Tarsalis Females Research Articles

Oviposition Behavior of Culex tarsalis (Diptera: Culicidae) Responding to Semiochemicals Associated with the Western Mosquitofish, Gambusia affinis (Cyprinodontiformes: Poecilliidae).

Considerable previous research has focused on predator-associated semiochemicals and how they affect mosquito oviposition behavior. However, most of this work has been done without taking into consideration either the natural density of the predators or how other semiochemicals in aquatic environments might affect the responses of gravid mosquitoes. The influence of mosquitofish density, source water (tap vs pond), presence of freshly laid egg rafts, and removal of a putative source of semiochemicals (bacteria) on oviposition by Culex tarsalis Coquillett (Diptera: Culicidae) was studied in laboratory bioassays. Culex tarsalis females were deterred from laying egg rafts on water that contained semiochemicals associated with Gambusia affinis (Baird & Girard), but this deterrence was not strongly associated with the density of fish used to condition aged tap water. The number of egg rafts laid onto Gambusia-exudate water made with either tap water (density < 1 fish per liter) or pond water was typically ≥ 50% of that onto water that did not house mosquitofish. Gravid mosquitoes tested individually did not reduce oviposition onto Gambusia-exudate water as compared to controls. Likewise, oviposition by females with ablated wings did not differ significantly between Gambusia-exudate water and controls. Oviposition onto filter-sterilized Gambusia-exudate water was reduced relative to unfiltered water, suggesting that semiochemicals deterring egg-laying were still present after bacteria were removed. Taken together, these findings suggest that the responses of gravid Cx. tarsalis to chemicals from habitats containing mosquitofish are complex and the origin of the semiochemicals present in the Gambusia-exudate water needs to be elucidated.

Oviposition of Culex tarsalis (Diptera: Culicidae) Differs on Water Conditioned by Potential Fish and Insect Predators.

The response of egg-laying Culex tarsalis Coquillett (Diptera: Culicidae) to water conditioned by three fish species used for mosquito control and three predatory aquatic insect species was examined in laboratory binary choice experiments. Oviposition by Cx. tarsalis was 72% less on water conditioned with the arroyo chub, Gila orcutti (Eigenmann & Eigenmann) (Cypriniformes: Cyprinidae) relative to control cups containing aged tap water, but no significant difference was found in the numbers of egg rafts laid on water conditioned with the fathead minnow (Pimephales promelas (Rafinesque), Cypriniformes: Cyprinidae) and the control treatment (water aged 24 h). Mosquito oviposition on water conditioned with the predominantly herbivorous/algivorous California Mozambique tilapia hybrid (Oreochromis mossambicus (Peters) × Oreochromis urolepis hornorum L. (Perciformes: Cichlidae)) or predatory insects (nymphs: Sympetrum corruptum (Hagen) (Odonata: Libellulidae); adults: Thermonectus basillaris (Harris) or Cybister fimbriolatus (Say) (Coleoptera: Dytiscidae)) did not differ significantly relative to that onto water aged for 24 h. As compared with water aged 24 h and water conditioned with diving beetles, oviposition by Cx. tarsalis was significantly lower (≥53%) when live predatory diving beetles were present in oviposition cups. Gravid Cx. tarsalis females do not respond equally to putative semiochemicals in water conditioned with the piscine or aquatic insect predators of immature mosquitoes tested here.

Using Undergraduate Researchers to Build Vector and West Nile Virus Surveillance Capacity

Vector surveillance for infectious diseases is labor intensive and constantly threatened by budget decisions. We report on outcomes of an undergraduate research experience designed to build surveillance capacity for West Nile Virus (WNV) in Montana (USA). Students maintained weekly trapping stations for mosquitoes and implemented assays to test for WNV in pools of Culex tarsalis. Test results were verified in a partnership with the state health laboratory and disseminated to the ArboNET Surveillance System. Combined with prior surveillance data, Cx. tarsalis accounted for 12% of mosquitoes with a mean capture rate of 74 (±SD = 118) Cx. tarsalis females per trap and a minimum infection rate of 0.3 infected mosquitoes per 1000 individuals. However, capture and infection rates varied greatly across years and locations. Infection rate, but not capture rate, was positively associated with the number of WNV human cases (Spearman’s rho = 0.94, p < 0.001). In most years, detection of the first positive mosquito pool occurred at least a week prior to the first reported human case. We suggest that undergraduate research can increase vector surveillance capacity while providing effective learning opportunities for students.

Mosquito and West Nile virus surveillance in northeast Montana, U.S.A., 2005 and 2006

Mosquito and West Nile virus (WNV) surveillance was conducted on a national wildlife refuge in northeast Montana in 2005 and 2006, during which outbreaks of WNV in a colony of American white pelicans (Pelecanus erythrorhynchos Gmelin) (Pelecaniformes: Pelecanidae) resulted in juvenile mortality rates of ∼ 31%. Both years, floodwater species Ochlerotatus dorsalis (Meigen) (Diptera: Culicidae), Aedes vexans (Meigen) (Diptera: Culicidae) and Ochlerotatus flavescens (Muller) (Diptera: Culicidae) comprised 78% of the total collection and heightened host-seeking activity was observed from mid-June to mid-July. Culex tarsalis Coquillett (Diptera: Culicidae) was most active from mid-July to mid-August and comprised 18% of the collection in 2005 and 20% in 2006. However, fewer than 10% of the Cx. tarsalis females collected in 2006 were obtained adjacent to the pelicans' nesting grounds. Minimum infection rates per 1000 Cx. tarsalis tested for WNV were 1.36 in 2005 and 1.41 in 2006. All pools in which WNV was detected in 2006 were composed of females collected 10 km from the nesting grounds. Substantial juvenile pelican mortality in 2006 despite reductions in the population of the primary vector and in mosquito infection rates near the colony suggests that the methods used to detect the introduction of WNV were too coarse and that amplification of the virus within the colony may reflect causes other than mosquito infection.

Mosquitoes and West Nile virus along a river corridor from prairie to montane habitats in eastern Colorado

We conducted studies on mosquitoes and West Nile virus (WNV) along a riparian corridor following the South Platte River and Big Thompson River in northeastern Colorado and extending from an elevation of 1,215 m in the prairie landscape of the eastern Colorado plains to 1,840 m in low montane areas at the eastern edge of the Rocky Mountains in the central part of the state. Mosquito collection during June-September 2007 in 20 sites along this riparian corridor yielded a total of 199,833 identifiable mosquitoes of 17 species. The most commonly collected mosquitoes were, in descending order: Aedes vexans, Culex tarsalis, Ae. dorsalis, Ae. trivittatus, Ae. melanimon, Cx. pipiens, and Culiseta inornata. Species richness was higher in the plains than in foothills-montane areas, and abundances of several individual species, including the WNV vectors Cx. tarsalis and Cx. pipiens and the nuisance-biter and potential secondary WNV vector Ae. vexans, decreased dramatically from the plains (1,215-1,487 m) to foothills-montane areas (1,524-1,840 m). Ae. vexans and Cx. tarsalis had a striking pattern of uniformly high abundances between 1,200-1,450 m followed by a gradual decrease in abundance above 1,450 m to reach very low numbers above 1,550 m. Culex species were commonly infected with WNV in the plains portion of the riparian corridor in 2007, with 14 of 16 sites yielding WNV-infected Cx. tarsalis and infection rates for Cx. tarsalis females exceeding 2.0 per 1,000 individuals in ten of the sites. The Vector Index for abundance of WNV-infected Cx. tarsalis females during June-September exceeded 0.5 in six plains sites along the South Platte River but was uniformly low (0-0.1) in plains, foothills and montane sites above 1,500 m along the Big Thompson River. A population genetic analysis of Cx. tarsalis revealed that all collections from the ≈190 km riparian transect in northeastern Colorado were genetically uniform but that these collections were genetically distinct from collections from Delta County on the western slope of the Continental Divide. This suggests that major waterways in the Great Plains serve as important dispersal corridors for Cx. tarsalis but that the Continental Divide is a formidable barrier to this WNV vector.

Seasonal Patterns for Entomological Measures of Risk for Exposure toCulexVectors and West Nile Virus in Relation to Human Disease Cases in Northeastern Colorado

We examined seasonal patterns for entomological measures of risk for exposure to Culex vectors and West Nile virus (family Flaviviridae, genus Flavivirus, WNV) in relation to human WNV disease cases in a five-county area of northeastern Colorado during 2006-2007. Studies along habitat/elevation gradients in 2006 showed that the seasonal activity period is shortened and peak numbers occur later in the summer for Culex tarsalis Coquillett females in foothills-montane areas >1600 m compared with plains areas <1600 m in Colorado's Front Range. Studies in the plains of northeastern Colorado in 2007 showed that seasonal patterns of abundance for Cx. tarsalis and Culex pipiens L. females differed in that Cx. tarsalis reached peak abundance in early July (mean of 328.9 females per trap night for 18 plains sites), whereas the peak for Cx. pipiens did not occur until late August (mean of 16.4 females per trap night). During June-September in 2007, which was a year of intense WNV activity in Colorado with 578 reported WNV disease cases, we recorded WNV-infected Cx. tarsalis females from 16 of 18 sites in the plains. WNV infection rates in Cx. tarsalis females increased gradually from late June to peak in mid-August (overall maximum likelihood estimate for WNV infection rate of 8.29 per 1000 females for the plains sites in mid-August). No WNV-infected Culex mosquitoes were recorded from sites >1600 m. The vector index for abundance of WNV-infected Cx. tarsalis females for the plains sites combined exceeded 0.50 from mid-July to mid-August, with at least one site exceeding 1.00 from early July to late August. Finally, we found that abundance of Cx. tarsalis females and the vector index for infected females were strongly associated with weekly numbers of WNV disease cases with onset 4-7 wk later (female abundance) or 1-2 wk later (vector index).

EVALUATION OF PYRETHRIN AERIAL ULTRA-LOW VOLUME APPLICATIONS FOR ADULT CULEX TARSALIS CONTROL IN THE DESERT ENVIRONMENTS OF THE COACHELLA VALLEY, RIVERSIDE COUNTY, CALIFORNIA

Eliminating infected female mosquitoes by aerial applications of ultra-low volume adulticides is the intervention strategy currently recommended to interrupt the epidemic transmission of encephalitis viruses, including West Nile. The current research optimized pyrethrin formulations and evaluated their efficacy in the desert environment of the Coachella Valley, Riverside County, California. After seven trials during 2004, a 1:2 by volume mixture of Pyrenone 25-5 in BVA oil optimized particle size, droplet descent to ground level, and kill of sentinel mosquitoes. Three subsequent experiments used 3 aerial applications of the 1:2 Pyrenone 25-5:BVA oil mixture on alternate nights to suppress Culex tarsalis Coquillett host-seeking abundance over a 1-square-mile target area. Mortality patterns among caged sentinel mosquitoes varied among sites and replicate sprays, indicating variable particle dispersion at ground level within the target area. In addition, mortality was observed for sentinels up to 1 mile downwind from the target area, indicating considerable particle drift. Geometric mean abundance of host-seeking Cx. tarsalis females collected at dry ice-baited traps within each of 3 sprayed and 2 unsprayed negative control strata varied similarly over time, indicating that our sprays minimally impacted the target population or that drift combined with other factors led to widespread area control. Experiments during March and June when recruitment rates were minimal showed general area-wide suppression of abundance following spray, whereas an experiment during September when recruitment rates were high from newly flooded marshes failed to prevent an area-wide increase in abundance. Clearly additional research is needed to standardize the efficacy of aerial applications of pyrethrins in hot dry desert environments.

Hunting for moving hosts: Cephalonomia tarsalis, a parasitoid of free-living grain beetles

Abstract The parasitic wasp Cephalonomia tarsalis parasitizes larvae of the saw-toothed grain beetle Oryzaephilus surinamensis, which feed on wheat grains. In contrast to most other host–parasitoid systems studied so far, the grain beetles are highly mobile within their habitat, bulk of grains in grain stores. This should increase the wasps’ problem to locate the hosts. To study the host-finding strategy of C. tarsalis females, the reaction of wasps to different grain and host-derived odour sources was tested in a four-chamber-olfactometer. These experiments revealed that wasps were attracted by healthy grains and mechanically damaged grains. In direct comparison, healthy and mechanically damaged grains are equally attractive. Both potential sources of host-derived odours, host faeces and trail-traces of larvae on filter paper were attractive to the wasps. The response to trail-traces vanished 30 min after larvae had been removed from the filter paper. With respect to the specificity of the odours, it turned out that wasps were attracted to odours from the seed–host complexes from O. surinamensis and Oryzaephilus mercator and the non-host complex of larvae of the granary weevil Sitophilus granarius in wheat grains. Odours from the seed–host complex were preferred. From these results, we hypothesize that host habitat location in C. tarsalis is achieved by using grain-derived odours. Within the habitat, wasps search for kairomones from host faeces and host trails. Following these larval trails finally leads wasps to their hosts.

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.

Multiple blood feeding by Anopheles freeborni and Culex tarsalis (Diptera:Culicidae): spatial and temporal variation.

Multiple blood feeding in field populations of Anopheles freeborni Aitken and Culex tarsalis Coquillet was detected using a histologic technique. Examination of 333 blood engorged An. freeborni and 41 Cx. tarsalis females revealed that 13 and 10% of these females imbibed multiple blood meals, respectively. Spatial and temporal distribution of multiple blood feeding events was evaluated against the percentage of blood fed females and the abundance of female mosquitoes. The percentage of blood fed An. freeborni females was higher among those collected in pasture and riparian habitats than those collected in rice field and mixed habitats. Conversely, spatial variation of multiple blood feeding in An. freeborni was not significantly associated with rice field, pasture, riparian, or mixed habitats. The decrease of multiple blood feeding for An. freeborni through the season was correlated inversely with the increase of the abundance of adult females; but not with the percentage of blood fed females. These results confirmed that multiple blood meals in An. freeborni, and perhaps in Cx. tarsalis, are a frequent phenomenon, and that the histologic procedure is an appropriate tool for longitudinal and region-wide detection of multiple blood feeding necessary in epidemiologic studies.

Spatial Distribution of Adult Mosquitoes (Diptera: Culicidae) in Habitats Associated with the Rice Agroecosystem of Northern California

The objective of this study was to determine whether abundance, blood feeding rates, and sex ratios of adult Anopheles freeborni Aitken and Culex tarsalis Coquillett were associated significantly with either rice field, pasture, riparian, or mixed habitats found within the rice culture agroecosystem of northern California. Significantly higher numbers of adult An. freeborni occurred in riparian and mixed habitats compared with rice and pasture habitats. Such a pattern was not evident for Cx. tarsalis. Riparian and pasture habitats contained significantly higher proportions of blood fed An. freeborni females than did rice and mixed habitats; however, the proportions of blood fed Cx. tarsalis females did not vary significantly among habitat types. The proportions of blood fed An. freeborni and Cx. tarsalis females in riparian habitats decreased with increasing abundance. There was no correlation between blood feeding rates and abundance for An. freeborni and Cx. tarsalis females in the other habitat types. The sex ratio of An. freeborni in pasture and riparian habitats was significantly female biased, unlike the other habitats which did not differ significantly from unity (1:1). Overall, riparian and mixed habitats contained greater numbers of adults mosquitoes; therefore, surveillance and control efforts of these mosquito species should be focused on such habitats.

Effect of temperature on the transmission of western equine encephalomyelitis and St. Louis encephalitis viruses by Culex tarsalis (Diptera: Culicidae).

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.

Mosquito Bionomics and the Lack of Arbovirus Activity in the Chino Area of San Bernardino County, California

Mosquito bionomics, including temporal abundance, metabolic status, and blood-feeding patterns, and arbovirus activity were studied at representative dairy and residential habitats in Chino, Calif., during 1987 and 1988. Host-seeking Culex tarsalis Coquillett and Cx. quinquefasciatus Say females were more abundant and appeared earlier each season at traps near dairy breeding sources than at traps in residential areas, whereas Cx. stigmatosoma Dyar, which also breeds in dairy effluent, was most abundant at residential traps. All three Culex species remained gonotrophically active throughout winter and did not appear to enter diapause. Dairy cows diverted the normally ornithophagic Cx. tarsalis and Cx. quinquefasciatus females from avian hosts in dairy, but not in residential, habitats. Cx. stigmatosoma fed almost exclusively on birds. The diversion of host-seeking Cx. tarsalis females to bovine hosts and reduced abundance because of mosquito abatement may have combined to reduce the receptivity of the Chino area to St. Louis encephalitis virus.

Overwintering Studies on Culex tarsalis (Diptera: Culicidae) in Kern County, California: Temporal Changes in Abundance and Reproductive Status with Comparative Observations on C. quinquefasciatus (Diptera: Culicidae)

Population ecology of Culex tarsalis Coquillett was studied at Hart Park, Kern County, Calif., during the winters of 1982-83 and 1983-84. Female relative abundance declined during autumn to a minimum in December and January. Most females collected during winter were empty, inseminated, and nulliparous, indicating a cessation of gonotrophic activity. However, observations on ovarian morphometry indicated that the hibernating states achieved included reproductive diapause, quiescence, and perhaps oligopause. Females remained vagile and readily repopulated resting sites after removal sampling. Relative abundance increased in late February and March in association with the renewal of gonotrophic activity, but before the emergence of the progeny of overwintering females. Overwintering arboviruses were not isolated from 1,176 C. tarsalis females collected from November 1983 to March 1984. Observations on ovarian dynamics of concurrently collected C. quinquefasciatus indicated that the hibernating states achieved were comparable with those observed for C. tarsalis. Relative abundance declined throughout winter and did not recover coincidentally with the renewal of gonotrophic activity in March.

Determining the mating ability of irradiated male Culex tarsalis (Diptera: Culicidae).

Significantly more virgin Culex tarsalis females were mated and fully inseminated by unirradiated males than by irradiated males when caged at a ratio of 1 male to 10 virgin females for 10 days. Females caged with irradiated males produced more unembryonated egg rafts than females caged with unirradiated males. Of 38 unembryonated egg rafts deposited by females caged with males irradiated at 5.5 kR, 50% were attributed to inseminated females. There was no difference in the size of egg rafts deposited by inseminated or uninseminated females.

Winter Survival of Culex Tarsalis (Diptera: Culicidae) Hibernating in Mine Tunnels in Boulder County, Colorado, USA

The mark-release-recapture technique was applied to the study of winter survival among hibernating Culex tarsalis for the first time. Three hundred and seventy-seven hibernating Cx. tarsalis females were collected from an abandoned gold mine tunnel near Jamestown, Colorado, on 19 October 1977, marked with a zinc sulfide dust, and released into the mine. Recovery of 8.7% of the marked population 3 to 5 months later provided only a crude estimate of survival rates; however, this study represents the first attempt to quantify survival rates among an unrestrained overwintering population of Cx. tarsalis . A total of 1088 female Cx. tarsalis were collected from this and surrounding mines from September 1977 through March 1978. The parity status of 606 specimens was 2.8% parous, 89.9% nulliparous, and 7.3% undetermined. A total of 905 Cx. tarsalis collected from September through March was tested for arboviruses, generally after being kept alive in the laboratory for 2 weeks at 26.7 °C and a 16-h photophase and after having fed on a chick, but no virus was isolated from either the mosquitoes or the chicks.

Oviposition Attractants and Repellents of Mosquitoes: Oviposition Responses of Culex1 Mosquitoes to Organic Infusions 2

The oviposition responses of Culex tarsalis and C. pipiens quinquefasciatus females to 2 organic infusions have been studied in the laboratory. A 1% lab chow infusion was significantly repellent to ovipositing C. tarsalis and C. p. quinquefasciatus females when tested in both one liter choice bioassay units and screen cage choice tests. C. p. quinquefasciatus females were attracted to a 1% chicken manure infusion while C. tarsalis females were repelled by this infusion when both were tested in one-liter units. These infusions were quite stable when stored at –20°C and remained active for 10 wk. Biological parameters including interval between blood feeding and oviposition, parity, and prior exposure to test infusions in bioassays did not affect the responses to the 1% lab chow infusion. Microorganisms were found to be involved in the responses to both infusions.

A chemical factor that stimulates oviposition by Culex tarsalis Coquillet (Diptera, Culicidae)

1. 1. Culex tarsalis females selected water containing pupae, exuviae and emerging adults of their own species for oviposition in preference to distilled water. 2. 2. Preference was shown for water which had contained these stages (emergence water) even after they had been removed by filtration. When this water was boiled and offered to C. tarsalis females with an unboiled sample of the same water, preference was shown for the boiled material. 3. 3. Evaporation of a volume of emergence water was followed by reconstitution of the dry material with distilled water or with 0·03M NaCl solution. The reconstituted samples were selected by ovipositing C. tarsalis females when offered together with distilled water or 0·03M NaCl. 4. 4. Ovipositing C. tarsalis females selected emergence water which had contained the same species in preference to emergence water obtained from Culiseta inornata, Aedes aegypti and Culex pipiens. 5. 5. Emergence waters which contained C. tarsalis males and females respectively were equally acceptable to C. tarsalis females. 6. 6. No discrimination was made by ovipositing females between water which had contained C. tarsalis 4th-instar larvae and emergence water of the species. 7. 7. No clear discrimination was made by ovipositing females between emergence water and water which had contained only pupae of the species. 8. 8. A similar type of behaviour was demonstrated by Culex pipiens but not by Aedes aegypti females.

Fatty Acid Composition during Postembryonic Development of the Mosquito Culex tarsalis1

Fatty acid composition, predominantly of triglycerides, was determined for a California-derived and largely anautogenous strain of Culex tarsalis Coquillett at 22°C and 16-hour daily photoperiod. Larval fatty acids are an approximate compromise between those found in dog food and microorganisms in the diet, with evidence that 16- carbon fatty acids are accumulated and stearic acid depleted. Pupal fatty acids are generally intermediate between those of larvae and newly emerged adults. Adult females fed 3 weeks on sucrose are characterized by large amounts of palmitoleic, palmitic, and oleic acids in that order of concentration. Females have much linoleic acid on emergence, but lose virtually all of this in 3 weeks. Males and females are compared for characteristic fatty acids. It is speculated that 16-carbon fatty acids are typical of culicine mosquitoes, and C. tarsalis females may require large amounts of palmitoleic acid for survival under cold conditions.