An Analysis of the Generation-Mean Life Table of the Mosquito, Culex tritaeniorhynchus summorosus, with Particular Reference to Population Regulation

Abstract

A new type of life table, of which parameters are given as mean values for all generations existing during a mean generation time, is proposed for populations with incompletely overlapping generations. This generation-mean life table is applied to the study of seasonal population changes of C. tritaeniorhynchus summorosus Dyar in 14 ha paddy fields. The estimation of developmental duration, which is essential to the construction of these life tables, is performed by temperature summation. The relationship between the duration of each stage and air temperature is examined. The mean generation times at different temperatures are calculated on the assumption that the ratio of lx (probability of surviving to the xth oviposition) to l1 and that of mx (mean number of female eggs laid by 1 female at the xth oviposition) to m1 are constant over the whole breeding season. The thermal constant and the developmental zero are 277.8 day-degrees and 10.1.degree. C, respectively. The population growth rate per generation and the survival rate of larvae and pupae are estimated directly from census data. The proportion of females is constant (0.527). Using these parameters, the expected fecundity (total number of eggs which 1 female is expected to lay during her complete life-time) is calculated on the assumption of a constant survival rate of eggs (0.951) and of equality between emigration and immigration. Analysis of the life tables shows that the present population is regulated by density dependent processes. There are 2 distinct equilibrium levels indicated by larval density. The 1st equilibrium level (133 m2 in 1966 and 404 m2 in 1967) is higher than the 2nd (37 m2 in 1967), and such a change is caused mainly by the increase of larval mortality due to the drainage of paddy fields, which is not a key factor. The most important regulatory mechanism may be intraspecific competition among larvae and it seems closely related to the spatial structure of the larval population.