1.0 liters/min. Host-seeking always commenced shortly after sunset and usually peaked during the succeeding 1-3 h, the hottest and driest time of the night. The time of maximal activity varied over time and space, because of increased mosquito abundance (presumably reduced blood feeding success), distance of the sampler from resting sites, adulticide applications, and perhaps weather.

Landscape ecology of arboviruses in southeastern California: temporal and spatial patterns of enzootic activity in Imperial Valley, 1991-1994.

Western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were detected in the Imperial Valley during the summers of 1991-1994 by isolation from the primary vector, Culex tarsalis Coquillett, and by the seroconversion of sentinel chickens. Enzootic transmission consistently was not detected first each year at sampling sites near specific landscape features such as a heron rookery and other riparian habitats along the New River, sites along the Mexican border, or saline and freshwater marshes along the southern shore of the Salton Sea. Despite mild winter temperatures and the elevated vernal abundance of Cx. tarsalis, WEE and SLE activity was not detected until June or July, indicating considerable amplification may be necessary before detection by testing mosquito pools for virus infection or sentinel chicken sera for antibodies. Results did not permit the spatial focusing of early season control efforts or research on mechanisms of virus interseasonal persistence.

Short-term reproductive diapause by Culex tarsalis (Diptera: Culicidae) in the Coachella Valley of California.

Culex tarsalis Coquillett from the Coachella Valley in southeastern California enter a short-term reproductive diapause that is terminated by changes in photoperiod at or shortly after the winter solstice. Reproductively active and unfed resting females exhibited respective increases and decreases in parity rates during December that were characteristic of diapause induction, although reproductively active females, larvae, and males were collected during every month throughout winter. Laboratory experiments demonstrated that females from Coachella Valley enter and maintain a facultative reproductive diapause similar to females from the more northern San Joaquin Valley when exposed to simulated winter conditions. In environmental chambers, diapause termination was not related to temperature accrual because females terminated diapause at approximately the same time regardless of temperature regimens. A field experiment showed that female cohorts emerging in the Coachella Valley from late October through December entered a reproductive diapause that was terminated synchronously by changing photoperiod in late December.

Population ecology and dispersal of Culex tarsalis (Diptera: Culicidae) in the Coachella Valley of California.

During 5 bimonthly experiments in 1993, 55,548 host-seeking and 22,563 newly emerged Culex tarsalis Coquillett females were marked with fluorescent dust and released at a marsh along the Salton Sea. Overall, 3,758 (6.7%) host-seeking and 37 (0.2%) newly emerged females were recaptured in dry ice-baited traps and walk-in red boxes operated for 7-12 consecutive days after release. The recapture of newly emerged females was unexpectedly low and insufficient for further analysis. The recapture rates and dispersiveness of females collected host-seeking within the study area were not different from females collected host-seeking at a site 16 km SE of the release site, indicating that Cx. tarsalis may not rely on memorized flight paths. Loss rates of host-seeking females were high, because many females readily dispersed from the marsh study area during host-seeking flights. Estimates of population size ranged from 914,000 in February to 4,000 in July and were well correlated with catch size in CO2 traps. The wing length and fructose positivity rate of released females did not vary as a function of age or dispersal distance at recapture. Parity of released females increased over time, but nullipars were collected during all recapture periods, perhaps indicating difficulty in blood meal acquisition. Cohort dispersal progressed at the rate of about 0.2 km/d and was sufficient to disseminate arboviruses in the southern Coachella Valley.

Landscape ecology of arboviruses in southern California: temporal and spatial patterns of vector and virus activity in Coachella Valley, 1990-1992.

Consistent temporal and spatial patterns in the activity of Culex tarsalis Coquillett and western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were delineated that were useful in developing a stratified surveillance program. Vernal increases in Cx. tarsalis abundance typically were associated with flooding of saline marshes along the north shore of the Salton Sea and were followed 6-8 wk later by the onset of WEE and SLE virus activity. Viruses then spread to managed marsh (duck club) and agricultural habitats in the Whitewater Channel flood plain and, depending upon the intensity of amplification, to agricultural and residential areas in the more elevated northwestern portion of the valley. Mean annual Cx. tarsalis abundance was correlated inversely with elevation and distance from the Salton Sea. Abundance was greatest at managed marsh habitats. Although spatially correlated with vector abundance among sites, virus transmission rates to sentinel chickens were asynchronous temporally with vector abundance. Seroconversion rates were related to flock location but not flock size (10 versus 20 chickens). Human cases were not detected during the study period, despite elevated transmission rates of both WEE and SLE viruses to sentinel chickens positioned in peridomestic habitats.

Landscape ecology of arboviruses in southern California: patterns in the epizootic dissemination of western equine encephalomyelitis and St. Louis encephalitis viruses in Coachella Valley, 1991-1992.

Temporal and spatial patterns in the initiation and dissemination of western equine encephalomyelitis and St. Louis encephalitis virus activity in Coachella Valley during 1991 and 1992 were detected by testing pools of host-seeking Culex tarsalis Coquillett for virus infection and sentinel chickens for seroconversions. Both viruses repeatedly were detected first at a salt marsh adjacent to the Salton Sea in the southeastern corner of the study area and then disseminated to the northwest to freshwater marsh, agricultural, and residential habitats. Virus dissemination was relatively slow (< 1 km/d) and may have been accomplished by dispersive host-seeking mosquitoes. Repeated early-season recovery of virus activity indicated that both viruses may persist interseasonally in salt marsh habitat.

Bionomics of Culex tarsalis (Diptera: Culicidae) in relation to arbovirus transmission in southeastern California.

Population dynamics and bionomics of host-seeking Culex tarsalis Coquillett 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 sentinel chickens. Female abundance was greatest during the spring and fall, before and after most virus transmission occurred and was not correlated with temperature, humidity, or rainfall. Parity rates were highest during late summer when virus activity peaked and were lowest during December when females may enter a short-term reproductive diapause. Although most likely underestimated, the proportion of older multiparous 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 the parity rate (adjusted to account for autogeny) was highest in winter; however, the proportion of females surviving to potentially transmit either WEE or SLE virus was highest in summer and early fall. Wing length decreased in summer as an inverse correlate of temperature and increased as a function of female age, implying that larger females lived 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 age. The proportion of females testing positive for fructose was greatest during winter and lowest during summer, perhaps affecting survivorship and blood-feeding avidity. The vector competence (infection, dissemination and transmission rates, and ID50) of females collected host seeking or emerging from field-collected pupae for WEE or SLE viruses remained similar over time, even though the wing length of females used in these experiments differed among samples. We conclude that in nature virus transmission progressed efficiently during midsummer because elevated temperatures shortened the extrinsic incubation period without markedly decreasing survivorship resulting in an increased proportion of females surviving extrinsic incubation to become infective.