Population dynamics and age grading of mosquitos*

Bull. Org. mond. Sant} 1974, 50, 317-321 Bull. Wid Hith Org. Population dynamics and age grading of mosquitos* E. M. UNGUREANU 1 The dynamics of a vector population follows a characteristic pattern determined by the normal or modified environmental conditions of the biotope and by vector behaviour. The wide use of the precise method of age grading developed by Detinova and Polovodova has led to the conclusion that the dynamics of the various age groups may indicate behav- ioural, environmental, and epidemiological trends and changes and their significance. Age grading is useful for the qualitative assessment of vector density in general and of man- mosquito contact from the operational and epidemiological points of view, but only when sampling in time and space and the processing of the material collected have been correctly performed. Ross (1) emphasized the epidemiological signifi- cance of age in a vector population. With the introduction ofnew antivector measures, the study of age composition within these populations has been further stimulated in order to facilitate the evaluation of the impact of insecticides. Macdonald (2), in further developing mathematical methods for epidemiological evaluation, underlined the importance of age grading as an epidemiological tool. During the last 60 years, age grading methods have gradually evolved and several of them have been described in the literature. The earliest method was based on the morphological aspects of the wing fringe for the separation-in a relatively imprecise way-of vector females in young and old groups; the latest methods are those of Detinova, Polovodova, and Schlein & Gratz (3). Mer (4) proposed examination of the dilatation of the oviducts (ampullae) for the differentiation ofparous and nulliparous females, and this method was used by numerous investigators until about 1950. The discovery, by the Soviet entomol- ogists Detinova (5) and Polovodova (6, 7), of more precise methods of age grading made such studies possible. Table 1 summarizes, in the form ofa key, the various morphological aspects now regularly used for age grading. The study of the age composition of a vector population is necessary for a better understanding of the dynamics of various age groups in time and space, * Presented at the Symposium on Malaria Research, Rabat, Morocco, 1-5 April 1974. 1 Professor of Medical Parasitology, Faculty of Medicine, Jassy, Romania. the longevity (physiological and calendar age) of the vectors, and their behaviour and contact with man in natural conditions or conditions modified by anti- vector measures. TIhe data collected by age grading are therefore used as parameters for assessing the epidemiological importance of vectors and vector populations under natural or modified conditions. Table 1. A simple key for the immediate differentiation of parous and nulliparous mosquito females during the gonoactive period (1) Presence of retained eggs . . . . . . . . parous Absence of retained eggs . . . . . . . . (2 or 4) (2) Ovarian follicle in Christopher's stage I or 11 early .... . . . . . . . . . . . . nulliparous Ovarian follicle in Christopher's stage 11 late (3) (3) Unfed or freshly fed females with the malpi- ghian tubes showing massive elimination of the excretion granules . . . . . . . i . parous Unfed or freshly fed females with the malpi- ghian tubes showing no signs of elimination of excretion granules . . . . . . . . . . . nulliparous (4) Presence of tracheole coilings in the ovary (with follicles in Christopher's stage I-11 late) nulliparous Absence of tracheole coilings in the ovary (with follicles in Christopher's stage ll-V) . (5) (5) a Absence of dilatations in the ovariole . nulliparous Presence of sacs showing signs that eggs have been laid recently . . . parous Presence of dilatations only, showing that eggs have been laid at least 24 h before. . . . . parous a While dissecting ovaries with follicles in Christopher's stages IV-V, an attempt should be made to find degenerated follicles; the easiest examination is carried out on ovaries with follicles in Chris- topher's stage 11. 3194 -317- E. M. UNGUREANU PHYSIOLOGICAL PROCESSES AND THE DYNAMICS OF VECTOR POPULATIONS An adult mosquito population, during the active period of development, changes continuously from both the quantitative and qualitative points of view. Such changes are produced by physiological needs and processes that determine the mating flight and the search for food and convenient resting and breeding places. The abundance of mosquito populations is regulated by deaths and by emergence from breeding places. When the daily mortality is higher than the daily emergence rate the general density decreases proportionally. Emergence and mortality regulate also the proportions of various age groups in a mosquito population. Under natural or modified conditions the dynamics of mosquito populations present two main aspects: (1) fluctuation in the density as a whole and (2) changes in the proportions of a mosquito population at different physiological stages and ages. In a particular area, the mosquito population mov- ing from breeding places to the host and to resting places is a mixture of newly emerged and older nulli- parous individuals and parous females of different ages. Fig. I shows that the age composition of vectors Fig. 1. Dynamics of adult mosquitos in various parts of their biotope, according to physiological stage and age. The continuous line represents nulliparous females; the broken line, parous females. N, Ni, and N2 = unfed, fed, and gravid nulliparous females, re- spectively. P, P1 and P2 = unfed, fed, and gravid parous females. during the active period of development depends on the proportions of nulliparous and parous females coming to bite from breeding places and from diurnal resting places. Some of the nulliparous females may have emerged recently, some may be a few hours old, and some may be one or more days old (especially if unfertilized). The biting parous fraction of a vector population is composed of: (a) parous mosquito females biting during the same night after egg-laying, easily recognized by the presence of ovariolar sacs, and (b) parous females biting at least one day after egg-laying, recognizable by the presence of well- contracted ovariolar dilatations. In some species, another category of biting females may be encoun- tered: gravid females (nulliparous and parous) taking a second bloodmeal. In view of the considerations mentioned above, the operational and epidemiological significance of vec- tor density may be better understood by age-grading representative samples and taking into consideration all influences on the dynamics of local vector popula- tions at the time of study. SAMPLING AND PROCESSING FOR AGE GRADING The amount and accuracy of information obtained on the age composition of a vector population depend on the sampling and processing of the material. In sampling, the following aspects should be taken into consideration: place, time, method, vector behaviour and dynamics, and the environmental conditions prevailing at the time of collection. Place of collection This should be carefully selected in order to include all representative situations. There are indications that, in the area close to the breeding places, the proportion of nulliparous females may be higher than it is in a distant area. Sevcenko (8) demonstrated this with Anopheles maculipennis. The present author made the same observation in Haiti, while age- grading A. albimanus populations. Time of collection This refers to the hours of collection, the number of days of successive collections, and the season of collection. These are important data, since the dy- namics of a vector population may show hourly, daily, and seasonal variations. Hamon (9), investigat- ing several anopheline species in Upper Volta, found that, during the first hour of biting, a small deficit of parous females occurs. The present author found, in an area of Haiti with very productive breed- ing places, that a high proportion of the A. albi- manus coming to bite during the first hour of the evening were nulliparous females, whereas this situa- 318 POPULATION DYNAMICS AND AGE GRADING OF MOSQUITOS Table 2. Uses of various age grading methods Aim Author Principle of method Technique Subjects of studies for whichresults could be used to establish the pro- Detinova (5) examination of ovary for (a) immediate examination 1. biting and resting behaviour portions of parous and presence or absence of of ovary in a drop of 2. dynamics of age groups in nulliparous females in tracheole coilings water or glycerine relation to dynamics of a vector population density(b) examination of ovary 3. daily mortality and life after drying expectancy of vector under natural or modified conditions (insecticide treatment, longevity studies) Polovodova (6) examination of ovarioles immediate 4. vector infectivity (simple method) for presence or absence examination of (dissection of parous of sacs or dilatations the ovaries, to liberate females only) the ovarioles 5. vectorial capacity to determine the Polovodova (7) examination of ovarioles isolation of ovarioles in 6. (more precise studies on proportions of various (detailed for presence and number water under dissecting 1-5)(physiological) age method) of dilatations microscope and counting 7. evaluation of physiological groups in a vector number of dilatations age population 8. indirect evaluation of calendar age 9. proportion of females of biting age to establish directly the Schlein & identification of cuticular examination of thoracic 10. correlation between calendar age of a vector Gratz (3) a layers on thoracic apodemes after physiological and calendar population apodemes maceration in potassium age hydroxide; staining and 11. duration of gonotrophic mounting in Canada cycle balsam. 12. (same as for 1-5) a This method is more laborious and difficult to use for age grading than Polovodova's method. tion was reversed during the following hours.b In an area where breeding places had decreased in number and size, the proportion of nulliparous females was smaller than that of parous females during all hours of collection. However, even in such situations, the proportion of nulliparous females was higher during the first biting hour than during the next hour. Daily and seasonal variations normally result from fluctua- tions in mortality and in the numbers of mosquitos emerging from breeding places. Collection The technique ofcollection should take into account all representative situations in time and space; vector and human behaviour; the location ofbreeding places; and density trends (increasing, stable, or decreasing). Usually baits are used in sampling, but valuable additional information may be obtained with other methods. If sampling is intended for age grading with the object of assessing the impact of spraying on the vector population and on the transmission (assess- ment of the vectorial capacity), collection on human bait is probably the most accurate method. b Unpublished report to WHO, 1968. Processing The method depends on the objective of the study and on the intended use of the data obtained. When the only information required is the proportions of parous and nulliparous females, Detinova's method may be applied, either immediately or after drying the ovaries. For the calculation of the physiological age, Polovodova's method should be used. Table 2 sum- marizes the methods used for age grading. When Detinova's method is used, immediate examination completed with Polovodova's technique yields far more information on the parous group than does examination performed after the ovaries have been dried, since it enables the proportions of parous fe- males with sacs and dilatations to be established (10). Few data are available on changes in the propor- tion of females with sacs, coming to bite in a sprayed area. However, an excellent example of this phenome- non was reported by Hamon (9). The present author observed a decrease in the proportion of females of A. gambiae, A. funestus, and A. nili with sacs, coming to bite in a sprayed area. With A. gambiae and A.funestus-both intradomestic species-the propor- tion of females with ovariolar sacs, in the control 319 E. M. UNGUREANU Table 3. Dynamics and significance of age groups and vector density during the gono- active period Anti- Proportion a Adult vector vector Possible explanation measures nulliparous parous denstity increasing decreasing increasing increase of output from breeding places increasing decreasing decreasing shortening of life of parous frac- none tion of mosquito population decreasing increasing decreasing reduction of output from breeding places stable stable stable output and mortality in equilibrium decreasing increasing decreasing large reduction of output from suddenly breeding places absent increasing decreasing stoppage of output from breeding anti- to 100 % continuously places larval stable and stable and low some operational defects persisting normal normal (constant) at same low level sudden decreasing increasing operational defects increase increasing decreasing decreasing good toxic effects of insecticides to very imago- low level cides normal normal increasing or inefficacy of insecticides hardly changed decreasing increasing increasing loss of efficacy of insecticides a The terms " decreasing and " increasing " refer to the trends of variations compared with given baseline data. area, was higher than that of females with dilatations. This indicates the predominance offemales coming to bite during the same night after egg-laying. After spraying, the proportion of females with sacs was smaller than that of females with dilatation. Thus the number of parous females coming to bite at least 24 h after egg-laying increased, so that they outnumbered the females coming to bite during the same night after egg-laying. This aspect was more pronounced with A. funestus-a species more endophilic than A. gam- biae. The data obtained on A. nili (which is much more pronouncedly exophilic than A. gambiae) showed that the proportion of females coming to bite during the same night in the control area was smaller than that of females coming to bite the next night, after egg- laying. In the sprayed area, however, the same phenomenon as mentioned for the other species occurred-i.e., a reduction in the parous females coming to bite during the same night after egg-laying. This indicates that the average duration of the gonotrophic cycle of the above-mentioned vectors was influenced by DDT. Such aspects are not without epidemiological significance. Table 3 shows the dynamics of age composition and vector density. CONCLUSIONS The biology and operational and epidemiological significance of the dynamics of a vector population may be illustrated by studying the age composition of the samples collected. Adequate sampling and processing are essential for obtaining relevant data. Age grading, if not accurately performed on repre- sentative samples of the vector population, is of little or no value. Adequate data on density and environmental con- ditions at the time of sampling are indispensable for a correct interpretation of the significance of the age composition. 320 POPULATION DYNAMICS AND AGE GRADING OF MOSQUITOS 321 RESUME DYNAMIQUE DE POPULATION ET CLASSEMENT PAR AGE DES MOUSTIQUES La determination de la composition selon l'age des populations de moustiques est aujourd'hui essentielle pour l'etude qualitative de la biologie d'une espece et de sa dynamique de population dans des conditions de milieu normales ou modifiees. La decouverte, par Detinova et Polovodova, de methodes precises de classe- ment par age a rendu possibles de telles etudes. La dynamique d'une population de vecteurs est d6ter- minee par des n6cessites et des processus physiologiques influences par le milieu. Dans les conditions naturelles ou modifi6es, elle presente deux aspects principaux: a) fluctuation de la densite globale, et b) modifications des proportions de moustiques d'ages et de stades phy- siologiques differents. La composition par age d'une popu- lation de vecteurs en divers points de son biotope est sou- mise a d'incessantes fluctuations durant la periode active de developpement. Le rythme d'eclosion et le taux de mortalite conditionnent non seulement la densite globale mais aussi la composition par age. La densite et le nombre des vecteurs subissent des variations horaires, quotidiennes et saisonnieres dans les diverses parties du biotope et des modifications analogues sont observ&es dans la composition par groupes d'age. Tout accroisse- ment ou toute diminution de la proportion des stades dangereux auront des repercussions du point de vue 6pid6miologique. I1 en resulte que pour etablir l'importance d'une popu- lation de vecteurs, sur les plans operationnel et 6pidemio- logique, il importe de determiner la composition par age au sein d'echantillons representatifs en tenant compte de tous les facteurs qui influencent la dynamique des populations locales au moment de 1'enquete. REFERENCES 1. Ross, R. The prevention of malaria, London, Mur- ray, 1910. 2. MACDONALD, G. The epidemiology and control of malaria, London, Oxford University Press, 1957. 3. SCHLEIN, J. & GRATZ, N. G. Bulletin of the World Health Organization, 47 (1): 71-76 (1972). 4. MER, G. G. Bulletin of entomological research, 23: 563 (1932). 5. DETINOVA, T. S. (1963) WHO Monograph Series, No. 47. 6. POLOVODOVA, V. P. Modifications dues 'a I'age dans I'appareil reproducteur femelle d'anoph6les et com- position par age des populations de moustiques, Rostov-on-Don, 1947 (thesis). 7. POLOVODOVA, V. P. Medicinskaja parazitologija i parazitarnye bolezni, 18: 352 (1949). 8. SEV6ENKO, A. K. Medicinskaja parazitologia i para- zitarnye bolezni, 22: 495 (1953). 9. HAMON, J. Bulletin ofthe World Health Organization, 28 (1): 83-109 (1963). 10. UNGUREANU, E. M. Bulletin of the World Health Organization, 47 (2): 239-244 (1972). DISCUSSION DLETZ: In a WHO project in Nigeria, the longevity of A. gambiae is estimated from data on human bait captures. Biting-density data from the project would indicate that control measures had reduced the vec- torial capacity by about 250 times if the populations were considered to be uniformly affected, but only by 20 times if it were assumed that 5% of the emer- ging female mosquitos had totally escaped the action. RAMSDALE: Detailed investigations of the dry-season ecology of A. gambiae have provided an explanation for the sudden appearance of adults before the onset of the rains. It is important to know the local vector population adequately before control operations are begun.