Reproduced from Wing Beats of the AMCA, the official publication of the Florida Mosquito Control Association. Please use the following citation when referring to this article:
Savage, H., and B. Miller. 1995. House Mosquitoes of the U.S.A., Cules pipiens complex. Wing Beats, Vol. 6(2):8-9.
House Mosquitoes of the U.S.A., Culex pipiens Complex
Dr. HARRY SAVAGE and Dr. BARRY MILLER
Importance
Members of the Culex pipiens complex are the principal vectors of St. Louis encephalitis (SLE) virus in the central and eastern United States with the exception of Florida, and in urban areas elsewhere in the U.S.A. Cx. quinquefasciatus Say and Cx. pipiens L. have been incriminated as vectors of dog heartworm,Dirofilaria inimitis. Members of the complex may also be important pest mosquitoes, particularly in urban areas and in association with feed lots and farms.
Geographic Distribution
The Culex pipiens complex in North America is represented by four members. In areas above 39 degrees N. latitude, only Culex pipiens, or the northern house mosquito, is usually encountered. At latitudes of less than 36 degrees N. only Cx. quinquefasciatus, the southern house mosquito, is generally present. Between 36 and 39 degrees N. latitudes, Cx. pipiens, Cx. quinquefasciatus, and hybrids between the two are encountered. The distribution o the fourth member, Cx. pipiens form molestus Forskal, remains poorly known.
Morphological Identification
Adults of the Cx. pipiens complex are light brown mosquitoes that lack distinctive markings on the proboscis and legs, and are not readily separated from other Culex (Culex) mosquitoes. Adult females of the complex are usually identified by the presence of distinctive, basal, pale abdominal bands. Abdominal bands are broadly rounded medially and distinctly constricted sublaterally before joining large, lateral scale patches. Male adults of Cx. pipiens and Cx. quinquefasciatus, and with less precision Cx. pipiens-quinquefasciatus hybrids, can be identified by use of the DV/D ratio of the genitalia. Larvae of the Cx. pipienscomplex can be identified by: the presence of a moderately long siphon that has 6-13 pecten teeth located on the basal 1/3, and 4-branched siphonal tufts, one of which is inserted laterally and out-of-alignment with the other three; and double-branched lateral setae on abdominal segments III-IV. The shape of the siphon and number of branches on setae I on abdominal segments 3-4 can be used to characterize Cx. pipiens and Cx. quinquefasciatus. However, no reliable means of specific larval identification is available in areas where hybrids may occur.
Molecular Identification
Recent molecular studies have lead to the development of specific polymerase chain reaction (PCR) primers that can be used in a PCR "cocktail" to identify Cx. salinarius, Cx. restuans, and the Cx. pipiens complex. Unfortunately, variation between Cx. pipiens and Cx. quinquefasciatus was insufficient to develop diagnostic primers for these taxa.
Breeding Sites, Development, Flight Range and Associated Species.
House mosquitoes are common in urban and suburban communities as well as on rural premises. Members of the complex readily breed in storm sewer catch basins, clean and polluted ground pools, ditches, animal waste lagoons, effluent from sewage treatment plants and other sites that are slightly to very eutrophic or polluted with organic wastes. Culex quinquefasciatus is generally associated with more eutrophic waters than Cx. pipiens. Females of Cx. pipiens and Cx. quinquefasciatus typically lay a single raft of 140-340 eggs after each bloodmeal. Eggs hatch in 1-2 days. Development from egg to adult is temperature dependent; requiring 8 to 12 days in summer. After bloodfeeding, females may return to the same or nearby larval habitats to oviposit and are often considered nonmigratory mosquitoes. However, females may travel considerable distances from resting sites to search for blood hosts, and marked females have been shown to travel up to 1100 m in a single night. Associated species include Cx. restuans, Cx. salinarius and Aedes albopictus in the eastern U.S., and Cx. tarsalis, Cx. restuans and Culiseta incidens in the western U.S.
Diapause and Seasonality
In southern regions, Cx. quinquefasciatus is active throughout the year, although larval growth rates may slow and adult populations may be reduced during cooler months. In northern areas, Cx. pipiens enters a facultative reproductive diapause and adult, inseminated females spend the winter in hibernacula, such as culverts and caves. Diapause is a genetically determined state of arrested development induced by environmental factors, primarily decreasing day length, that allows adult females to overwinter in cold climates.
Bloodfeeding
Both Cx. pipiens and Cx. quinquefasciatus readily utilize birds as bloodmeal hosts; however, both will feed on mammals including humans and dogs when these hosts are abundant. Culex pipiens appears to display a stronger preference for birds and to be less opportunistic than Cx. quinquefasciatus. Autogeny, or the ability to mature a batch of eggs without a bloodmeal, has been observed in populations which also do not diapause and are referred to as Cx. pipiens form molestus.
Dr. Harry M. Savage and Dr. Barry R. Miller are Research Entomologists at the Division of Vector-Borne Infectious Diseases, Centers for Disease Control & Prevention, Ft. Collins, CO.