ABSTRACT: There are a variety of factors that influence eastern equine encephalitis (EEE) epizootics and the maintenance of eastern equine encephalomyelitis virus (EEEV) in bird populations. Water temperatures in the habitat of immature Culiseta melanura, the enzootic vector of EEEV, substantially influence the emergence patterns of this mosquito. It is hypothesized that the timing of emergence explains the susceptibility of certain geographical areas to EEE epizootics. The purpose of the study was to compare the population dynamics of Cs. melanura in areas that frequently demonstrate EEEV activity versus areas in which EEEV activity is infrequent. Our objectives were to compare the seasonal population dynamics of Cs. melanura at 2 widely separated geographic areas, to determine the number of generations completed at each geographic area, and to correlate the timing of emergence peaks with nesting patterns in local birds.

There are a variety of factors that influence EEE epizootics in the northeastern region and the maintenance of viable EEEV in bird populations along the Atlantic flyway. Culiseta melanura (Coquillett) has been documented as the primary enzootic vector of EEEV (Crans et al. 1994, Crans et al. 1986, Nasci and Edman 1981, Maloney and Wallis 1976, and others). Larval Cs. melanura inhabit permanent freshwater swamps along the entire eastern seaboard. Culiseta melanura is one of the few mosquitoes that overwinters in its larval stage. Growth rate of the immatures is temperature dependent and larvae take months to mature even at warm temperatures. Overwintering larvae emerge as water temperatures moderate in April and May. Emergence is earliest in the southern distribution of this species and occurs later in areas to the north.

As the season progresses and water temperatures increase in the subterranean larval habitats, a second peak in emergence takes place during the summer. It has been suggested that Cs. melanura completes three (Morris et al. 1976) to four generations in a year (Nasci and Edman 1984). Heat summation models developed by Mahmood and Crans (1998), however, suggest that Cs. melanura larvae develop at a rate more apt to produce only two generations of adults per year.

Epizootic transmission of EEEV appears to involve three critical phases: 1) recrudescence of virus in resident birds, 2) amplification of virus within local bird populations, and 3) transfer of virus to mammalian hosts via bridge vectors. In the spring, EEEV recrudesces in birds (Crans et al. 1994, Crans et al. 1986) and is transferred to Cs. melanura adults. Virus circulates between susceptible bird hosts and Cs. melanura adults, and is amplified within bird populations during the summer. Over the course of the summer, various other mosquitoes serve as bridge vectors, which transfer EEEV from birds to mammalian hosts. The mosquitoes Coquillettidia perturbans (Walker) and Aedes sollicitans (Walker), in particular, have been implicated as bridge vectors of EEEV (Crans et al. 1994).

Adult emergence and voltine nature of Cs. melanura play an important function in the amplification of EEEV in birds. An abundance of nulliparous adult Cs. melanura and the presence of hatchling year (HY) birds (susceptible bird hosts) are limiting factors for successful amplification. The timing of Cs. melanura adult emergence is a function of water temperatures that exist during larval development. It is hypothesized that cool water temperatures in northern areas of this mosquito's distribution delay emergence of adult mosquitoes and preclude nulliparous Cs. melanura from making contact with resident HY birds. In short, temperature regulates the northern limits for EEEV amplification each year.

Culiseta melanura populations are being monitored at two locations. The northern sampling site is located in High Point, New Jersey (fig 1). The southern site is located in Dennisville, New Jersey. Cape May County has a history of horse cases, and virus isolations in adult Cs. melanura appear frequently in late summer. A population of Cs. melanura exists in an area of white pine/spruce trees and cedar swamp that is part of Belleplain State Forest. The High Point site is a cedar swamp located at high elevation and should be representative of habitats that occur in much of New England. The Dennisville site is decidedly southern and representative of habitats in Delaware, Maryland and northern Virginia.

Adult Cs. melanura were collected from 25 resting boxes at both locations on a weekly basis from mid April through October, 1998. Parity status of adult Cs. melanura (blood fed, unfed, and gravid females) is in the process of being determined using the methods of Mahmood and Crans (1998) and Detinova (1962), and is summarized by the following. Ovaries from adult female Cs. melanura are dissected with fine point forceps. Individual ovarioles are removed and stage of follicular development is recorded. In unfed females, parity is determined by drying an ovary and observing the condition of tracheal skeins. In blood fed females, parity is determined by the presence or absence of true dilatations on the follicular pedicels. Parity status is ascertained in gravid females by the presence of true dilatations on degenerating follicles that are found in ovarian tissue. From this information an overall parity profile will be developed, and comparisons will be made between northern and southern populations.

The intent of this study is to evaluate the impact that habitat temperature has on the population dynamics of Cs. melanura. A great deal of geographic variation exists between the northern and southern-most areas of the northeast. It is hypothesized that water temperatures in crypt habitats regulate the emergence patterns of this species and indirectly impact the amplification of EEEV on a geographic basis. In the southern range of this mosquito's distribution, the summer generation emerges when recrudescing virus is present in adult birds and when recently fledged juveniles are locally abundant. In the northern range of this mosquito's distribution, the summer generation emerges later in the season and makes contact with migrating birds that are unsuitable for sustained local amplification. If this hypothesis is correct, water temperatures in underground crypt habitats regulate the northern limits for EEE activity each year. This would explain why EEEV makes contact with birds and equines virtually every year in southern New Jersey and occurs sporadically in areas to the north.

The author would like to thank John Keator, Superintendent of High Point State Park; Scott Crans, Superintendent, Sussex County Department of Health and Public Safety; and Jim McNelly of the Cape May County Mosquito Extermination Commission for their assistance. The author would also like to thank Dr. Farida Mahmood for her laboratory assistance, as well as Eugene Fuzy and Marco Vargas for their efforts in the field. This is New Jersey Agricultural Experiment Station Publication No. E-40400-02-99 supported by State funds and funds from the New Jersey State Mosquito Control Commission.

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