Mosquito seasonality and arboviral disease incidence in Murray valley, southeast Australia

Adult female mosquito populations were monitored at weekly intervals during spring-autumn (November-March) for 4 years (1991-95) using dry-ice-baited light traps at forty sites in the Murray Valley of Victoria, Australia. Among twenty species of mosquitoes collected, Culex annulirostris was the most abundant (66.6 +/- 9.3%) followed by Cx australicus (15.3 +/- 7.7%). From a total of 476,682 mosquitoes collected, nearly all were females and only 1295 (0.27%) were males. Mosquito population densities were generally higher in 1992-93 and 1993-94 seasons than in 1991-92 and 1994-95 seasons. Greatest densities of Cx annulirostris and Cx australicus occurred in 1992-93, coinciding with outbreaks of Ross River (RR) and Barmah Forest (BF) arboviruses causing human polyarthritis. In the majority of shires, Cx australicus was the predominant species from spring to early summer (November and December), then was replaced by Cx annulirostris from mid-summer to autumn (January-April). In three shires, Aedes bancroftianus and Ae.sagax predominated during the early part of the season. Densities of both Cx annulirostris and Cx australicus were related to temperature. Cx australicus adults were found to be trapped when the mean ambient temperature exceeded 6 degrees C, with peak population recorded at 20 degrees C. Cx annulirostris adult density increased when the mean temperature rose above 12 degrees C, reaching a peak during February and March when temperature exceeded 25 degrees C. Cx annulirostris declined rapidly from April onwards, with no adult activity evident from May to November. Population densities of Aedes spp. were generally less than reported from earlier studies, possibly due to lower rainfall in spring and summer as well as reduced flood irrigation practices. In each year, a significant correlation was detected between Cx annulirostris density and RR virus incidence in humans. As Cx annulirostris is the predominant local mosquito species and feeds on a wide spectrum of hosts including man, it seems Likely that Cx annulirostris is the major vector of RR in the inland Murray Valley region.