Culex restuans Theobald

by Wayne J. Crans, Rutgers University


Subgenus: Culex

Type of Life Cycle: Culex pipiens Type

Typical Habitat: Stagnant pools of ground water, artificial containers, breeds in water that ranges from clear to grossly polluted

Larvae Present: Early spring through late fall

Head Hairs:

Upper: 4-8 branched

Lower : 4-8 branched

Antenna:

Length: Shorter than head

Tuft: Multiple, inserted near middle of shaft

Abdominal Hairs (Segments III-VI): 1-1-1-1

Comb Scales: Patch

Siphon:

Index: 4.0-4.5

Tufts: Represented by 3 irregularly placed single hairs and one small pair of 2-3 branched subapical tufts

Pecten: Evenly spaced on basal 1/3 of siphon

Anal Segment:

Saddle: Complete ring

Precratal tufts: None

 

GEOGRAPHIC DISTRIBUTION: Culex restuans has a distribution that ranges from central Canada south into Mexico. The mosquito is very common in the eastern and central United States. Collection records range west into California but its distribution becomes spotty west of the Continental Divide. Culex restuans is widespread throughout New Jersey and is considered common in every county of the state.

SEASONAL DISTRIBUTION: Culex restuans undergoes a life cycle that is typical for domestic Culex. Inseminated adult females enter hibernation in fall and pass the winter in a period of quiescence. Hibernating females of this species are common in basements, spring houses, outbuildings and subterranean enclosures. The females congregate near moisture and move frequently during the winter to remain in a humid atmosphere. Females that survive the winter emerge from hibernation and begin depositing egg rafts during the month of April in southern New Jersey and in May further north. Populations of this mosquito usually peak by July. A second peak is often evident in the fall to produce the adults for the overwintering generation. Culex restuans larvae remain in suitable habitats throughout the breeding season, usually mixed with one or more associate species. Larvae often persist in some habitats after females enter hibernation and specimens can often be collected well into October.

LARVAL HABITAT: Culex restuans utilizes an exceptionally wide range of larval habitats. The water used by this species can vary from nearly clear to grossly polluted. A partial list of larval habitats includes: temporary ground water, the edge of grassy swampland, sphagnum bogs, road side ditches, tire ruts, hoof prints, discarded buckets, tires, catch basins, sewage effluent and septic seepage. Culex restuans regularly colonizes temporary ground pools that remain flooded after they have produced broods of floodwater Aedes. In this habitat, the water is generally extremely dark and rank. In May, Culex restuans larvae use woodland pools that have produced the spring brood of Aedes canadensis. Later in the summer, the species almost always colonizes shaded ground pools that have produced Ae. trivittatus and Psorophora ferox. Culex restuans is also the first species to utilize water that collects in discarded tires. The species can often be found in tire water that is absolutely clear and devoid of leaf litter. In some cases, a thin bloom of algae provides the only nutrient in this tire habitat. In some cases, larvae may also be present in nearby buckets that emanate a rank odor from rotting vegetation that has collected over the years. In habitats that are rich in organic matter, Cx. restuans can develop from egg to adult in about 10 days, Complete development can be tripled in water that contains limited organic matter and the adults that emerge may be only half the size.

COMMON ASSOCIATE SPECIES: Cx. pipiens, Cx. salinarius, An. punctipennis Cs. inornata, Ae. canadensis

LARVAL COLLECTION: No special techniques are required to collect Cx. restuans. This species is extremely ubiquitous and can usually be found in significant numbers from early spring through late fall. Culex restuans egg rafts will appear in buckets of prepared straw infusions within 24 hrs. Any samples taken from discarded tires are certain to produce representatives of this species in numbers.

LARVAL IDENTIFICATION: Culex restuans larvae look very much like Aedes in the dipper because of their extremely short antennae and solid body structure. If you draw several specimens into a dropping pipette and examine them from the side, air tube length eliminates most of the Aedes sp. as a tentative identification. Under the microscope, the single hairs on the siphon are the easiest character to use for confirmation. Culex restuans is the only Culicine in the northeast with single hairs on the air tube. The only larva that shares this characteristic is Wyeomyia smithii, a Sabethine that completes its larval life cycle within the leaves of Pitcher Plants. It is advisable to confirm the identification by looking at the head capsule after the air tube has been examined. Most Culex sp. have prominent antennae that are constricted toward the tip. Culex restuans is the only Culex in the northeast with short Aedes-like antennae.

REPRESENTATIVE COLLECTION RECORDS

Northern New Jersey

Location: Glen Gardner, Hunterdon Co.

Date: May 10

Habitat: Wheel Barrow

Instar: 1st - 4th & Pupae

Southern New Jersey

Location: Ongs Hat, Burlington Co.

Date: June 3

Habitat : Woodland Pool

Instar: : 1st - 4th & Pupae

 

IMPORTANCE: There is considerable disagreement in the literature concerning the pest status of Culex restuans. Most authors consider the mosquito to be a bird feeder that rarely, if ever, bites humans. Others, however, have described the species as a significant pest with an annoying bite. It is possible that some of the confusion is the result of misidentification of adults in the bite counts. A large proportion of adult female Cx. restuans lack the white spots on the thorax that most keys use to separate this species from Culex pipiens. Accurate identification requires holding females that have blood fed on humans and allowing them to oviposit. Identification to species would require hatching the egg rafts and looking for larval characteristics. It is just as doubtful that competent culicidologists like Dyar, McLintock and Barr could have misidentified Cx. pipiens or Cx. molestus as Cx. restuans. Under certain circumstances, this mosquito appears to accept humans as a blood meal host to the point where it can function as a pest. In most cases, however, Cx. restuans is not attracted to humans and the species is not regarded as a significant nuisance. Eastern equine encephalitis virus (EEEV) is occasionally isolated from Cx. restuans at study sites that are monitored for virus activity in New Jersey. It is possible that this species plays a minor role as a secondary vector of EEEV and accelerates transmission among birds during periods of virus amplification.

 

 


Center for Vector Biology