Aedes stimulans (Walker)

by Wayne J. Crans, Rutgers University


Subgenus : Ochlerotatus

Type of Life Cycle : Univoltine Northern Aedes

Typical Habitat : Woodland Pools

Larvae Present : Very Early Spring,

Head Hairs

Upper: Double (Often, one is single)

Lower: Single (Rarely double)

Antenna

Length: Less than half as long as head

Tuft: Small, multiple, inserted near middle of shaft

Abdominal Hairs (Segments III-VI) : 2-2-2-1

Comb Scales : Patch

Siphon

Index: 3.0 - 3.5

Tuft: 3-4 Hairs, inserted beyond pecten

Pecten: Evenly spaced

Anal Segment

Saddle: Incomplete

Precratal tufts: 2-4

Other : Saddle spiculate along posterior border

GEOGRAPHIC DISTRIBUTION: Aedes stimulans is a univoltine Aedes of the northeastern United States and southern boundaries of eastern Canada. Its range extends from Newfoundland south to Maryland, west to eastern Kansas and north to the upper Great Lakes region. A cryptic species, Aedes mercurator, previously reported as Ae. stimulans, replaces the species in the western United States, Northwest Territory of Canada and southwestern Alaska. Aedes stimulans is widely distributed in New Jersey but is most common in the northern third of the State. Scattered populations have been reported from most of the southern counties but the species cannot be considered common in any area of the Pine Barrens.

SEASONAL DISTRIBUTION: Aedes stimulans is a univoltine species with a typical northern Aedes life cycle. The single generation hatches from overwintering eggs in very early spring and the larvae develop in cold water habitats. This is one of the first mosquitoes to appear in northern New Jersey, preceded only by Aedes communis and Aedes provocans. Aedes stimulans shares habitat with these snow pool Aedes in the extreme northwestern corner of Sussex County, but is generally about one instar behind. If Aedes canadensis is also present, the smaller species is two or more instars behind. Aedes stimulans larvae are usually present by late March in woodland pools within Stokes State Forest and High Point State Park. Pupation at these higher elevations takes place toward the end of April. Further south, in Morris, Hunterdon and Warren counties, the populations are about a week ahead. Populations that develop in the Pine Barrens pupate in early April. Aedes stimulans females seek a host during the month of May. The pools that produced the single generation dry down quickly as the surrounding trees leaf out and females lay a band of eggs around the perimeter. Embryos within the eggs of this univoltine species enter diapause and do not hatch if the pool refloods with summer rains. The onset of cold breaks diapause and the eggs hatch in very early spring to initiate the next generation.

LARVAL HABITAT: Aedes stimulans has an affinity for shaded woodland pools with a lining of heavy leaf litter. Temporary pools formed by spring overflow of rivers and streams can be very productive. Drainage ditches that are cut through forested areas are another habitat utilized by this species. The deep snowpools filled with dark colored water that are favored by Ae. communis usually have minimal numbers of Ae. stimulans. When the two species coexist, the water is generally shallower and covered with a heavier lining of leaves. Aedes stimulans larvae favor shade and frequently congregate in the darker portions of the larval habitat.

COMMON ASSOCIATE SPECIES: Ae. communis, Ae. excrucians, Ae. canadensis

LARVAL COLLECTION: The larvae of Ae. stimulans are usually very abundant and the species is fairly easy to collect in numbers. On a bright day, large numbers of larvae can be seen in shallow pools and collecting can be selective. Walking slowly through deeper pools with the dipper submerged allows the collector to watch for larvae that are swimming beneath the surface. Fourth instar Ae. stimulans larvae favor the more shaded areas of the pool and may all be congregated at one end. Sampling one small portion of the larval habitat can result in no specimens even though collections of 30 or more per dip are possible from a nearby shaded area. Late instar congregations of this species have been shown to move about the pool to compensate for the changing position of the sun. Microsporidian infections are common in 4th instar larvae which take on a lumpy appearance as a result of the parasite.

LARVAL IDENTIFICATION: The larvae of Ae. stimulans are fairly difficult to identify with a key because most keys do not separate the species out until the last several couplets. This requires observing extensive characters, many of which are difficult to discern. Aedes stimulans larvae resemble Ae. communis and both key out at the very end of the key. The two species occupy similar habitat, although Ae. stimulans are normally one or more instars behind in their development. Under the microscope, both species have either single or double head hairs, comb scales in a patch, evenly spaced pecten teeth and an incomplete saddle. The gills of Ae. communis larvae almost always have a characteristic rusty-red color which may be evident in the dipper. The gills retain their color after preservation but gills are fragile and more often than not, break off in preserved specimens. Most keys use the spicules on the saddle as the major character to separate these two species. Aedes stimulans has well developed spicules on the posterior portion of the saddle. The saddle of Ae. communis is almost smooth. Proper placement of the specimen is essential to accurately assess the presence or absence of spicules. Spicules are tiny teeth that protrude from the saddle. In most cases, they cannot be seen by looking directly down on the saddle. If the specimen is positioned on a white background with the siphon pointing directly up, the spicules on the posterior margin of the saddle stand out as sharp little teeth. They are very obvious in Ae. stimulans and totally absent in Ae. communis. As a result, the posterior margin of the saddle is raggedly toothed in Ae. stimulans and completely smooth in Ae. communis.

Aedes stimulans possesses a character that does not appear in any key. New Jersey specimens almost always have one upper head hair single and the other double. Although this characteristic cannot be considered diagnostic, if a substantial percentage of the collection has the 1-2 upper head hair pattern, there is a very good chance that the collection is comprised of Aedes stimulans rather than Ae. communis or Ae. grossbecki (the other larva that appears at the end of most keys).

REPRESENTATIVE COLLECTION RECORDS

Northern New Jersey

Location: Montegue, Sussex Co.

Date: April 20

Habitat: Flooded Woodland

Instar: 4th

Northern New Jersey

Location: Hackettstown, Warren Co.

Date: April 22

Habitat: Woodland Pool

Instar: 4th & Pupae

Central New Jersey

Location: Great Swamp National Refuge, Morris Co.

Date: April 25

Habitat: Woodland Pool

Instar: 4th & Pupae

IMPORTANCE: Aedes stimulans is recognized as one of northern New Jersey's most important early season pests. The mosquito bites avidly in the woodlands close to its larval habitat but will also enter communities in numbers in quest of a blood meal. Housing developments that are built in wooded settings have severe problems if the woodlands harbor breeding habitat for this species. In areas near flood plains, the species is joined by Aedes sticticus to cause added annoyance. Aedes stimulans is exceptionally long lived and can be collected in light traps into the month of August. Because of its longevity, the mosquito is considered a probable vector of dog heartworm in northern New Jersey.

 


Center for Vector Biology