Exophilic behavior of Anopheles darlingi Root in a Southern Region of Brazil
Oswaldo Paulo Forattini
Department of Epidemiology of the "Faculdade de Saúde Pública Universidade de São Paulo" Av. Dr. Arnaldo, 715 01255 São Paulo, SP Brazil
Fortnightly 25-hour catches, with human bait, were carried out in a modified environment of the "Jacaré-Pepira" River, Dourado County, S. Paulo, Brazil which is the original region of the "Araraquara" strain of Anopheles darlingi. The exophylic biting activity was mostly nocturnal with bimodal rhythm, showing two clear peaks corresponding, respectively, to dusk and dawn. Going into crepuscular details two secondary peaks were observed, an eocrepuscular preceding the intracrepuscular one As, by means of chromosome arrangement studies, this population was found to have low polymorphism, it is supposed that those rhythms are, in a good measure, of endogenous command. The number of mosquitoes biting increased during the hot, wet season and decreased remarkably during the dry, cold one. There was some evidence that An albitarsis may also show a bimodality in its nocturnal biting activity rhythm.
Uniterms: Anopheles darlingi. Malaria, transmission. Anopheles albitarsis. Mosquitoes. Ecology.
Despite Anopheles darlingi being the major vector of malaria in the larger part of Eastern South America, the number of more recent observations on the biology of this mosquito have been relatively small. However, some of these have indicated the geographical variability of its behavior. Its anthropophily, exophily and even the 24-hour rhythm, have revealed regional variations of greater or lesser degree (Giglioli12, 1956; Charlwood and Hayes5, 1978).
The investigations on chromosomic inversions suggested the existence of a greater degree of polimorphism in Northern than in Southern populations of Brazil (Kreutzer and coll.17, 1972). With regard to these, such evidences were obtained from a population denominated "Araraquara" but which, in fact, originated from "Dourado" county, S. Paulo State, Brazil. This lesser degree of polimorphism could lead to the hypothesis of greater populational stability and, arising from that, greater uniformity of behavior. In so far as such aspects are held to be necessary concomitants of the transmission of malaria it was thought to be opportune to make observations along these lines, focussing on this population. The results obtained are the subject of the present study.
As has been mentioned, the place chosen for these observations is located in the county called "Dourado" which corresponds to the valley of the "Jacaré-Pepira" river, a tributary of the "Tietê" river which, in its turn, is at present dammed up in this region to form the "Bariri" reservoir. Both belong to the "Paraná" river system, in the central highplain of the State of S. Paulo. The geographical location of the area under study is 22°09' S. latitude and 48°21' W. longitude, at an altitude of 1,500 feet above sea level. The regional climate corresponds to Köppen's "Cwa" mesothermic type, that is to say, high altitude tropical with a dry winter and a hot and rainy summer.
The present landscape features are those of an environment extensively and intensively modified by agricultural and cattle-raising activities, particularly the latter. As a result, one may observe the predominance of pastures and the presence of residual forests of an essentially secondary character. The "Jacaré-Pepira" river has, in this area, a sinuous course, with marshy banks subject to flooding, accompanied constantly by gallery or shallow edge river woods, of low height.
For the carrying out of this project, a place at the edge of the above-mentioned water-course was chosen, where the county authorities had installed an area for public leisure, with ample structures to shelter meetings and picnics. The place is situated near the bridge on the secondary road which links the counties of "Dourado" and "Jaú". There are no dwellings there and the nearest houses are scattered and situated at not less than 550 yards from the point at which the observations were made.
The reasons for the choice of this site were, as has been mentioned, the study of the behavior "in loco" of the population from which the "Araraquara" strain originated. Figures 1 to 5 are provided to give some idea of the site, as well as of the panoramic aspects and some details of the above description.
MATERIAL AND METHODS
Collections were made with the use of human bait. The collectors operated under the abovementioned roofed structures, situated on the banks of the river "Jacaré-Pepira", at the point described in the preceding paragraph. The rhythm was twice monthly, covering two alternate weeks, and with a duration of 25 uninterrupted hours for each collection. The period covered was that from 10.00 a.m. to 11.00 a.m. on the following day. The methodology used followed that described in a previous study (Forattini and coll.9,10,11, 1981, 1986). The macroclimatic data are those corresponding to those of the Hydrometeorological Station of "Bariri" of the State Department of Water and Electrical Energy (DAEE), situated in the county of the same name and near "Dourado" county (SEADE1,2,3, 1981, 1982, 1983).
Though recognizing that Anopheles albitarsis is a markedly polimorphic species and that it probably, therefore, is constituted of a complex of cryptic species, the name of An. allopha suggested by Faran and Linthicum8 (1981) for the Southern populations of South America cannot be adopted, in face of being nomen nudum (Oliveira and Deane18, 1984). Under these circumstances, in this study the former denomination will continue to be used.
In the period from March 1980 to April 1982, 17,611 mosquitoes were collected, of which 9,925 (53.4%) were anophelines. Of these, 9,523 (95.9%) were represented by Anopheles darlingi. The specific distribution of the genus Anopheles was as follows:
It can be observed that An. darlingi represented 54.1% of the total number of mosquitoes collected, which gave it the dominant position in these collections. Together with An. albitarsis, the two species made up a total of 97.2%, that is to say, practically the total of anophelines obtained in these observations.
Hourly activity The results of the twice monthly collections, according to the time intervals, are shown in Table 1. In the 52 collections carried out, a total of 9,648 specimens of the two anophelines was obtained, confirming their essentially nocturnal activity, with a clear crepuscular increase (Figure 6).
Crepuscular and pericrepuscular activity
The relative percentual distribution, according to the "crep" intervals of the spaces of time used in the collections as divided between the two crepuscular periods, can be seen in Figure 2. They corresponded to 52 evening captures and the same number of morning ones, in the periods from 5.00 p.m. to 8.00 p.m. and from 4.00 a.m. to 7.00 a.m. respectively. Such data, with reference to An. darlingi, demonstrate the occurence of evident peaks of activity, as much intra as pericrepuscular (Figure 7). There were considerable increases within the crepuscular periods, properly so-called, the evening peak being slightly more intense and clear than the corresponding morning one. With regard to the pericrepuscular periods: in both one could observe the existence of a second peak, of pre-crepuscular character, after which there follow the respective intracrepuscular increases.
Seasonal variation A greater density of An. darlingi was observed in the periods corresponding to the hot months from December to April, with a peak in March. Their presence diminished in the cold, dry months of July and August. Correspondence to the annual rainfall rhythm was thus observed, though this does not exclude the possibility of occasional peaks, as was observed in the month of June 1980 when the occurrence of non-typical intense rains reached a total of 121.1 mm., influenced the calculation of the total for the observations in this period (Figure 8).
As stated above, the Anopheles darlingi is a species of wide geographical distribution. It stretches from the South of Mexico to the North of Argentina, and from the Eastern slopes of the Andes chain to the coast of the Atlantic Ocean. It is to be understood, therefore, that there should occur variations of behavior, possibly showing the existence of geographical population varieties or even of cryptic species. With regard to the variation in endophily and anthropophily, Giglioli12 (1956), in a revision very advanced for that epoch, considered that, from the eclecticism which it presented in the center of its area of distribution, this mosquito would tend to become more specialized by the intensification of its endophilic and anthropophilic habits towards the periphery of its geographical distribution. However, the feeding activity of the mosquito tends to be carried out in accord with the 24-hour cycle which, in its turn, is subject to the influence of both exogenous and endogenous factors. These are most clearly shown by observations carried out over a period of time sufficiently long to reveal the rhythmic aspect, characteristic of each species. In the light of this it is admitted that the daily periodicity of the anopheline hematophagy is normally distributed with one or two peaks during the 24-hour period. With regard to An. darlingi, observations have been carried out with a view to detecting regional differences in this cycle. And this on the assumption of the possibility that these expected diversities of behavior allow the hypothesis of the existence of distinct populations. In other words, the occurrence of regional diversities of behavior, susceptible of attribution to endogenic command, reveals the possible existence of cryptic species. In reviewing this subject, Elliot7 (1972) considers that this anopheles presents a unimodal cycle but with a certain geographical variation, regarding the placing of the time of peak activity, which is situated between 10.00 p.m. and 12.00 p.m. in Colombia and Peru and between 0.00 a.m. and 2.00 a.m. in the Amazonian Region of Brazil. Such observations, as well as others that followed them, had the domiciliar environment as their scene of execution, both intra and peridomiciliary. This last being understood either of the varandas of the houses as well as the neighborhood of the dwellings. In this way, observations carried out in French Guiana revealed the presence of a typical bimodal rhythm, including both twilight periods, to which however is added a clear nocturnal peak, between 1.00 a.m. and 2.00 a.m.. In this case, there was occurring, apparently, a trimodal cycle, parallel to a considerable daily exophagy (Pajot and coll.19, 1977). In a nearby area, though apparently distinct from the biogeographical point of view, a unimodal rhythm was observed, with a variable peak positioned between 21.00 and 23.00 hours (Hudson14, 1984). In some places in Brazilian Amazonia, observations have been carried out which revealed the occurrence of appreciable variations. In general terms, one can detect a unimodal rhythm in the region to the north of the Amazon river with the occurrence of the peak of activity varying from immediately after midnight at the northernmost locality (Uauaris), to before this time, that is to say, between 21.00 and 24.00 hours at the southernmost point (Km 137 of the BR-174 highway) of this region. In a place (Aripanã) situated to the south of that river, the rhythm observed was of a bimodal character, with the first peak occurring either at the beginning of or in the first half of the night, and the second during the morning twilight period and the beginning of the morning (Charlwood and Hayes5, 1978; Hayes and Charlwood13, 1979; Charlwood and Wilkes6, 1979). The result of the present work, of a clearly bimodal and crepuscular rhythm, leads one to suggest certain considerations in the light of the knowledge at present available and quoted above.
One must take into consideration, preliminarly, that the data here reported were obtained in an extradomiciliary environment. And this, in the true sense of the word, that is to say, in an uninhabited area and at a distance of not less than 550 yards, in a straight line, from the nearest house. Therefore, the aspect observed has reference to essentially exophilic activity, in the strict sense. One thus obtained a clearly bimodal cycle of biting activity, with the peaks occurring in each crepuscular period, as is to be found in the graph of Figure 6. Beyond this, the detailing of the collections in these periods, allowed one to detect a subdivision in each of these two peaks, there being one lower one of an eocrepuscular nature, prior to the other, much greater and clearly intracrepuscular, as can be seen from the graph in Figure 7. The slight rise in the curve to be observed in the first hours after midnight, does not characterise a peak, as in the example reported from French Guiana and which, according to Pajot and coll.19 (1977), could represent an inheritance from the "wild" populations, with no contact with human activity. Thus there remains the celar bimodal crepuscular aspect the interpretation of which should be sought in the composition by age and the feeding preferences of this population. In a similar cycle observed in Aripuanã, Charlwood and Wilkes6 (1979) discovered that the crepuscular peaks are predominantly constituted by nulliparous females. As one is dealing with observations carried out in a peridomiciliar environment it would be unwarranted to invoke them in the attempt to establish some analogy aplicable to these present investigations. On the other hand, in French Guiana, Pajot and coll.19 (1977) consider that, with rare exceptions, the age of the females appears to have no influence on the cycle of activity, either in the domiciliary or the extradomiciliary environment.
If these variations in the rhythm of the 24-hour cycle of An. darlingi are indicative of the existence of diverse populations and, consequently, of distinct behavior, is an open question. It is necessary to comment that the description of some of them was based on a limited number of observations, which makes it difficult to discount the influence of exogenous factors. The research resulting from a larger number of collections, carried out regularly and over a prolonged period of, at least, one year, allows one to distinguish between two fundamental types of cycle. The first being unimodal with a nocturnal peak. This peak, probably by virtue of the influence of exogenous factors, characteristics of the region or locality, shows variations in its time of occurrence, now in the first half of the night, now immediately after midnight (Elliot7, 1972; Hudson14, 1984). The second type is of bimodal characteristic with crepuscular peaks. This is that observed in French Guiana and in this, the Southern, region of Brazil. One must draw attention to the fact, for the first of the two regions, even in this type of cycle, of the persistence of a nocturnal peak, thus suggesting that the unimodality could represent the primitive character of this kind of behavior (Pajot and coll.16, 1977).
In any case, the existence of distinct populations within the species identified as An. darlingi seems to be beyond doubt. The investigations, even though still few, into the chromosomic polymorphism of this species, shows, as was mentioned in the introductory section of this text, the occurrence of inversions which permit the identification of northern and southern populations of this species. The former, endowed with more notable polymorphism, and the latter, more stable (Kreutzer and coll.17, 1972; Tadei and coll.20, 1982). However, there is a lack of observations which would permit an association of such aspects with behavioral characteristics. This present study constitutes the first in this direction and is related to the population which, as already stated, is denominated "Araraquara", the chromosomic arrangements of which are, apparently, fixed. In this case, it is to be concluded that the clearly bimodal character of its 24-hour cycle is stable and that it presents an aspect of the exophilic behavior of this southern population of Brazil, which is under endogenous command.
As regards seasonal variation, the annual cycle of activity is more exposed to the influence of exogenous factors and, in a special way, to those which can affect the productivity of the breeding places. More recent studies have revealed some variability in this rhythm. It is generally admitted that the occurrence of heavy rains can flood the breeding places and create flood currents that carry away the immature forms. Even in the coastal region of French Guiana, where the macroclimatic conditions seem not to exercise any perceptible influence on the results of the collections of An. darlingi, the occurrence of heavy rains accompanies a decrease in and sometimes even total absence of the mosquito, whatever the time of capture (Pajot and coll.19, 1977). Probably, and in accordance with the regional frequency and intensity of this climatic phenomenon, one may attribute to such a mechanism some influence in the increase in the density of this anopheline in the dry season or in the transitional period between the wet and dry seasons. This is what has, in fact, recently been reported for some Amazonian localities in Brazil (Hayes and Charlwood13, 1979; Charlwood4, 1980). However, and because of the dependence on diverse factors, such as the topography of the terrain and others, in a general way the density tends to increase in the periods of greatest rainfall. This was what was observed in recent research, with the result represented by the graph in Figure 8, similarly to that reported from other regions quite distinct from this one, such as Surinam (Hudson14, 1984). In the region here studied, the dry months include the period from May to August/September, in contrast with the others, of heavier rainfall, when greater density of capture was achieved.
Finally, the presence of An. albitarsis must be mentioned, even though found in much smaller numbers than the other species. The result of its collections was sufficient to permit some tendency to crepuscular bimodality in its 24-hour cycle to be perceived (Figure 6 ). However, further observations are necessary before one can arrive at more significant results in this respect.
The identification of the strain of Anopheles darlingi, presumably of low variability, to which the name "Araraquara" was given, opened up the possibility of the study of the behavior of this population. In this study one sought to observe its exophilic activity in relation to human bait and this revealed a clearly bimodal crepuscular 24-hour cycle. In the circumstances which guided the research, it would seem to be correct to conclude that one is dealing with behavior, at least in large part, under endogenous command. The significance of this bimodality constitutes an open field for investigation. Under laboratory conditions, with species such as An. gambiae, indications of the existence of peaks for other kinds of activity, apart from hematophagy, such as copulation and egg-laying (Junes and Cribbins16, 1978; Jones15, 1981) were obtained. However, the fact that the present results have been obtained with human bait leads one to suppose that the bimodality registered reflects the feeding rhythm of An. darlingi in this region.
Though it was not possible to obtain sufficient data, there are indications to suggest the occurrence of similar local behavior of An. albitarsis.
In considering both the evening and morning crepuscular periods, the clear existence of two peaks was observed, one eocrepuscular immediately before the other, intracrepuscular. This bimodality can be regarded as a subdivision of the 24-hour bimodality. An adequate interpretation of this can only be achieved by means of further researches.
The seasonal variation of exophilous An. darlingi in this region follows the rhythm of the rainfall regime. Thus greater density is to be observed during the rainy months and lower density in the dry months.
To Prof. Dr. Jair Licio Ferreira Santos for his valuable help in the computer programing and the statistical calculations on the data obtained.
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Received for publication in 14/4/1987
Accepted for publication in 10/8/1987
Carried out with financial support of the "Fundação de Amparo à Pesquisa do Estado de São Paulo FAPESP" (Process Ecology 82/0286-1) and of the "Financiadora de Estudos e Projetos FINEP" (Agreement B/76/80/238/00/00).