Potential WNV Mosquito Vectors


The following mosquitoes are known or suspected mosquito vectors of West Nile virus that are found in Fairfax County. To date, the only mosquito species that have tested positive for West Nile virus are Culex restuans and Culex pipiens. For information regarding the frequency and distribution of West Nile virus activity in these mosquitoes, click here.

Culex pipiens: Among the more common mosquito pests in Virginia, Cx. pipiens is found in urban and suburban communities as well as on rural premises. It breeds in underground storm sewer catch basins; artificial containers; gutters; polluted ground pools; bird baths; discarded tires; animal waste lagoons; farm animal wallows; clogged, grass choked ditches; in effluent from sewage treatment plants; and in other sites that are slightly to very eutrophic or polluted with organic wastes. This mosquito is a nighttime flyer that will invade houses where it will hide in corners and in dark places during the day. This mosquito feeds principally on birds, but will feed on humans, particularly when inside a house. Cx. pipiens over-winters as adult mosquitoes, hiding in culverts, under houses, and inside outbuildings. In the spring, eggs are laid on top of water, in suitable habitats. The eggs hatch into larvae that mature within two weeks when water temperature and nutrient levels are relatively high. Cx. pipiens females may travel up to 3/4 of a mile in a single night in search of a blood meal, but generally are found within ½ mile of their breeding habitat. Cx. pipiens is the most important vector for WNV. Culex pipiens were the most common field collected mosquito to be found carrying WNV in during 1999, 2000 and 2001. For example, in 2001 there were five times as many Cx. pipiens that tested positive for WNV as Cx. restuans; Culex restuans were the second most common field collected mosquito to test positive for WNV. Cx. pipiens are generally not readily trapped in large numbers with CDC traps, so low numbers in a CDC trap do not necessarily indicate low numbers in the environment. Their populations are best monitored with gravid traps baited with an appropriate infusion bait. Cx. pipiens are found anywhere in Virginia and can be among the most abundant species in urban environments.

Culex restuans: Among the more common mosquitoes in Virginia, Cx. restuans is found in most of the same breeding habitats as Cx. pipiens. Cx. restuans becomes active in the early spring and is the most common Culex species found during the springtime. Cx. restuans becomes less common during the warm summer months when Cx. pipiens is most active. Cx. restuans is a night time biter that feeds exclusively on birds. Laboratory trials show them to be moderately competent WNV vectors, but they are probably the second most important primary vectors that amplify WNV in the bird population and they were the second most common field collected mosquito species to test positive for WNV during the 1999 through 2003 seasons. Cx. restuans are more readily trapped with properly baited gravid traps than with CDC traps. Low numbers in a CDC trap do not necessarily indicate low numbers in the environment.

Culex salinarius: This species was identified as the most likely bridge vector for human WNV cases on Staten Island, NY in 2000. It is the third most common mosquito species to test positive for WNV. Cx. salinarius breeds in temporary flood-pools/puddles on the margins of freshwater, brackish or saltwater marshes. It may also be found breeding in any grass choked, stagnant puddle, or in large artificial containers containing a mixture of water and grassy organic matter or other vegetation. It can be found anywhere in Virginia and can be among the most abundant species in some areas during the summer. It is attracted to CO2 baited CDC traps and is often caught in large numbers in coastal communities. It is rarely collected in gravid traps. Cx. salinarius is a nighttime flyer that may invade houses. It feeds readily on both birds and large mammals including humans. The flight range of Cx. salinarius is approximately 2-3 miles. It overwinters as an adult in rodent burrows, culverts or other sheltered areas, and may become active even during the winter months on days when the air temperature is high enough to allow flight (e.g., >55°F).

Culex erraticus: Cx.erraticus is a small dark, chocolate-brown colored mosquito with white markings on its abdomen. It breeds in semi permanent pools and along pond margins in among overhanging vegetation or willow roots. It may have a flight range of several miles and is known to take the majority of its blood meals from large mammals such as horses and livestock, but also bites birds. It has been found infected with WNV in a number of states. It can only be collected in CDC light traps and will rarely come to gravid traps.

Ochlerotatus triseriatus: Oc. triseriatus known as the "eastern tree hole mosquito" is found throughout Virginia. Oc. triseriatus favors laying eggs in tree holes (holes in tree trunks that hold water), but also breeds in artificial containers that are in shady locations and are polluted with some tree debris (leaves, etc.). It is often found in association with tire piles. Oc. triseriatus feeds during the day and its flight activity generally ends at dusk. Oc. triseriatus typically does not travel more than about 500 ft from its breeding habitat, so if tree hole mosquitoes are an identified problem, their breeding habitat is generally going to be close by. Oc. triseriatus has been identified as a moderately efficient WNV vector in the laboratory, but field data in 2000 and 2001 have shown it to be the fifth most common mosquito species found carrying WNV. Oc. triseriatus are not readily trapped with CDC traps, so detection of low numbers with a CDC trap may mean that there are many more in the environment.

Ochlerotatus japonicus: Oc. japonicus was first discovered in the United States in New York in 1998. Since that time it has been discovered throughout the states of Connecticut, New Jersey, and Pennsylvania. More recently it was discovered in central and western Maryland, Washington D.C., and northern Virginia and has been found in counties along the Shenandoah Valley and Blue Ridge mountains as far southwest as Roanoke Virginia. Oc. japonicus is associated with rock pools (holes in rock outcrops that hold water), but it breeds equally well in all forms of artificial containers. It can be found sharing its breeding habitat with Oc. triseriatus and like that species, becomes active relatively early in the season. Little is known about Oc. japonicus' flight range, or its feeding preferences, but it will bite humans. It was the sixth most common field collected mosquito to be found carrying WNV in 2001. In laboratory trials it has been shown to be a highly efficient vector of WNV. Oc. japonicus are not readily trapped with CDC traps, so detection of low numbers with a CDC trap may mean that there are many more in the environment..

Aedes albopictus: Ae. albopictus, the Asian Tiger Mosquito, is a container breeder that is found throughout Virginia and is easily the most common urban, suburban and rural nuisance mosquito associated with artificial breeding habitats. Ae. albopictus feeds during the day and its flight activity generally ends at dusk. However, it will enter homes at dusk, and once inside, will bite during the night even in darkness. Ae. albopictus does not become active until mid spring (mid May) and is most common during the summer months. This mosquito favors laying eggs in artificial containers that are in shady locations and are polluted with some tree debris (leaves, etc.), but it will also breed in tree holes and rock pools. It is often found sharing larval habitats with Oc. triseriatus and Oc. japonicus. Ae. albopictus does not frequently travel more than about 500 ft from its breeding habitat, so if tiger mosquitoes are an identified problem, their breeding habitat is going to be close by. Populations of this mosquito are usually greatest around bushes and foliage nearest to the breeding habitat. Ae. albopictus has been identified as one of the most efficient WNV vectors in the laboratory, but its role in field transmission is still unclear. Ae. albopictus may also be an important vector of LaCross Encephalitis. Ae. albopictus are not readily trapped with CDC traps, so detection of low numbers with a CDC trap may indicate that there are many more in the environment.

Ochlerotatus atropalpus: Oc. atropalpus, known as the "rock pool mosquito" breeds mostly in rock pools (holes in rock outcrops that hold water), but have been known to occasionally breed in artificial containers, away from the rocky stream or river habitats where they are usually found. Like most other container breeding mosquito species, this mosquito feeds during daylight hours and does not venture far from its breeding habitat. They are known to bite humans and can be persistent biters near their habitat. Laboratory trials have shown Oc. atropalpus a to have one of the highest WNV vector competencies, but due to their limited distribution (mostly in places where rock pools occur) few have been captured and tested, and none have been found carrying WNV.

Aedes vexans: Ae. vexans is a floodwater breeder found throughout Virginia. It breeds in a great variety of temporary flood-pools, usually in woodlands or grassy sites (fields, ditches), and can be produced in large numbers in these habitats. Eggs are laid in moist depressions, especially those containing moist organic debris, and hatch when they are flooded by water. Eggs may hatch during the same season in which they were laid, or may also sit for several seasons until stimulated to hatch by floodwater. Several generations of this mosquito can occur each year. Ae. vexans has a flight range of 5 to 10 miles and feeds during dusk and after dark. Laboratory studies have shown it to be relatively inefficient as a WNV vector, but this mosquito can occur in large numbers and was suspected of being an important WNV bridge vector to horses and humans in New Jersey in 2000. Aedes vexans is readily collected in CDC light traps.

Ochlerotatus sollicitans: Oc. sollicitans is a saltmarsh breeder found primarily in coastal Virginia, but may occur anywhere in the state where water habitats and salinity levels are adequate. It is a fierce aggressive biter that can be produced in large numbers in saltmarsh habitats. Oc. sollicitans has a flight range of 5 to 10 miles, but may travel 40 or more miles. It takes feeding flights during dusk and after dark and is strongly attracted to lights, so it may migrate from salt marshes toward nearby towns. However, it will feed at any time during the day when its resting sites are invaded. Laboratory studies have shown it to be a moderately efficient WNV vector, but field studies have also shown that this mosquito feeds mainly on large animals and only a small proportion of its blood meals come from birds. Thus, only a low proportion might be infected with WNV. However, this mosquito can occur in huge numbers and care should be taken to avoid excessive exposure to them. Ochlerotatus sollicitans is readily collected with CDC light traps.

 


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