EVIDENCE FOR A BI-VOLTINE LIFE CYCLE IN CULISETA MELANURA

INTRODUCTION

Culiseta melanura is a bird feeding mosquito that serves as the primary maintenance vector of eastern equine encephalitis virus(EEEV) in the Atlantic flyway. The mosquito has an affinity for acid water and develops as a larva in subterranean crypts (Orrel 1997). Crans and McNelly (1997) used Cs. melanura as a model for a life cycle type that lays non-desiccation resistant eggs directly on water, develop as a larva in freshwater swamp habitats, have multiple generations each year and overwinter in the larval stage. Mahmood and Crans (1998) followed the life cycle of Cs. melanura under controlled conditions in the laboratory and obtained evidence to suggest that the species is probably bi-voltine, rather than multi-voltine in most of the northeastern United States. This paper summarizes the results of those studies and shows how bi-voltinism may regulate EEEV activity each year.

RECRUDESCENCE OF INFECTIOUS VIRUS IN BIRDS

The overwintering mechanism(s) for EEEV remained an enigma for many years. Crans et al. (1994) sampled birds and mosquitoes simultaneously at a site in southern New Jersey and found that virus repeatedly appeared in birds before it cycled in mosquitoes. They hypothesized that EEEV probably overwintered as a latent infection in previously infected resident birds and recrudesced in spring and early summer during the stress of the nesting season. They also hypothesized that an influx of newly emerged Cs. melanura was needed in the presence of non-immune juvenile hosts to initiate amplification in any geographic area. Their studies did not suggest a mechanism that would explain the appearance of large numbers or nullipars at any point in the season.

AVIAN NESTING IN THE NORTHEAST

In most of the northeastern United States, summer resident bird species (Wood Thrush, Catbird) return from their wintering sites in the south in late April and early May to join permanent resident bird species (Cardinal, Chickadee, Tufted Titmouse) that remain on site all winter long. These resident species nest during the month of June and fledge their young by early July. The juveniles remain on site in large numbers until September when migration takes place. The summer residents then join large numbers of migrants that fly south to their overwintering sites. At the same time, the permanent resident bird species that remain on site are joined by winter resident birds that nested and fledged their young further north. Crans et al. (1994) showed that recrudescence of latent virus took place as early as May or June, but amplification never occurred until July at the earliest. This meant that there appeared to be a 1-2 month lag in the cycle between appearance of virus and amplification in juveniles. At no time, during their investigations, did virus appear in Cs. melanura during the month of June.

EVIDENCE FOR A BI-VOLTINE LIFE CYCLE IN CS. MELANURA

Mahmood and Crans (1998) showed that Cs. melanura was a mosquito with an exceptionally slow metabolism. Egg hatch to pupation takes 1 month at 79^o F, 3 months at 61^o F and 7 months at 50^o F. The species had a thermal minimum that approached 50^o F but was able to survive temperatures well below that range in a state of torpor. The mosquito develops significantly slower than other species and appears well suited to the cold water crypts that make up its primary breeding habitat. Burbutis and Lake (1956) took water temperatures from Cs. melanura crypts in New Jersey and found that they remained below 50^o F from November to April and rarely exceeded 60^o F during the summer. Joseph and Bickley (1969) made similar measurements in Maryland and showed variations of only several degrees in that area to the south. Using the models of Mahmood and Crans (1998), Cs. melanura would take 3 months to develop in those cold water habitats. Eggs laid from adults that emerged from overwintering larvae in May would not produce adults until August. Eggs laid in August would overwinter as larvae and not emerge until the following spring. This would allow only 2 generations per year and result in a bi-voltine life cycle.

IMPACT OF A BI-VOLTINE LIFE CYCLE

Considering the structure of the avian nesting season in the northeast, the 2 generations of Cs. melanura would encounter very different bird populations. The spring generation of Cs. melanura that emerge from overwintering larvae would emerge to feed on adult birds that are in the process of setting up nesting territory. Adult birds are poor amplification hosts because most already possess immunity. As a result, the spring generation would encounter recrudescing virus in the absence of susceptible amplification hosts. The summer generation emerges after large numbers of juveniles have joined the population. As a result, virus could race through the population just before the fall migration takes place. If the summer generation of Cs. melanura emerges late (cold water temperatures in the crypts), it would encounter a migrating bird population. This would negate virus amplification on site by constantly moving the virus southward. As a result, temperature appears to regulate the northern limits for virus amplification each year. In geographic areas where 2^nd generation of Cs. melanura encounters large numbers of juvenile birds that remain on site, amplification of virus takes place. In geographic areas where the 2^nd generation of Cs. melanura encounters the fall migration, the virus cycle is essentially aborted.

Further field data are needed to test the validity of the hypothesis presented here. Studies are currently underway to make the comparisons needed.

ACKNOWLEDGMENTS

This is New Jersey Agricultural Experiment Station Publication No. E-40101-01-99 supported by state funds and funds from the New Jersey State Mosquito Control Commission.

REFERENCES CITED

Burbutis, P. P. and R. W. Lake. 1956. The biology of Culiseta melanura (Coq.) in New Jersey. Proc. N. J. Mosq. Exterm. Assoc. 43: 155-161.

Crans, W. J. and J. R.. McNelly. 1997.

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Crans, W. J., D. F. Caccamise, and J. R. McNelly. 1994. Eastern equine encephalitis virus in relation to the avian community of a coastal cedar swamp. J. Med. Entomol. 31:711-728.

Joseph, S. R. and W. E. Bickley. 1969. Culiseta melanura (Coquillett) on the eastern shore of Maryland (Diptera: Culicidae). Univ. Maryland Ag. Exp. Sta. Bull. 161: 1-83.

Mahmood, F. and W. J. Crans. 1998. Effect of temperature on the development of Culiseta melanura (Diptera: Culicidae) and its impact on amplification of eastern equine encephalomyelitis virus in birds. J. Med. Entomol. 35:1007-1012.

Orrell, E. P. 1997.

The population dynamics of Culiseta melanura larvae at an endemic focus for eastern equine encephalitis virus in a New Jersey Coastal cedar swamp. Proc. Northeastern Mosq. Control Assoc. 43:38-39.