LIFE-TABLE STUDIES IN CULEX SPECIES AND THE SUSCEPTIBILITY OF CULEX QUINQUEFASCIATUS SAY TO INSECTICIDES COMMONLY USED IN ACCRA,

Abstract

Interest in Cutex qu£nque fa so i atus Say ( = f atigans Wied)

has been increasing with the growth of urban areas in the

tropical zones of the world because this rjmosquito is an

important vector of bancroftian filariasis. When increased

urbanization is accompanied: by inadequate sewage treatment,

conditions are ideal for proliferation of Cm qu'inquefa&ei a t u sm

No other species of mosquito has benefitted rfiore from these

acute sanitary problems than has C• qu£nquefaso'£atus» 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 Chris ti an sborg concluded that

Aedes aegif-pt'L and Cutex quinquefaso'Latus are two dominant

species but that Am a&gypti- 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 Cu lex ti'g ripes* The other was: a small

active surface-feeding fish RapZochtZus- gvaham'Zm 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 Cm quznque fasci-atus population biology and disease relationships

ti.e. filariasis) were initiated in Rangoon, Burma.

The results of these studies were reported by de Meillon

et. aZ- (.1 967). More recently, Chinery (.1 965 , 1 968a, 1 968b)

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 Cm quCnquefasczatus

( = 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 Cm q u in qu & fa&ciatu s in West Africa was conducted

by Subra (.1 970), Patterson et. aZ-(1 970). They studied the

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seasonal variation of the population. Weidhass et a t •

(1 97 1 , 1 973) also studied C. qu tn q u e fa s s ta tus in an

effort to suppress and eliminate the species using

s; te ri le m a le s •

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

1 9 7 0 ), 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

1 97 1 , 1 973, Southwood e tm a l , 1 972).

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

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area removed as practised by Cambournac (1939), Bates

(1941] and Goodwin and Eyles (1 942) are more suited to the

study of extensive areas of shallow water with much vegetation,

where larvae are numerous and evenly distributed.

Zn 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 An ophele s g ambiae 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

ab so lu te d e n s i t y 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

r e l a t iv e d e n s i t y 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 (.1 975) ,

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. Prom experience by WHO, (Anon, 1975a)