Reproduced from the Proceedings of the 76th Annual Meeting of the NJMCA. Please use the following citation when referring to this article:
Romanowski, M. 1989. Aedes dorsalis in New Jersey: larval habitat and identification. Proc. N. J. Mosquito Control Assoc. pp. 58-62 .
INTRODUCTION: Aedes dorsalis (Meigen) is a rare mosquito in New Jersey but has distribution that extends over the greater portion of North American northern Europe and into Asia (Rees and Nielsen 1947). In the United States, the mosquito reaches greatest abundance from the plains states to the Pacific coast. In the east, it has been reported in lesser numbers across the Great Lakes states to the east coast states of Massachusetts, Connecticut and New Jersey.
Aedes dorsalis was first reported from New Jersey by Burbutis and Lake (1959). The record was presented by a single adult female that was collected by light trap in Passaic County on July 26, 1957. The first specimen was believed to be a migrant after attempts to collect larvae and adults failed. Since that time, adults have been collected in Bergen, Cumberland, Essex, Monmouth, Morris, Ocean, Somerset and Sussex Counties. To date, Ae. dorsalis larvae have only been collected from Cumberland and Ocean counties.
BIOLOGY OF Aedes dorsalis: Ae. dorsalis was classified by Pratt (1959) as having a "Temporary Pool" type of life cycle. Females produce an average of about 130 eggs which are deposited in 1-6 batches in the moist soils of shallow depressions that are exposed to periodic flooding. The majority of females live less than 90 days and males rarely live for more than 30 days (Rees & Nielsen 1947).
Aedes dorsalis overwinters in the egg stage and the eggs hatch after flooding during the first warm weather in the spring. Hatching has been shown to be dependent on a reduction of dissolved oxygen within the breeding pool (Gjullin et al. 1941). Egg hatch is dependent upon the reflooding of the larval habitat, thus, breeding can be continuous throughout the warm season. In Utah, 10 successive broods in a single season were common (Rees & Nielsen 1947). The eggs of Ae. dorsalis require partial dryness for a period of at least 5 days before hatching. Some do not hatch on the first flooding, providing eggs that will hatch during subsequent flooding (Rees & Nielsen 1947).
Ae. dorsalis is well known for its capacity to migrate long distances. The adults are strong fliers and have been traced for 22 miles in Utah and more than 30 miles in California (Rees & Nielsen 1947). As a result, the mosquito has been recognized as a chance migrant in some areas of its range. The adult is regarded as an important pest species in western North America and avidly feeds on humans and domestic animals. According to Rempel (1950), Ae. dorsalis is the most annoying pest species in western Canada. The bite of the female has been described as vicious, and the mosquito will attack any time of the day or night. Peak activity occurs toward evening or during calm, cloudy days (Carpenter & LaCasse 1955). Periods of high biting activity have also been observed when weather conditions are humid (Rees & Nielsen 1947).
Man and other large mammals seem to be the preferred host of Ae. dorsalis, however, the mosquito will feed on large birds if no mammalian hosts are available. If a female is interrupted taking a bloodmeal, it will repeatedly return to the same or other nearby host until feeding is complete. Ae dorsalis is also a species of significant medical importance having been implicated as a vector of both California encephalitis and western equine encephalitis (Carpenter & LaCasse 1955). In the western United States, the reproductive cycle of Ae. dorsalis begins in March and concludes with the advent of freezing weather in October or early November. Adults are most active at temperatures ranging from 50-95 degrees F with activity ceasing at temperatures below 42 degrees F (Rees & Nielsen 1947).
The length of time required to complete the larval and pupal stages is temperature dependent. A mean temperature of 75-80 degrees F is optimum for rapid development. In this range, growth from egg to the emergence of the adult can be completed in 51/2 -7 days. As the temperature declines, the growth period lengthens correspondingly (Rees & Nielsen 1947).
LARVAL HABITAT OF Aedes dorsalis: Aedes dorsalis larvae occur in a variety of habitats including both brackish and freshwater. They are found in large numbers on tidal marshes of the Pacific coast. The species is also common along the margins of the Great Salt Lake in Utah.
Aedes dorsalis can be found in a variety of freshwater habitats including marshes, temporary pools formed by precipitation, natural springs and irrigation water. The larvae have also been found in waste lagoons in Indiana and seepage from a pulp lagoon in Wisconsin (Siverly 1972). Aedes dorsalis larvae are also common in saline water situations. In Utah, it was found in saline pools with as high as 12% salt content (Rees & Nielsen 1947). In New York, larvae have been collected from tidal salt marshes, creek beds and semipermanent pools near salt mines and roadside ditches and ponds containing salt from roadway ice removal during the winter (Means 1981). The larvae of Ae. dorsalis are most often found in shallow grassy water exposed to direct sunlight but they occasionally occur in densely shaded pools (Carpenter & LaCasse 1955). Breeding habitat varies greatly in size. They have been found in depressions as small as cattle hoof prints and as large as shallow marshes encompassing 10,000 acres (Rees & Nielsen 1947).
The larvae Ae. dorsalis have been found in association with numerous other mosquito species. In Utah, the species has been found breeding with 18 other species of mosquitoes including Aedes vexans, Culex tarsalis and Culiseta inornata (Rees & Nielsen 1947). In New York, under saline conditions, the larvae have been found with Aedes sollicitans.
Two counties, Cumberland and Ocean, have identified the larval habitat of Ae. dorsalis in New Jersey. In Cumberland County, the larvae were found in a Spartina patens marsh that was used for salt hay farming. In Ocean County, the larvae were found in a small section of a S. patens marsh that was partially surrounded by housing developments. The microhabitat consisted of several small clear pools that were interdispersed with tussocks of S. patens. No larvae were found in the old grid ditches present on the site.
An obvious question we attempted to answer was; What do these sites have in common that would make them better suited for a rare species like Aedes dorsalis in our geographic area? A possible answer has to do with changes in the salinity of the larval habitat. Both sites were S. patens marshes that had been altered to allow intrusion of freshwater onto a salt water marsh system. At both sites, barriers were created that reduced tidal activity. In Cumberland County, a man-made dike was created encircling a portion of salt marsh. In Ocean County, the construction of roads and plugging of bayfront ditches blocked tidal flow. In each case, a section of marsh with lower than normal salinity was created, which in turn, produced more favorable Ae. dorsalis habitat.
The degree of dry-down in the habitat where Ae. dorsalis larvae were found may also be a possible answer to the question. The tidally blocked areas where Ae. dorsalis breeding was documented experience fewer periods of complete dry-down than a typical salt marsh. This "puddled" situation is probably less favorable for other salt marsh Aedes sp. and may have allowed for colonization of a mosquito like Ae. dorsalis that is normally found over a wider range of habitats.
SURVEILLANCE FOR Aedes dorsalis: Since Ae. dorsalis has a propensity to bite at any time of the day or night and will migrate long distances, the species should be readily collected in the Landing Rate and Bite Counts that are employed in most surveillance programs. Adults can also be collected in light traps but they are not as positively phototrophic as other species of Aedes (Rees & Nielsen 1947).
Identification of adult females can be done with relative ease. A key characteristic to look for is pale bands on the tarsus that extend across the joints of the tarsal segments. Aedes canadensis exhibits this characteristic; however, it is a darker mosquito without the mixed pale and dark wing scales of Ae. dorsalis (Stojanovich 1961).
Larval collection is slightly more difficult. Later instar larvae are easily driven to the bottom of a pool where they often remain still for an extended period. Larvae are most easily collected by waiting for them to return to the surface which usually happens within five minutes (Means 1981).
Field identification of larvae is considerably more difficult and all suspect samples should be brought back for closer examination. However, if found with Ae. sollicitans larvae, the noticeable difference in airtube length should be obvious enough to warrant further examination.
CONCLUSIONS: Since Ae. dorsalis seems to be a marginal species in New Jersey, special efforts are required to collect this species. Findings to date indicate that two distinct larval habitats appear most favorable for collection. In coastal areas, look for a high salt marsh area in which the normal tidal flow has somehow been interrupted. These include salt hay farms with dikes and marshes isolated by natural berms or man-made developments. In upland areas look for ditches or ponds that have been artificially inundated with salt. Ditches that are adjacent to heavily salted roads may provide suitable habitat as would wet areas near a road salt storage facility.
The low populations of Ae. dorsalis in New Jersey make routine adult surveillance procedures an unlikely method for location of specimens. Adult surveillance efforts are, therefore, best centered around suspect larval habitats. With this in mind, counties that have not yet located Ae. dorsalis may eventually be able to include this lesser known mosquito to their species list.