Elsevier

Acta Tropica

Volume 132, Supplement, April 2014, Pages S35-S41
Acta Tropica

Occurrence of natural Anopheles arabiensis swarms in an urban area of Bobo-Dioulasso city, Burkina Faso, West Africa

https://doi.org/10.1016/j.actatropica.2013.12.012Get rights and content

Highlights

  • We investigated the natural swarming system of Anopheles arabiensis in urban area of Bobo-Dioulasso.

  • An. arabiensis swarms occurred indoors as well as outdoors above visual markers such as rooftops, waste or bare ground.

  • An. arabiensis also swarmed near the breeding sites contrary to An. gambiae.

  • The future works will address the male dispersion, the mating and feeding patterns of this species in urban area.

Abstract

The swarming behaviour of natural populations of Anopheles arabiensis was investigated by conducting transect surveys on 10 consecutive days, around dusk, from March to April and from September to October 2012 in Dioulassoba, a district of Bobo-Dioulasso city in Burkina Faso (West Africa). Swarms were observed outside, around identified larval breeding sites on the banks of the Houet River, as well as in the open-air courtyards found at the centre of many homes in the region. Swarms were found to occur in open sunlit spaces, mostly located above physical or visual cues somehow visually distinct from the surrounding area. Overall 67 and 78 swarms were observed, respectively, during the dry season (March–April) and the rainy season (September–October) of 2012, between 1.5 m and 4.5 m above the ground at their centre. 964 mosquitoes were collected and analysed from dry season swarms, of which most were male, and all were An. arabiensis, as were the few resting mosquitoes collected indoors. Larvae collected from breeding sites found on the banks of the Houet River mostly consisted of An. arabiensis and only a minority of Anopheles coluzzii (formerly identified as An. gambiae M form). Of 1694 mosquitoes analysed from 78 swarms in the rainy season collections, a few An. gambiae (formerly known as An. gambiae S form) males were identified, and the remainders were An. arabiensis. The majority of larvae collected during the wet season from the same breeding sites were identified as An. arabiensis followed by An. coluzzii and An. gambiae. The same pattern of species composition was observed in resting mosquitoes, though the proportion of An. arabiensis was less overwhelming. These data support the conclusion that An. arabiensis is the most prevalent species in this area, though the difference in species composition when using different population sampling techniques is noteworthy. Further studies are required for more detailed investigations of male dispersal, feeding behaviour and mating patterns in this urban setting.

Graphical abstract

Anopheles arabiensis swarming system was described at the first time in Burkina Faso (West Africa) surprisingly in a central urban area crossed by a River.

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Introduction

In the less humid countries of tropical Africa, Anopheles arabiensis Patton, a member of the Anopheles gambiae Giles species complex, is a major malaria vector, particularly in southern Africa, Madagascar and along a wide East–West belt fringing the Sahel (Della Torre et al., 2005). Generally, An. arabiensis is regarded as being more exophilic, exophagic and zoophilic than An. gambiae Giles sensu stricto, but even so it remains a highly efficient vector of malaria (White, 1974). This species is thus an important subject of research, with a particular need for a greater understanding of its biological, genetic and ecological characteristics as they relate to malaria control (Petrarca et al., 2000, Costantini et al., 2005, Tirado et al., 2006, Taye et al., 2006). The release of sterile male mosquitoes into malaria and dengue endemic regions is proposed as a technique to control mosquitoes and reduce incidence of disease, and known as the Sterile Insect Technique, or SIT (Benedict and Robinson, 2003). This might be an effective means to reduce malaria cases in this region of Burkina Faso, but this approach relies on an appropriate knowledge of mosquito mating biology (Ferguson et al., 2005). In the past ten years the swarming and mating characteristics of natural populations of An. gambiae s.s. have been relatively well studied, especially in West Africa (Tripet et al., 2005, Diabaté et al., 2006, Diabaté et al., 2009, Diabaté et al., 2011, Dabiré et al., 2013). However, despite its medical and epidemiological importance, little is known about An. arabiensis natural swarming and mating patterns in the field (Marchand, 1984, Charlwood et al., 2002). In particular, there is currently very little basic information about An. arabiensis swarming behaviour, including swarm location and composition, factors influencing mating success, and the relative competitiveness of laboratory grown and wild male mosquitoes following release of the former into the field. The availability and diversity of plant nectar, as well as factors such as the nature of ground cover and climatic conditions are all likely to impact male dispersal patterns, which in turn are likely to affect the nature and locations of swarms. Before basic knowledge is gained about these and other elements of the mating behaviour of disease vectors it will be difficult to achieve successful population control using SIT.

Recent studies have shown that An. coluzzii and An. gambiae (formerly An. gambiae M and S molecular forms, respectively, (Coetzee et al., 2013), mainly have distinct, separate swarm sites (though shared sites have been reported, REFs), suggesting that the environmental and endogenous cues that determine the nature of swarming/mating sites must be under strong genetic control (Diabaté et al., 2009, Diabaté et al., 2011, Dabiré et al., 2013). There is concern therefore, that the mating competitiveness of released sterile males may be compromised by selection pressures during laboratory rearing, that alter their responsiveness to swarming cues under natural conditions (Bartlett, 1984, Peloquin and Asman, 1988, Howell and Benedict, 2009). These observations about the selection of swarm sites also indicate that males consistently arrive at predictable sites every day, and local knowledge of these sites may be of great use when monitoring overall population densities during mosquito control campaigns. As part of a series of several studies into the adaptation of An. arabiensis to an urban setting, the goal of the current study was to gather data on the locations and characteristics of swarms of mosquitoes in an urban setting within the city of Bobo-Dioulasso, Burkina Faso. Previous work has shown that An. gambiae s.s. and An. arabiensis are sympatric in Dioulassoba, with the proportion of An. arabiensis increasing over the last decade (Dabiré et al., 2012). Particular emphasis was placed on investigating species composition and the visual cues used by males to select a swarm site. Selected houses were sampled for resting adult mosquitoes and larval breeding sites were also sampled; collected individuals were identified by species.

Section snippets

Study area

Dioulassoba (11°10′42″ N; 4°17′35″ W) is a longstanding residential district in the centre of Bobo-Dioulasso, the second largest city in Burkina Faso located in the Southwest of the country. This district is crossed by a permanent stream along which are dotted small plots of vegetables and herb plants grown by the residents, and which is polluted with domestic waste litter and waste water (including sewage). Small puddles along the banks of the stream and the surrounding borders constitute

Physical and visual markers for swarm formation

All swarm sites and breeding sites where larvae were sampled were geo-referenced, which allowed a map of swarms and the dispersal of breeding sites across the district to be produced both from dry and wet season surveys (Fig. 1). During both the dry and the rainy seasons swarms were observed along the length of the transect and fairly evenly distributed, though concentrated slightly more on the edges of blocks of residences and along the banks of the Houet river. Larval breeding sites were

Discussion

The desire to investigate mating and swarming behaviour in wild populations of anopheline mosquitoes is not new. In fact the state-of-the-art research began early (Downes, 1969) and since these first observations several types of insects whose mating systems require swarms have been closely studied (e.g. Sullivan, 1981, Cooter, 1989) and with more relevance to this current study by Yuval et al. (1993), who provided a thorough description of swarming and mating behaviour within natural swarms of

Conflict of interest

Authors declare no conflict of interests.

Authors’ contributions

SPS conducted fieldwork and contributed to data analyses. HDF and BSNS conducted fieldwork and contributed to molecular analysis in the lab. SDD conducted the PCR analysis. MA drew the swarm map. BT, GLC, SF, TL, DA and LRS, contributed to drafting and revising the manuscript. GJ and DKR designed the study and wrote the manuscript.

Acknowledgments

Authors are grateful to the people of Dioulassoba who were very cooperative and allowed this work to be carried out in a very good working ambiance. Authors thank Ouari Ali, field technician of IRSS/Centre Muraz who initiated the fieldwork and geo-referenced the swarm's sites. This work was supported by an IAEA grant, contract no 16190/R0, accorded to Dabiré K. Roch under CRP G34002.

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