Estimated mortalities of the immature stages of Aedes cantans (Mg.) (Diptera, Culicidae) in a natural habitat

For three consecutive years the eggs, the immature stages and the emergent adults of Aedes cantans (Mg.) populations were sampled from a small woodland ditch in southern England with a view to constructing the life-table of the species. Estimates of the numbers of ‘viable’ eggs available for hatching and the size of the resultant adult population yielded an estimate of the probability of a ‘viable’ egg resulting in an adult. Estimating the mortality rates in different immature stages which occur simultaneously is known to be difficult. It was considered reasonable to rely on a generalised exponential survivorship model based on the assumption that the relative mortality rate during the duration of each instar of A. cantans is constant, but this rate may vary from instar to instar. The parameters of the model were estimated using experimental data on the duration and total incidences through regular sampling of different instars together with the estimate of the probability that a ‘viable’ egg would result in an adult. The survivorship picture thus obtained agreed closely with the survivorship pattern obtained using a graphical approach based on the construction of the stage-specific age distribution of the pre-adults, and then drawing a smooth curve through the age distribution to produce an approximation to the survivorship curve. Because mortality was similar in all three years, the three years' data were pooled to give estimates of the life-table parameters. These clearly showed that substantial population loss occurred in the first two larval instars of A. cantans.