Reproduced from the Proceedings of the 77th Annual Meeting of the NJMCA. Please use the following citation when referring to this article:
O'Malley, C. M. 1990. Aedes vexans (Meigen): An old foe. Proc. N. J. Mosquito Control Assoc. pp. 90-95.
CLAUDIA M. O'MALLEY
Burlington County Mosquito Extermination Commission, 49 Rancocas Road, Mount Holly, New Jersey 08060
INTRODUCTION: Aedes vexans (Meigen) is one of the most widespread pest mosquitoes in the world. Its distribution includes Nearctic and Palearctic regions, the African west coast and Oriental regions south and east to Samoa (Horsfall, 1972). In North America, it is common in southern Canada and is found throughout the United States, with the exception of Hawaii. Aedes vexans has been recognized as an important pest mosquito for quite a long time, although under a variety of names. In its original description in 1830 it was named Culex vexans, and in 1904, John B. Smith referred to it as Culex sylvestris, the "swamp mosquito" (Carpenter and LaCasse, 1955; Headlee, 1945). More recently, in an AMCA Newsletter, the regional directors were polled as to which mosquito species constituted the most serious pest in their area. Without exception, Ae. vexans was given that dubious honor (Russo, 1977). Here in New Jersey, Ae. vexans is considered to be the most important nuisance mosquito in Burlington and Middlesex Counties, and is of major importance in Bergen, Cape May, Ocean, Salem and Warren Counties. In fact, in the coastal counties only Ae. sollicitans and Cx. salinarius outrank Ae. vexans in importance, and in Warren County, Ae. trivittatus is the only mosquito considered to be more important as a pest species.
BIOLOGY: Aedes vexans overwinter in the egg stage. Eggs are laid singly in sites subject to inundation by rain water, overflow, seepage or tidal water. There is some, but not conclusive, evidence that eggs are sometimes laid directly on water (Headlee, 1945). It is known that Ae. vexans eggs need some moisture for embryonation. The selection of suitable oviposition sites by Ae. vexans greatly affects its survival. Females place their eggs directly on the soil at sites which are (1) likely to retain sufficient moisture for successful embryonation and (2) likely to be inundated at some future date (Strickman, 1980). Ovipositing females select suitable sites by means of cues based on a number of environmental factors. One of these is moisture in the soil. According to Horsfall (1975), substrate moisture is probably the single most important factor acting as an oviposition attractant. Eggs are laid on soil that is moist but not water-logged, in a zone above the water table.
Another environmental factor that affects selection of oviposition sites is that of dense cover over the soil. Detritus, piles of twigs and low herbal canopy are usually associated with the greatest numbers of eggs within an oviposition site (Horsfall et al., 1973).
Eggs of Ae. vexans hatch when inundation of the oviposition site occurs; however, they do not all hatch at one time. Also, eggs must go through a period of drying before flooding, in order for hatching to take place. Most of the eggs of one laying will hatch after the first flooding, but some remain for the second and subsequent floodings. In fact, Ae. vexans eggs have been found to survive in numbers for three years when kept moist (James and Harwood, 1969). Hatching is due directly to a reduction of the dissolved oxygen content of the water. Bacteria, yeast and other organisms stimulate Ae. vexans eggs to hatch by reducing the water's level of dissolved oxygen. This may contribute to the survival of the species, since natural water with a low content of dissolved oxygen would have a relatively large number of bacteria, and thus, an ample supply of food for the hatching larvae (Bates, 1970).
In New Jersey, the first appearance of Ae. vexans larvae varies considerably throughout the state. In Burlington County, Ae. vexans are routinely collected in early April, and sometimes in late March, depending upon climatic conditions. Middlesex and Warren Counties also report collecting this species in early April, and Ocean County usually records its first appearance in early to mid-April. Salem County reports the first appearance of Ae. vexans to be in mid-May, and Bergen and Cape May Counties do not see this species in any appreciable number until late May. Larvae are present throughout the state well into October.
The larvae of Ae. vexans are found in a wide variety of habitats. Larval development varies according to the time of year when the egg hatch. During the summer months, a six- to eight-day period is require for larval development. Ae. vexans larvae feed by grazing and filtering, and probably live on a variety of materials. Larvae develop normally in a relatively dispersed state, but are often greatly concentrated as result of receding water. Since all Ae. vexans eggs do not hatch at the same time, it is sometimes possible to find young larvae (i.e. first or second instar) present within the same site as pupae, especially if reflooding of the habitat has taken place. This author has observed than those larvae hatching in late September and October are usually much smaller in size than those found earlier in the breeding season.
The pupal stage of Ae. vexans is unremarkable. As with larvae, this stage varies in length according to temperature, but usually only last two to three days during the summer (Headlee, 1945).
Adult Ae. vexans feed on the nectar of flowers and, in the case of the female, blood. Nectar feeding occurs any time after emergence. Blood is apparently taken from whatever hosts are available, and blood feeding begins the second day after emergence and is very marked on the third day (Horsfall, 1972). Ae. vexans is a serious nuisance pest. Females will feed in shady places during the day; however, they are very active a dusk and vigorously seek blood meals at this time. Peak activity appear to be thirty to forty minutes after sunset (Thompson and Dicke, 1965) Female Ae. vexans bite readily, but not very viciously, and the result are less painful than from either Ae. trivittatus or Ps. ferox.
In New Jersey, emergence of Ae. vexans usually begins in mid-May and adult populations normally reach nuisance levels early in June Adult activity persists through September and well into October, when autumn temperatures remain warm. The average life span of adult Ae vexans in nature is three to six weeks (Horsfall et al., 1973). Various experiments involving staining to determine longevity produced female Ae. vexans collected as long as 55, 104 and 113 days after staining (James and Harwood, 1969).
Male Ae. vexans form swarms after sunset in meadows, fields an woods. Females approach the swarms singly, and copulation takes place on the wing. The Canadian form of this species produces from 108 to 182 eggs for deposition at one time. These are deposited singly and in rapid succession. They are white on first appearing, but soon turn a shade of steel blue. Embryos are usually fully developed in eight to ten days (Horsfall, 1972). There are several generations, or broods, per year.
Ae. vexans is a mosquito which disperses for considerable distances from its breeding sites. Rees found that this species has a flight range of five to eight miles in Utah, Hearle claims that in British Columbia it migrates a distance of ten miles (Carpenter and LaCasse, 1955), and in New Jersey, Headlee (1945) noted movements of this species of five, ten and even fifteen miles from its breeding site. Thus, nuisance from this species can be quite widespread. This is in contrast to Ae. canadensis, which remain localized around their breeding site, and also remain for much longer periods of time.
LARVAL HABITATS AND ASSOCIATED MOSQUITO SPECIES: As mentioned earlier, Ae. vexans is one of the most widespread pest mosquitoes in the world. This could be due, in part, to the wide range of habitats it utilizes. Here in New Jersey, Ae. vexans larvae are collected from a number of different habitats. Among these are: sheet water or open rain pools, tire ruts, stormwater management facilities (this includes detention, retention and infiltration basins), dredge spoil sites, salt marsh impoundments, ditches, areas in which streams or creeks have flooded over their banks, flooded woodlands, around the edges of semipermanent swamps and bogs that are subject to some drying down, and woodland pools or any type of temporary rain pool. Larvae do not seem to exhibit a marked preference for either sunlight or shade within these habitats.
As can be expected from such an extensive list of larval habitats, there is a variety of mosquito species found in association with Ae. vexans. In New Jersey these include: Ae. canadensis, Ae. cantator, Ae. sollicitans, Ae. sticticus, Ae. stimulans, Ae. trivittatus, An. bradleyi, An. punctipennis, An. quadrimaculatus, Cx. pipiens, Cx. restuans, Cx. salinarius, Ps. call and Ps. columbiae.
In the northern United States and Canada, Ae. vexans is also associated with nearly all of the early-season Aedes, and in Virginia, it has been associated with Cs. melanura (Horsfall, 1972).
SURVEILLANCE: The light trap is a very effective tool for sampling adult Ae. vexans, as is the CO2-baited CDC trap. Since it is such a ready biter, landing rates and bite counts are also a good means of sampling adult female populations. However, if these are to be carried out during daylight hours, the collector should be sure to remain within shaded areas. Larval Ae. vexans are easily collected, when the proper dipping technique for each particular habitat type is employed.
IDENTIFICATION: Identification of Ae. vexans is relatively easy in both the larval and adult stages. Larval Ae. vexans are characterized by the anal segment being incompletely ringed by the saddle, pecten teeth detached distally and antennae which are shorter than the head. Upper head hair 5 has three or more branches and the upper, lower and preantennal head hairs form a triangle-they are not inserted in a straight line, as is the case with Ae. cinereus. Finally, in Ae. vexans larvae, the comb of the eighth segment consists of eight to 16 scales in an irregular single or double row, rather than a patch.
Adult identification is similarly uncomplicated. This is a medium-sized species. Tarsi have white bands on some or all of the segments, and these bands are the base of the segments only. The proboscis lacks a pale band, and is of uniform coloration. The white bands on the hind tarsi are narrow, less than 1/3 the length of the tarsal segment. This last is probably the key identification feature-the third, fourth and fifth abdominal segments are dark-scaled, with basal white bands with a v-shaped notch posteriorly; the seventh abdominal segment is usually completely dark-scaled (Siverly, 1972).
ROLE IN DISEASE TRANSMISSION: In addition to being a widespread pest species, Ae. vexans has also been implicated in the transmission of several important diseases. Lewandowski et al. (1980) determined that Ae. vexans is a suitable host for Dirofilaria immitis, and it appears to be one of the primary vectors of dog heartworm in central Michigan. In 1976 in Maryland, Jankowski and Bickley found 68% of Ae. vexans that fed on an infected dog to develop infective stage larvae of D. immitis, and consider this species to have high vector potential.
In the case of western equine encephalitis, transmission has been obtained with Ae. vexans in laboratory experiments; this has also occurred with St. Louis encephalitis. Laboratory transmission of eastern equine encephalitis has also been obtained with this species (Bates, 1970). This virus is most consistently isolated from Cs. melanura, but has also been found in field collected specimens of Ae. vexans. In 1981, an EEE virus strain was recovered from Ae. vexans in Michigan (Encephalitis Surveillance, 1981). The 1959 outbreak of EEE in New Jersey was associated with heavy rainfall in July and high temperatures resulting in extremely large mosquito populations. It was suggested that Cs. melanura was the primary sylvan vector bringing virus to epidemic centers. Ae. sollicitans was the primary vector in coastal areas where most human cases occurred, and Ae. vexans was largely responsible for inland equine outbreaks (James and Harwood, 1969).
In conclusion, I would like to briefly mention something which could be taken into consideration when planning control strategies for Ae. vexans. Earlier, soil moisture was mentioned as being one of the primary factors in determining Ae. vexans oviposition sites. This knowledge may be used to advantage by mosquito control agencies. Following heavy rains that flood more sites than the agency can treat, an examination of previous rainfall records and of characteristics of individual sites may aid in determining which locations are likely to produce the most larvae. If, during the last opportunity females had to lay eggs, rainfall was scarce or fell over a short period of time, then the drier sites (i.e., those more elevated, with less detritus and canopy) would probably contain few eggs. If, on the other hand, rain fell during a number of days while gravid females were present, even the driest sites would probably have been moist enough to receive a large proportion of the eggs deposited (Strickman, 1980). Using this information, mosquito control personnel would know which particular sites were likely to contain the most Ae. vexans eggs, and personnel and equipment could be deployed in a more efficient manner.