Differences in host choice between the sibling species of treehole mosquitoes Aedes triseriatus and Aedes hendersoni.

Adult treehole mosquitoes were collected by vacuum-sweeping of vegetation in urban, suburban, and rural woodlots in northern Indiana. The sibling species Aedes triseriatus and Ae. hendersoni were identified by polyacrylamide gel electrophoresis. Blood meals were identified by the modified precipitin method. Ae. triseriatus fed predominantly on chipmunks and deer, and Ae. hendersoni fed mainly on tree squirrels and racoon. The relative rates of feeding on the major hosts were variable depending on the location of collection, and probably reflected differences in host density. No blood-feeding on humans was detected.

Detection of eastern equine encephalitis virus in infected mosquitoes using a monoclonal antibody-based antigen-capture enzyme-linked immunosorbent assay.

Surveillance of mosquito populations for virus activity is not often performed by small, vector-control districts because they do not have the financial resources to use virus isolation, or newer methods such as the polymerase chain reaction. Consequently, development and refinements of rapid, sensitive, and simple enzyme-linked immunosorbent assays (ELISAs) applicable to a wide variety of public health settings are justified. We have developed an antigen-capture ELISA for the detection of eastern equine encephalitis (EEE) virus in mosquitoes that uses both monoclonal capture and detector antibodies. The sensitivity of this assay is 4.0-5.0 log10 plaque-forming units/ml, which is comparable to previously published EEE antigen-capture assays developed with polyclonal antibody reagents. This test identifies only North American strains of EEE virus and does not react with either western equine encephalitis or Highlands J viruses. Test sensitivity was enhanced by sonicating mosquito pools, treating them with Triton X-100, and increasing the time and temperature of antigen incubation. The conversion of this ELISA to a monoclonal antibody-based format should result in a readily standardizable and transferable assay that will permit laboratories lacking virus isolation facilities to conduct EEE virus surveillance.

First field evidence for natural vertical transmission of West Nile virus in Culex univittatus complex mosquitoes from Rift Valley province, Kenya.

West Nile virus is a mosquito borne flavivirus endemic over a large geographic area including Africa, Asia, and the Middle East. Although the virus generally causes a mild, self-limiting febrile illness in humans, it has sporadically caused central nervous system infections during epidemics. An isolate of West Nile virus was obtained from a pool of four male Culex univittatus complex mosquitoes while we were conducting an investigation of Rift Valley fever along the Kenya-Uganda border in February-March 1998. This represents the first field isolation of West Nile virus from male mosquitoes and strongly suggests that vertical transmission of the virus occurs in the primary maintenance mosquito vector in Kenya. A phylogenetic analysis of the complete amino acid sequence of the viral envelope glycoprotein demonstrated a sister relationship with a Culex pipiens mosquito isolate from Romania made in 1996. This unexpected finding probably reflects the role of migratory birds in disseminating West Nile virus between Africa and Europe.

Relationship of wing length to adult dry weight in several mosquito species (Diptera: Culicidae).

The relationship between dry weight at adult emergence and wing length cubed was compared by regression analysis for three mosquito species reared in the laboratory and five species collected from the field. The regression equations were compared by ANCOVA between sexes and sources within species, and among species. Results indicated that the proportional relationship of wing length to teneral adult dry weight is highly variable and that unique regression formulae are required for each mosquito species and sex. Also, the relationship between wing length and dry weight differed within species among specimens reared in the lab versus those collected from the field.

Influence of larval and adult nutrition on biting persistence in Aedes aegypti (Diptera: Culicidae).

Biting persistence was measured for Aedes aegypti (L.) mosquitoes given different levels of larval and adult nutrition. Persistence was measured as the total number of times during a 60-min trial that a mosquito returned to land on a defensive host after being disturbed and dislocated from the host, and as the rate of decline of landings over the 60 min test (pattern of persistence). Large females were significantly more persistent than smaller females, regardless of whether the females were provided water or 5% sucrose for 3 d before the test. Access to sucrose for 3 or 7 d did not influence the total number of landings of large females. Large females starved for 7 d exhibited significantly reduced persistence compared with large females starved for only 3 d. Water-fed specimens displayed a more rapid decline in the pattern of persistence than sucrose-fed specimens. Field-collected females, intermediate in size between the large and small laboratory females, had dry weights indicating they were relatively starved. They had a total number of landings similar to the small laboratory females and a persistence pattern similar to sucrose-fed specimens. Size-related differences in persistence may be one factor causing differences in parity rates and vector potential observed between large and small females in Ae. aegypti field populations.

Larval diet, adult size, and susceptibility of Aedes aegypti (Diptera: Culicidae) to infection with Ross River virus.

The relationship of larval nutrition and adult body size to the susceptibility of Aedes aegypti (L.) to Ross River virus infection was examined. Large adult mosquitoes produced by feeding larvae a high-level diet consumed significantly more virus particles than did smaller mosquitoes. However, when a correction for body size was made, smaller mosquitoes were found to consume significantly more virus per unit of body weight. A host viremia of 2.4 log10 PFU/ml failed to infect mosquitoes of any size. Large Ae. aegypti mosquitoes were significantly more susceptible than small mosquitoes when fed on hosts with viremias of 4.8, 5.7, 6.4, and 7.5 log10 PFU/ml, but differences in susceptibility were less apparent at higher viremias.

Aedes albopictus (Diptera: Culicidae) population density and structure in southwest Louisiana.

Pupal density, wing length at emergence, host-seeking female abundance, and host-seeking female wing length and parity were determined monthly for a population of Aedes albopictus (Skuse) in a southwestern Louisiana tire dump from January 1988 through January 1989. Pupae and host-seeking females were first collected in April. Pupal densities were consistently high from June through September. Host-seeking female abundance was highest during July and August. Average female wing length was longest during spring, declined in midsummer, and increased slightly during fall. Average wing length was negatively correlated with average pupal density during the month of collection and with average air temperature recorded the previous month. Host-seeking females had consistently longer wings than emerging females, and parous host-seeking females had longer wings than nulliparous host-seeking females. Average wing length of the parous host-seeking females was consistently long throughout the season. Parity of host-seeking females parity was highest during July when approximately 21 parous host-seeking females were collected per hour. These results indicate that seasonal patterns in immature population density influence average body size and parity structure in Ae. albopictus.

Effect of topically treating cattle with permethrin on blood feeding of Psorophora columbiae (Diptera: Culicidae) in a southwestern Louisiana rice-pasture ecosystem.

The percentage of resting mosquitoes that had fed on blood or were gravid-parous was determined in two pastures associated with the riceland agroecosystem in southwestern Louisiana. In one pasture, all of the cattle were treated with permethrin every 14 d over a 22-wk period, whereas cattle in the other pasture were untreated. The percentage of blood-engorged Psorophora columbiae (Dyar & Knab) was significantly lower in the vicinity of the treated herd. The effectiveness of the permethrin treatment in reducing blood feeding decreased over the interval between applications and as a result of increased rainfall. The proportion of Ps. columbiae that was gravid or parous was not significantly affected by the treatment. Anopheles quadrimaculatus Say engorgement rates also were significantly reduced by the treatment; however, Anopheles crucians Wiedemann and Culex salinarius Coquillett engorgement rates did not differ between the two areas. The results indicate that host management by permethrin treatment has potential for controlling Ps. columbia and An. quadrimaculatus populations in the Louisiana riceland-pasture agroecosystem.

La Crosse encephalitis virus habitat associations in Nicholas County, West Virginia.

Aedes triseriatus (Say) population density patterns and La Crosse encephalitis virus infection rates were evaluated in relation to a variety of habitat parameters over a 14-wk period. Ovitraps and landing collections were used in a La Crosse virus-enzootic area in Nicholas County, WV. Study sites were divided into categories by habitat type and by proximity to the residences of known La Crosse encephalitis cases. Results demonstrated that Ae. triseriatus population densities were higher in sugar maple/red maple habitats than in hemlock/mixed hardwood habitats or in a site characterized by a large number of small red maple trees. Sites containing artificial containers had higher population densities than those without. La Crosse virus minimum infection rates in mosquitoes collected as eggs ranged from 0.4/1,000 to 7.5/1,000 in the 12 study sites, but did not differ significantly among sites regardless of habitat type or proximity to human case residences. La Crosse virus infection rates in landing Ae. triseriatus mosquitoes ranged from 0.0/1,000 to 27.0/1,000. La Crosse virus was also isolated from host-seeking Ae. canadensis (Theobald) in two study sites, at rates similar to those found in the Ae. triseriatus populations. The Ae. triseriatus oviposition patterns and La Crosse virus infection rates suggest that this mosquito species disperses readily in the large woodlands of central West Virginia. The La Crosse enzootic habitats in Nicholas County, WV, are contrasted with those studied in other geographic regions where La Crosse virus is found.

Susceptibility parameters of Aedes albopictus to per oral infection with eastern equine encephalitis virus.

Aedes albopictus (Skuse) mosquitoes were fed on snowy egrets, Egretta thula (Thayer and Bangs), that had been infected by subcutaneous inoculation of eastern equine encephalitis (EEE) virus. Freshly fed mosquitoes were frozen and tested to determine how much virus they had ingested. Other fed mosquitoes from the same lots were incubated for 7 d at 27 degrees C before testing. Seven lots of Ae. albopictus fed on viremic birds. Based on average amounts of virus ingested and day 7 virus infection rates in mosquitoes from the same lots, the amount of virus required to infect 50% of the mosquitoes was calculated to be 10(2.8) Vero cell plaque-forming units (PFU). The infection threshold (i.e., the amount of virus required to infect from 1 to 5% of mosquitoes) was determined to be < or = 10 PFU per blood meal. These parameters indicate that Ae. albopictus is sufficiently susceptible to infection with EEE virus to enable it to acquire infectious doses from a wide variety of viremic birds and possibly from equines.

Eastern equine encephalitis virus in Ohio during 1991.

During August and September of 1991, an epizootic of eastern equine encephalitis (EEE) virus in horses occurred in Wayne and Holmes countries, OH. This was the first recorded epizootic of EEE virus in the state. Twelve horses were confirmed positive for EEE virus through virus isolation or seroconversion, and seven additional horses with compatible symptoms were in close spatial and temporal proximity to the confirmed cases and were presumed to have died from EEE virus. The outbreak was centered around the Killbuck Wildlife Area, a 2,147-ha tract maintained by the state, half of which consists of wooded swamp and marsh. Mosquitoes were collected in upland areas before the epizootic and in the swamp basin at the end of the epizootic to identify the mosquito species involved in EEE virus transmission. We collected and tested 22,095 specimens for the presence of virus. EEE virus was isolated from one pool of the most likely epizootic vector, Coquillettidia perturbans (Walker). The minimum infection rate for EEE virus in this species was 0.1/1,000. Dense populations of Aedes vexans (Meigen) and Culex salinarius Coquillett occurred in the area, but their densities peaked after the epizootic. It is unlikely that these species were involved in epizootic transmission. IgM antibody to EEE virus was detected in three bird species collected in the swamp.

Control of Aedes albopictus from the perspective of North America.

Aedes albopictus is a serious nuisance species and a potential vector of domestic arboviruses in the U.S. Public health and mosquito control agencies have taken numerous actions to address the problems posed by this species. Federally mandated programs of disinsection and inspection of tire casings imported into the U.S. are aimed at preventing additional introductions of this and other exotic mosquito species. Control programs, including the use of traditional chemical larvicides and adulticides, as well as the introduction of predatory copepods into larval habitats, have been developed and tested for population management. A few of the established populations in the northern areas of Ae. albopictus distribution in the continental U.S. have been successfully eradicated through source reduction and insecticide application.

The size of emerging and host-seeking Aedes aegypti and the relation of size to blood-feeding success in the field.

The average wing length of Aedes aegypti females collected as pupae was 2.47 mm, which was significantly smaller than the 2.64 mm average wing length of the host-seeking females collected in the field. The average wing length of nulliparous host-seeking females was 2.62 mm, which was significantly smaller than the 2.76 mm wing length of parous host-seeking females. Thus, small Ae. aegypti females exhibited reduced blood-feeding success and, most likely, reduced survival when compared with large adults.

Effects of sublethal exposure to Bacillus thuringiensis var. israelensis on larval development and adult size in Aedes aegypti.

The effects of exposure to sublethal concentrations of Bacillus thuringiensis var. israelensis (Serotype H-14) on second instar Aedes aegypti larvae were investigated. A test system was developed in which adverse effects would be detected as increased duration of larval development and decreased adult body size. No evidence of negative effects on survivors could be detected when sufficient B.t.i. dosages were applied to kill approximately half of the larvae in the treatment groups. However, when larval density was not controlled, and competition for food decreased as a result of larval mortality in the B.t.i.-treated groups, adult wing length was greater in the B.t.i. survivors than in the untreated controls. In addition, a residual mortality was noted in larvae that had been exposed to B.t.i. for 24 hr and then removed to habitats without B.t.i.

Arbovirus titer variation in field-collected mosquitoes.

Patterns in the distribution of titers in arbovirus-positive mosquito pools were examined. Virus isolation records from the Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, from 1974 through 1993 were used to estimate virus titers in field-collected pools. Pools were classified as either low titer (< or = 3.0 log10 plaque-forming units [PFU]/ml) or high titer (> 3.0 log10 PFU/ml). The proportion of virus-positive pools that had high titers varied among the different domestic arboviruses, within viruses among field sites and years, and within viruses among mosquito species tested. Alphaviruses produced a greater proportion of pools with high titers than did the flaviviruses. Variation in the proportion of pools with high titers among sites and years suggested variation in mosquito and/or virus strains. Variation in the proportion of pools with high titers among mosquito species indicated species-specific differences in vectorial capacity. The results show that information about the titer of virus in mosquito pools can complement other parameters, such as the minimum infection rate, currently used in mosquito-based arbovirus surveillance programs.

Habitat associations of mosquito and copepod species.

Copepods and mosquitoes were collected and identified over a 12-month period from three woodland ponds, discarded tires and a salt marsh. The species distribution of both mosquitoes and copepods varied among habitats and seasonally. Acanthocyclops vernalis was the predominant copepod in all of the habitats except the discarded tires, where Thermocyclops dybowskii was the predominant species. Amblyospora sp.-infected mosquitoes and copepods were found on several occasions in one of the woodland ponds and in the salt marsh. The results indicate that several copepod species have the potential to influence larval mosquito populations either directly as predators or indirectly as intermediate hosts of parasites.

Effect of per os Edhazardia aedis (Microsporida: Amblyosporidae) infection on Aedes aegypti mortality and body size.

Infection with Edhazardia aedis uninucleate spores had less effect on Aedes aegypti larval mortality and adult body size than did larval diet. Larval mortality averaged 60-81% in starved larvae and 2-16% in well-fed larvae. No significant amounts of larval mortality could consistently be attributed to exposure to the parasite at dosages of 1.5 x 10(3) or 1.5 x 10(5) spores/ml. Infection rates in adults surviving exposure to the parasites as larvae ranged from 30 to 59%. Infected adults had significantly smaller body sizes than uninfected adults or controls. Storage of spores in water reduced infectivity gradually over the course of 36 h. By 48 h, the spores were not able to infect mosquito larvae. Spore infectivity was eliminated by drying.

Effect of permethrin treatment of cattle on Psorophora columbiae populations: preliminary field test of a host management concept.

The results of a field test conducted in cooperation with the Jefferson Davis Parish Mosquito Abatement District (JDPMAD) during 1988 in southwestern Louisiana suggested that the biweekly treatment of cattle with permethrin reduced the number of adult Psorophora columbiae in nearby areas. Routine mosquito control operations by JDPMAD were similar from 1987 to 1988 in the cattle-treated area but increased an average of 41% in control areas. In spite of this, during the year of cattle treatment, captures in New Jersey light traps averaged 86 and 26% of the previous 4-year average in control and treated areas, respectively. Similarly, the proportion of trap nights in 1988 when Ps. columbiae captures exceeded 500 averaged 73 and 30% of the previous 4-year average in control and treated areas, respectively. Simulation model projections that took into account differences in JDPMAD operations, weather, cattle density, agricultural practices and the presence of treated cattle were accurate within an average of 9% of observed values for the treated and control areas. The simulation studies indicated that the treatment of cattle with permethrin reduced Ps. columbiae populations by 83%. These results suggest that host management may be a viable method for suppressing populations of Ps. columbiae and that some form of host management be considered for inclusion in integrated control programs in this environment. Further evaluation of this concept is warranted.

Control of Aedes albopictus larvae using time-release larvicide formulations in Louisiana.

The ability of time-release formulations of larvicides and insect growth regulators (IGRs) to provide long-term control of Aedes albopictus was investigated in the field. Larvicides used in the study were Bactimos pellets (Bacillus thuringiensis var. israelensis, active ingredient) and Abate pellets (temephos, active ingredient). The IGR Altosid (methoprene, active ingredient) was used in pellet and sand formulations. Application rates were higher than label recommendations. In a preliminary test, clay flower bowls were treated with 2 g of material. Bactimos pellets failed to provide control after 60 days. Abate pellets and the Altosid formulations provided essentially 100% control for 150 days. After 360 days in the field, the Abate pellets produced 100% larval mortality, and significant levels of control were provided by the Altosid formulations and the Bactimos pellets. In a small-scale operational trial of this technique, 1 g of Altosid pellets was applied to every container that could be located in 2 urban residential neighborhoods in Lake Charles, LA. Aedes albopictus biting populations were monitored weekly in the treated areas and in an untreated control area. Biting population densities declined significantly in treated areas compared with the control area. Results suggested that long-term control of Ae. albopictus populations can be achieved economically with one application of Altosid pellets or Abate pellets in containers.

Interspecific mating between Louisiana strains of Aedes albopictus and Aedes aegypti in the field and laboratory.

Interspecific mating between Aedes albopictus males and Ae. aegypti females was detected in the field using mark-release-recapture techniques. By 3 days after the release of virgin Ae. aegypti females into a field site containing only Ae. albopictus, 100% of the captured females were inseminated. Laboratory investigations indicated that male Ae. albopictus were very proficient at inseminating Ae. aegypti females and that Ae. aegypti males rarely inseminated Ae. albopictus females, especially if Ae. aegypti females were available. Most of the Ae. aegypti females inseminated by Ae. albopictus males contained only small amounts of dead sperm in their spermathecae, while inseminated females from the converse interspecific mating and from intraspecific matings contained only large amounts of live sperm. The results are discussed in relation to the decline in Ae. aegypti densities observed since the introduction of Ae. albopictus into the southern USA.