RESEARCH State during the period 2000–2003, totaling ≈7,195 trap-nights and 378,000 mosquitoes. Mosquitoes were col- lected by using Centers for Disease Control and Prevention (CDC) light traps baited with CO2 (dry ice) from evening until the next morning, which includes the peak activity periods for the mosquitoes considered here. While some mosquitoes are underrepresented in CDC light trap collec- tions (e.g., Ochleratus trivittatus [6,7]), baiting traps with CO2 and trapping during both dusk and dawn minimizes this bias (3). Mosquitoes from these traps were identified to species with 1 exception; Cx. pipiens and Cx. restuans adults are difficult to distinguish in the field and are usually counted and submitted for testing after being pooled and labeled as Cx. pipiens/Cx. restuans. As a result, we present the 2 species as a pair. We averaged their vector competencies and fraction of blood meals from mammalian hosts, which were examined for each species separately by using a molecular identification protocol (8) and identification of mosquitoes as larvae to separate the 2 species. Identification of a subset of trapped adult mosquitoes by experienced entomologists (9) suggests that although year- to-year variability occurs, these species had approximately equal abundance averaged over the past 4 years (L. Kramer et al., unpub. data). Both species have similar feeding behavior (15%–22% of blood meals come from mammalian hosts, averaging the data weighted by sample size from [9,10] and L. Kramer et al., unpub. data) and breed in similar habitats (containers such as tires, gutters, catch basins, polluted surface pools). As a result, combin- ing these species in our analysis should not detract from our ability to determine which vectors transmit WNV to humans; nor should it alter strategies that should be taken to control vector populations. To increase the sensitivity of our analyses, we used WNV testing data from all of New York State from 2000 to 2003 to estimate the infection prevalence for each species. However, we only included data from mosquitoes trapped with CDC light traps because prevalences were higher from mosquitoes caught in gravid traps, and these traps primarily capture Culex mosquitoes that have already fed at least once. Although the large-scale averaging we performed ignores important spatial and temporal varia- tion, the larger dataset is required to accurately estimate prevalence for species that are rarely infected (i.e., all non- Culex species). A correlation analysis of mosquito species’ prevalences at the county and state level suggested that the 2 datasets were comparable (Rockland vs. New York State, r = 0.92 and Suffolk vs. New York State, r = 0.97). Mosquitoes were tested for WNV RNA by using reverse transcription–polymerase chain reaction (RT-PCR) (11) in groups (pools) of 20 to 50, and infection prevalence of each species is expressed as the minimum infection rate (MIR), where MIR = 1,000 × (pools testing positive for WNV/total number of mosquitoes tested). This calculation assumes that each pool contains only 1 infected mosquito, which is >99% likely for MIRs <3 and pools of 50. Occasionally, MIRs >3 have been recorded for Cx. pipi- ens/Cx. restuans (5), which could lead to an underestimate of the true prevalence and Risk for this species pair. Blood meals from mosquitoes trapped in New York and New Jersey were identified to vertebrate order by using PCR or the heteroduplex method (9,10,12). We calculated the fraction of each species’ blood meals that came from mammals as a relative estimate of the probability that the species would feed on humans (13). We believe this approximation is valid because identification of mam- malian blood meals to the species level showed that all of the species considered here feed on humans (10,14). In addition, all of the identified mammalian blood meals from Cx. pipiens and Cx. restuans were from ground-dwelling mammals (9,10, Kramer et al., unpub. data). As a result, we believe that mammalian blood meals identified from Cx. pipiens and Cx. restuans were not a result of these nor- mally ornithophilic mosquitoes’ occasionally biting arbo- real mammals. Lastly, recent research showed that North American Cx. pipiens are actually hybrids between more ornithophilic Cx. pipiens and more opportunistically feed- ing Cx. molestus and Cx. quinquefasciatus (15), which might help explain their feeding on humans and other mammals. Finally, previous research has shown that mosquitoes may be infected with WNV (i.e., test positive) but not transmit virus when feeding, at times because the virus is not present in the salivary glands (4). The probability that the virus will be transmitted with a bite, given that a mos- quito tests positive for WNV, differs among species and has been incorporated into the analysis through the vector competence Cv (see Table for sources). Three species, Culiseta melanura, Ochlerotatus canadensis, and Oc. trivittatus, have not been tested for vector competence. For these species, we used values for their congeners and also present the Risk for these species if their Cv was 1, i.e., if every infected mosquito transmitted the virus when feed- ing (see Table). Results The species-pair Cx. pipiens + Cx. restuans accounts for >80% of the total Risk, a surrogate for human WNV infec- tions in this region, over this time period (Table). The threat of this species-pair is ≈16 times higher than that for the 4 other important species, Oc. japonicus, Ae. vexans, Oc. trivittatus, and Cx. salinarius. This finding is a result of the high WNV prevalence and abundance of this species-pair, which more than compensates for the relatively small frac- tion of mammalian blood meals of these primarily bird- 426 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 3, March 2005