Ochlerotatus japonicus (Theobald) 

Jamesina J. Scott, Rutgers University/Placer Mosquito

(See also Morphological Comparisons of several Aedes/Ochlerotatus species)

Subgenus: Finlaya

Type of Life Cycle: Multivoltine Aedes, Ae. triseriatus Type

Typical Habitat: Artificial Container, Treeholes and natural rockpools

Larvae Present: All Season

Head Hairs:

Upper: Multiple

Lower : Multiple

Antenna:

Length: Much shorter than head

Tuft: Multiple, very short, inserted at middle of shaft

Abdominal Hairs (Segments III-VI): 2-2-2-2

Comb Scales: Patch

Siphon:

Index: 2.5

Tuft: 4-6, inserted within pecten row 

Pecten: Detached

Anal Segment:

Saddle: Incomplete ring, highly spiculated at distal margin

Precratal tufts: 2

Other: Upper and lower head hairs are arranged in a straight line.  

GEOGRAPHIC DISTRIBUTION: The first North American specimens of Ochlerotatus japonicus were adults recovered from light trap collections in Ocean County, NJ and Suffolk County, NY in September 1998.  The larvae were first discovered in automatic horse watering devices in Ocean County, NJ the following spring.  Presently, breeding populations of Oc. japonicus are known in 18 of NJ's 21 counties.  At the end of 2003, Oc. japonicus had been collected from 19 states in the USA (CT, DE, GA, MA, ME, MD, NC, NH, NJ, NY, OH, PA, RI, SC, TN, VA, VT, WA, and WV) and Quebec, Canada.

SEASONAL DISTRIBUTION: Present all season long.  The earliest recorded larval collection was made on March 6, 2002 in Bergen County, NJ.  The latest larval collection was made on 07 January 2003 in Somerset County, NJ.  In central New Jersey, the adults have been collected with gravid traps from early April through late November. 

LARVAL HABITAT:  The larvae of Oc. japonicus are typically found in small-volume containers of relatively clean, clear water.  They are most often recovered from artificial containers, including bird baths, buckets, plastic milk jugs, wheelbarrows, animal watering containers, and tires.  They have also been collected from natural containers such as treeholes in Sussex County, and rockpools in Hunterdon, Sussex, and Warren Counties.  Within their native range, they are occasionally collected from ground water, and Bergen County has collected Oc. japonicus larvae from standing water in tire ruts.  It has also been collected from cement catch basins in Warren County, NJ and New York.  

COMMON ASSOCIATE SPECIES:  Ochlerotatus japonicus larvae have been found in container habitats with: Aedes albopictus, Anopheles barberi, An. punctipennis, An. quadrimaculatus, Oc. atropalpus, Oc. hendersoni, Oc. triseriatus, Culex pipiens, Cx. quinquefasciatus, Cx. restuans, Cx. salinarius, Cx. territans, Culiseta melanura, Cs. incidens, Orthopodomyia signifera and Toxorhynchites rutilus septentrionalis. More than likely, the larvae of Oc. japonicus will eventually be found with other container breeding mosquito species as its range continues to expand in North America.

LARVAL IDENTIFICATION: There are two major characters which separate Ochlerotatus japonicus larvae from all other North American mosquitoes: its highly spiculated anal saddle, and the upper and lower head hairs which are multiple (tufts) and arranged in a straight line.  

   Ochlerotatus japonicus larvae are relatively easy to separate from associated container species.  The Culex species are easily recognized and can be separated in the dipper by their longer air tubes.  

    Ochlerotatus atropalpus most closely resembles Oc. japonicus  in general body shape, and, using the standard North American mosquito identification keys,  Oc. japonicus will be misidentified as Oc. atropalpus based on their detached pecten teeth and the tuft inserted within the pecten row.  Fortunately, these two species are easily separated under the microscope by their head hairs and the difference in spiculation on the anal saddle as described above.  

    Ochlerotatus triseriatus larvae have a darker coloration, a characteristic serpentine motion and an elongate body shape which are useful in screening field collections but should not be relied upon for separation of early instars.  There are several useful characteristics to quickly isolate the Oc. japonicus larvae from field populations of Oc. triseriatus.  Ochlerotatus  triseriatus has much smaller gills and the ventral pair is considerably shorter than the dorsal pair. The anal gills of Oc. japonicus are much longer than the saddle and are equal in size. Be aware, however, that gills frequently break off in preserved specimens. As a result, gill characteristics are most useful when observing living specimens.  Ochlerotatus triseriatus has a single row of comb scales that are arranged in an extremely irregular fashion, while the comb scales of Oc. japonicus are arranged in a patch. The upper and lower head hairs of Oc. triseriatus are single and arranged in a box-like formation, while in Oc. japonicus they are all multiple and aligned in row.  The lateral hairs on the saddle are useful because they can be observed in living specimens without special orientation. The lateral hairs are very long in Oc. japonicus and 5-7 branched in Ae. triseriatus. 

IMPORTANCE: We do not yet know what the impact of Oc. japonicus will be in New Jersey. It does not seem to be an aggressive human-biting mosquito like New Jersey's other exotic mosquito, Aedes albopictus.  Laboratory studies have show that Oc. japonicus is a very efficient vector of West Nile virus, but its actual role in the natural transmission of this virus has yet to be determined.  It is worth noting, however, that several pools of Oc. japonicus  were positive for West Nile virus during the 2000, 2001, and 2002 surveillance seasons.  This may indicate that Ochlerotatus japonicus could provide an important link between people and West Nile virus in the United States.