Seasonal patterns of age-composition of sylvan mosquito populations in Uganda (Diptera, Culicidae)

Abstract

Fifty-two standardised weekly collections of female mosquitos were made by human catchers at ground-level in lake-side forest near Entebbe, Uganda. The numbers of nullipars and pars in each sample were recorded, in order to determine whether the capacity of these mosquito populations to transmit virus varied seasonally. Methods are described for handling large catches for the purpose of identifying and age-grading. The total sample comprised 31,978 mosquitos belonging to 37 or more species. Results for 15 species are presented in detail.The nulliparous rate (i.e., the number of nullipars as a percentage of total females) differs markedly between species, but relatively little within species. Certain species of Mansonia (Coquillettidia), mainly swamp breeders, have the highest nulliparous rate, and species of Aedes and Eretmapodites, which breed in small containers, have the lowest. In some species, biting females show a lower nulliparous rate than those caught resting.Culex annulioris Theo. and species of Mansonia, which are swamp breeders, have a main peak of abundance between May and July, during the main rains and when lake-level is highest. Species of Aedes (Stegomyia), which are tree-hole or small-container breeders, show small, irregular, short-term fluctuations in numbers.The number of pars, an epidemiologically important parameter, is absolutely highest at or soon after the time of greatest abundance. For certain bird-biting mosquitos, namely, Culex annulioris and species of Mansonia (Coquillettidia), most pars are found during the main rains, when susceptible hosts are also most numerous. Implications of these findings on virus-isolation programmes are briefly discussed.