Life-Table Studies in Culex Species and the Susceptibility of Culex Quinquefasciatus Say to Insecticides Commonly Used in Accra

Abstract

Interest in Culex quinquefasciatus Say (= f atigans Wied) has been increasing with the growth of urban areas in the tropical zones of the world because this mosquito is an important vector of bancroftian filariasis. When increased urbanization is accompanied: by inadequate sewage treatment, conditions are ideal for proliferation of C. quinquefasciatus. No other species of mosquito has benefitted more from these acute sanitary problems than has C. quinquefasciatus. For its egg deposition and larval development, this species finds optimum conditions in water with a high degree of organic pollution. Usually, mosquito larval and pupal surveys are made to provide guidance for control programmes. They can be designed for species, geographic and habitat distribution, parasitism and density. In Ghana, Macfie and Ingram (1916] working on the seasonal distribution of the adult and larvae of mosquitoes in the various districts of Accra and Christiansburg concluded that Aedes aegypti and Culex quinquefasciatus are two dominant species but that A. aegypti- was more common. Earlier, Graham (1910) had studied the mosquito larvae found in water - receptacles at Lagos, Nigeria and found that a seasonal variation in numbers was shown to occur, but the causes were obscure. In this study, 2 active natural enemies: of mosquitoes were found to exist. One natural enemy of mosquito larvae was Culex tigripes. The other was: a small active surface-feeding fish Haplochilus grahami. This fish has a great capacity for populating flooded land by leaping from one pool to another. Catfish, however, preys on these fish. In recent years, many basic studies on various aspects of C. quinquefasciatus population biology and disease relationships (i.e. filariasis) were initiated in Rangoon, Burma. The results of these studies were reported by de Meillon et. Al (.1 967). More recently, Chinery (1965, 1 968a, 1968b) studied the mosquitoes in Accra with emphasis on their breeding places, in relation to the mosquito control in an urban environment. He found that proliferation of quinquefasciatus (= fatigans) is due mainly to urbanization associated, with the use of insecticides in preference to conventional sanitary- measures. The most recent study on pre— adult population of C. quinquefasciatus in West Africa was conducted by Subra (1970), Patterson et al (1970). They studied the seasonal variation of the population. Weidhass et al (1971, 1973) also studied C. quinquefasciatus in an effort to suppress and eliminate the species using sterile males. Referring to the previous studies in Accra, one observes that life-table studies and other quantitative studies on the population dynamics are lacking. It is known that the quantitative determination of mosquito larval abundance is a primary requirement of both mosquito control and research programmes. Generally, studies on mosquito dynamics are deficient on two accounts (Anon, 1968). 1. There are few ‘adequate studies' of total numbers of eggs, larvae, pupae and adults in any sample. 2. The many effects of the environment on survival and reproduction have not been fully explored, still less adequately quantified. One aim of entomologists is to develop technologies that will give the required degree of control most effectively, efficiently and economically. Such an aim implies that the control of diseases caused by mosquitoes should be based on the management of total populations rather than the continual reduction of high density populations at times when the insect or disease becomes a problem. In most cases, it is possible to visualize the way in which a control technique will act on an insect population. However, the development of the potential of new or integrated approaches to control is less available. As a contribution to planning integrated control measures, scientists have in recent times relied on life-table construction. The purpose of this is to summarize the survival and mortality rates of a population. Its application in recent times is found in agricultural and forest entomology (Morris 1963, Southwood 1966, Varley and Gradwell 1970), and in fisheries biology (Ricker 1944, 1948, Wohlschlag; 1954, Beverton and Holt 1957). Only recently has it been applied to mosquitoes (Lakhani and Service 1974, Service 1971, 1973, Southwood et al, 1972). Varley and Gradwell (1970) emphasized that the most instructive life-table will usually be based on the continuous and intensive study of a population in a single habitat, not by sampling different populations in a number of similar habitats in different years. Where sampling is for life-table construction, differences in sensitivity of larval instars to disturbance produce biased samples. The ‘enclosure method’ whereby a given area is enclosed and all the larvae contained in the area removed as practised by Cambournac (1939), Bates (1941) and Goodwin and Eyles (1942) are more suited to the study of extensive areas of shallow water with much vegetation, where larvae are numerous and evenly distributed. In the types of breeding site indicated this method is difficult to perform, because the larvae accumulate along the ‘shore line’ and not in the open area of the water habitat. The method used by Christie (1954) for recovery and enumeration of Anopheles gambiae from pools is however quite accurate and efficient and it shows that about 95% of the larvae of all stages except the first are recovered. In general, the specimen— collecting devices used in qualitative and quantitative studies are the same (reviewed by Bradley and Goodwin, in Boyd 1949) but, where in qualitative work the specimens collected are most frequently in themselves the end-goal, the number of organisms taken per unit of effort or area is important in quantitative studies. Additionally, quantitative surveys methods differ in that a sampling design and a mathematical model for data analysis must be developed. In some cases it is also required that a method be developed for determining the numerical relationship between the specimens collected and total population else. In connection with quantitative considerations, Andrewartha (1961a) recognizes two types of population densities. If it is possible to count or estimate all the animals in an area, the results are spoken of as the absolute density of a population. Where the size of a population is known only as a ratio to the size of another population either in time or space, it is said that the relative density is, known. In order to understand the factors that determine mosquito numbers, knowledge of the effects of environment upon survival, fecundity and dispersal is necessary. The data required are the total numbers of eggs, larvae, pupae and adults, in the area under study at different times, together with an analysis of environmental factors. Changes in numbers are then related to changes in environment. The analysis of such data is the subject of a paper by Hayes and Hsi (1975). The sampling of larval density should aim at roughly estimating the relative density for quantitative evaluation. For this reason, the relative abundance of larvae of a particular species at different places can be determined, provided that the methods of sampling are standardized as far as possible. Forom experience by WHO, (Anon, 1975a) the number of larval dips should be standardized as units of 10 dips or multiples of 10 dips (ie, 20, 30, etc) per capture station depending on the density of larvae. The most recently designed larval trap (Service, 1981) is very efficient and may replace other aquatic light traps. Probably this is because the mosquito larvae and pupae easily get attracted to the two ends of a cylinder within which the ‘Cyalume’ chemical lightstick is suspended. It is therefore hoped that this light trap may be very useful for future work on Life-Table. Quantitative approach to Life-Table studies is very important in larval control. If the numbers of each stage (e.g. eggs laid, eggs hatching, larvae, pupae and emerging adults) in a population are counted, and the logarithm: of each number subtracted from that for the preceding stage, the results (the ‘natalities’ over the different age intervals) are called K-values. The sum of the K-values gives the total 'mortality' for the generation (Southwood, 1978), When a series of Life—Table are available for successive generations of a population, the quantitative relationships of such K-values to population fluctuations, to population density, and to various environmental factors may be determined. This involves the quantification of significant environmental factors as they change throughout the period of study. The objectives of such a Life-Table analysis make possible the recognition of (1) Environmental factors that are important for the prediction of population fluctuation and (2) The relationship of mortality (and dispersal) and each stage of the life cycle, and of natality, to population density. Knowledge of this relationship is essential for building a model of the way in which numbers are determined. Thus information derived from this type of analysis provides the basis for the study of population dynamics. Studies of the effects of environmental factors on the mosquito throughout its life-cycle are also important. In this study, the sites at Malam (plate A) and Nima (plate B) were selected for the following reasons. Nima drain is one of the most polluted areas in Accra, with human and animal excreta, and the breeding site, apart from proximity to Legon is also permanent throughout the year. What is more, it is an area noted for the breeding of Culex quinquefasciatus (=fatigans). The pond at Malam on the other hand, is an isolated area and inaccessible to the general public. The water is free from organic pollution, and also permanent throughout the year. Finally, the two sites were free from exposure to pesticides over the period of study. Also inhabitants of the 2 sites are of the same ethnic groups (Hausas). However, while Nima drain is used for sewage disposal, there is improved sanitation at Malam. Therefore, there is a vast contrast between the two sites. The first aim therefore is to compare the incidence of C. quinquefasciatus (=fatigans) in the two sites. Secondly, it is the aim of this study to find the other mosquito species occurring with C. quinquefasciatus in the two sites. The physico-chemical nature of the breeding sites was investigated with a view to determining the best time to control the vector- - thus reducing its mean density below the Economic Threshold. Since there has been little publication on insecticide susceptibility tests on mosquitoes of Accra, it was also considered worthwhile to study the susceptibility of C. quinquefasciatus to some insecticides for comparison with levels obtained elsewhere. Even though this project of 12 months' duration is not long enough to confirm the seasonal fluctuation, it is however, hoped that it would serve as a guideline for future work. Furthermore, the work as a whole is extensive - covering many aspects — but the section of Life-Table studies, seasonal fluctuation and insecticide susceptibility tests are topics of great interest and much quantitative work has been carried out.