Surveillance of mosquito larvae (Diptera: Culicidae) in microhabitats of a University Campus in Southwestern Nigeria

Introduction: Mosquitoes, vectors of vital diseases, are among the most deadly animals in the world. Their faunistic composition and prevalence of these mosquitoes are dependent on the characteristics of larval habitats. Objective: To determine how habitats influence the composition, abundance and distribution of the mosquito larvae on Obafemi Awolowo University, Ile Ife campus. Methods: We sampled larvae monthly between 07: 00 and 10:00 (August, 2017 January, 2019) using a standard dipping method. The microhabitats included ground pools, discarded containers, drainage channels and tree holes in each of the study sites. Results: We identified seven species distributed in three genera: Anopheles spp. (Anopheles gambiae, Anopheles funestus); Aedes spp. (Aedes aegypti, Aedes albopictus); Culex sp (Culex pipiens, Culex quinquefasciatus, Culex poicilipe). Culex was the dominant and most abundant genus (48%). Larvae were significantly (p<0,05) more abundant in the dry season. The highest abundance of mosquitoes was recorded in Akintola Hall (50,1%) and drainage channels were the main microhabitat. Location suggests that these species have affinity for humans and their dwellings. Conclusion: We recommend elimination of breeding sites (un-kept drainage channels, leakages and discarded domestic containers).

Over the years, mosquito infestation has been a major public health concern in Nigeria. Infact, mosquitoes have been implicated for the transmission of important diseases such as; Malaria, Filariasis, Yellow fever, dengue fever etc. These diseases have been reportedly responsible for the death of about one billion people yearly (WHO, 2014). Little wonder the mosquitoes have been described as the deadliest creature on earth (WHO, 2015). The high prevalence of mosquitoes has impacted the social well-being of individuals living in such areas of high prevalence by the diseases transmitted by the mosquitoes (Amiruddin, Sidik, Alwi, Islam, & Astuti, 2012). Furthermore, malaria and these other diseases vectored by mosquitoes has been identified as one of the leading causes of childhood and maternal morbidity, low productivity and reduced school attendance in Nigeria. Although, there has been improvement in health facilities in most developing countries and this is expected to yield a corresponding reduction in the transmission of malaria but on the contrary, the disease persists in several African cities. In fact, the prevalence is much higher in the urban areas than in the rural areas (Mbanugo & Okpalaononuju, 2013).
Climatic and environmental variables have been identified as major factors influencing the distribution of mosquitoes in the environment (Amusan, 2004). Also, it has been discovered that the vectoral capacity of mosquitoes for the transmission of diseases is dependent on the rate of larval production from the breeding habitats (Depinay, Mbogo, Killeen, Gnols, & Carlson, 2004). Mosquitoes have not only shown preference for all sorts of stagnant water, they are also known to require only a few tablespoons of water to breed. As a result, tyres, ponds, ground pools, streams erosion pits, leaking pipes, tree holes and discarded containers have been mentioned as important breeding sites for mosquitoes. Environmental factors such as temperature, rainfall and humidity are also considered essential in the duration of mosquito development and longetivity. For instance, temperature is regarded as the most important factor as it is responsible for variations in the maturation period of the mosquito larva while rainfall does not only affect relative humidity, it also increases the available breeding sites (Ojo & Mafiana, 2001).
Surveys and collection studies have been identified as important steps in the determination of the risks involved with diseases and possibly the development of control and mitigating plans for vector-borne diseases. In Nigeria, previous studies on the distribution and diversity of mosquitoes larvae have revealed Anopheles, Aedes, Culex and Mansonia as the commonly encountered group of mosquitoes in the country. These genera have been listed among the deadly mosquitoes in Africa (WHO, 2014). This is the essentiality of the study of the ecology of this important group of insects. This study therefore seeks to provide information on the abundance, distribution and ecology of the various mosquitoes within the selected microhabitats on Obafemi Awolowo University, Ile-Ife campus. This study will further seek to ascertain if the environmental factors associated with the microhabitats are important for the accurate predictions of the occurrence of mosquitoes. The information provided here-in will be useful in planning more effective vector control strategies.

METHODOLOGY Study Area:
The study was conducted within the campus of Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. The University lies between 07°31'14.76"N & 04°31'49"E. The total land mass is about 5,605 hectares. The area stands at an average height of 300masl with an average altitude of 1200m. Ile -Ife receives about 1500 mm of rainfall annually and the ambient temperature ranges between 20°C and 30°C with a mean of 27°C (Komolafe & Arawomo, 2008). The campus community has rich vegetation typical of tropical rainforest belt and grassland with few swampy areas. The major source of water within the University campus is Opa reservoir which provides potable water and other ancillary functions to the University community. Apart from the reservoir, there are other streams and ground pools with intermittent flows. These ground pools are usually stagnant and conducive for breeding of mosquitoes especially during the dry season.
Study sites: Three different locations within the campus were selected for mosquito larvae sampling. The Study sites selected include; Akintola Hall (A residential hostel), BOO park (A semiforest) and Opa reservoir (The University water scheme area). These study sites are characterized by different vegetation gradient and they are located about 4km apart.

Ecological description of the study sites
Akintola Hall: This study site comprised a student hostel which accommodates about 1 400 students living in eight blocks of three storey building. The site is surrounded by tall trees, shrubs and abundant grasses. The area is characterized by numerous leaking pipes, drainage channels and patched of ground pools with stagnant water. A stream named Aho stream runs through the back of the hostel.
Boo Park: This is a semi-forested area located close to the academic area of the University campus. There are lots of tall trees forming dense canopies in the area. This area is also characterized by varieties of ornamental plants. Lots of tree holes, ground pools, and patches of stagnant waterbodies are found within this study site.
Opa Reservoir: This area houses the University's water scheme. The reservoir provides potable water and other ancillary functions to the University community. This site is characterized by lots of submerged and terrestrial vegetation. Water treatment tanks, pipes, storex tanks, drainage channels and containers are also in abundance in this study sites.
Sampling Techniques: Mosquito larvae were collected using standard dipping method with a mosquito scoop (Bioquip, Gardena, USA) in the various selected microhabitats. Where dippers could not be used (especially in containers), the content of the containers were emptied into plastic bowls for easy collection of the mosquito larvae (Service, 1993). Larvae sampling was conducted monthly between 07: 00 and 10:00 for six months (August, 2017 -January, 2019) representing three months each of wet and dry seasons. The various microhabitats sampled included; ground pools, discarded containers, drainage channels and tree holes in each of the study sites.
Identification of Mosquito larvae specimens: The collected mosquito specimens were identified to species level by standard identification keys and guides such as Gillet (1972), Oyerinde (1999) andHarbach (2008).
Data analysis: Diversity indices were used to estimate the diversity of mosquito larvae among the study sites. Pearson correlation analysis was used to test the relationship between the mosquito species and the environmental variables recorded. These analyses were done using Patheological Statistics (PAST, Version 3,14).
Ethical, conflict of interest and financial statements: The authors have fully complied with all pertinent and legal requirements both during the study and in the production of the manuscript. We state that there are no conflicts of interest of any kind. The financial sources are fully and clearly stated in the acknowledgements section and we fully agree with the final edited version of the article. A signed document has been filed in the journal archives.

RESULTS
A total of 611 individuals of mosquitoes which comprised three genera (Anopheles, Aedes and Culex) were collected for the entire study period. These three genera were represented by seven species as thus; Anopheles gambiae, Anopheles funestus, Aedes aegypti, Aedes albopictus, Culex pipiens, Culex quinquefasciatus, Culex poicilipes. Culex was dominant over the two other genera as it accounted for 48% of the total collection while Aedes and Anopheles represented 26% and 25% respectively. All the three genera were recorded in all the investigated microhabitats within the study area. Culex pipiens was the dominant mosquito species in the study area as it accounted for the highest number of individuals (159) collected and this represented 26% of the entire collection. Other species that also occurred in relatively high numbers include; Anopheles gambiae, Aedes aegypti and Culex quinquefasciatus. Culex poicilipes accounted for the least number of individuals (48) and this represented 7,9% of the entire collection (Table 1). Amongst the study sites that were investigated, the highest abundance of mosquitoes was recorded in Akintola Hall. A total of 306 mosquito larvae were collected from this study site and this represented 50,1% of the entire collection. Opa reservoir accounted for 223 mosquito larvae which represented 36,5% of the total collection while BOO park accounted for the least number of individuals (82) which accounted for 13,4% of the total collection. Seasonal analysis of the mosquito larvae collection revealed that more larvae were collected in the dry season than in the wet season (Fig. 1).
A total of 330 mosquito larvae were collected in the dry season while 281 were collected in the wet season which represented 54% and 46% of the total collection respectively ( Table 2). It was also observed that significantly higher (p<0,05) number of mosquitoes larvae were recorded in the dry season in the microhabitats sampled except in the leaking pipes where the difference was not statistically significant (p>0,05) (Table 3). Investigations further revealed that the highest abundance of mosquito larvae was recorded in the drainage channels as it accounted for 277 ind. (which represented 45% of the entire collection). Leaking pipes and ground pools accounted for 89 (15%) and 87 (14%) individuals respectively. The least abundance of mosquito larvae was recorded in the Tree holes where 54 (9%) individuals were collected.

DISCUSSION
This study revealed that seven species distributed in three genera were contained in the study area as at the study period. This suggests a relatively high diversity of mosquitoes within the study area. This observation is similar to earlier reports such as; Oguoma and Ikpeze (2008), Egbuche, Ezihe, Aribodor, and Ukonze (2016), Onyekachi, Carmelita, Ohaeri, Amaechi, and Collins (2018) and Okwa, Omolade, Savage, and Adetutu (2018) in which similarly high diversity of mosquitoes were recorded within relatively similar landscapes. The high species richness in this study area may be attributed to availability of diversed suitable breeding microhabitats such as domestic containers, drainage channels, leakages, tree holes and ground pools. The availability of suitable microhabitats in the study area is further corroborated by the occurrence and abundance of the seven species in each of the tree study sites (Opa Reservoir, Akintola Hall and BOO Park). The occurrence of all the recorded species in the three study sites suggested that each of the study sites provided suitable breeding habitats for the mosquito species recorded in this study.
Culex was the dominant and most abundant genera in this study. The occurrence of Culex sp as dominant and most abundant species is not strange as this group of mosquitoes have been reportedly dominant and abundant in studies in different part of the country. Such studies include; Mafiana, Anaeme and Olatunde (1998), Anosike et al. (2007), Onyido, Ezike, Ozumba, Nwankwo, andNwankwo (2009). According to Farjana, Ahmmed, Khanom, Alam, and Begum (2015) and Okwa et al. (2018), Culex mosquitoes are usually common in polluted waters and sites with foul smells. The high prevalence of Culex in this study area could possibly be attributed to their ability to breed in polluted waterbodies (Mafiana et al., 1998). It is worth noting that the highest abundance of Culex sp was recorded in Akintola Hall, which could be regarded as the dirtiest of the three study sites. This further confirmed the preference of this group of mosquitoes for dirty and polluted habitats.
Aedes also occurred in relatively high number as it accounted for about 26% of the entire collection. This group of mosquitoes has been known to breed very well in shallow waterbodies and domestic containers (WHO, 2002). Apart from shallow waters, ambient temperature, relative humidity and water quality have also been mentioned as factors that influence the occurrence and abundance of the Aedine mosquitoes in any particular habitat (Chen, Lee, Stella-Wong, Lau, & Sofian-Azirun, 2009;Dejene, Habte, Teshome, Meshesha, & Akalu, 2015). Aedes (which is the principal vector of dengue viruses) is known to be closely associated with humans and their dwellings. They are known to breed very well in water holding containers in and around the home (Senthamarai & Jebanesan, 2016). Little wonder a large number of the Aedes mosquitoes were recorded around Akintola Hall of residence.
Anopheles mosquitoes were recorded in relatively fewer numbers in this study. This is surprising as these mosquitoes have been previously mentioned as dominant and abundant in several mosquito surveys in Nigeria. This dominance has been attributed to their ability to breed successfully in discarded containers and other microhabitats. In fact, Opoku et al. (2007) and Aigbodion and Odiachi (2003) recorded large number of Anopheles mosquitoes in containers and domestic run off. However, Anopheles mosquitoes are known to have preference for cleaner and well oxygenated waters. Therefore, the relatively fewer number of individuals collected in this study may be due to the inadequacy of such clean waterbodies in the study sites.
Overall, more mosquito larvae were collected in the dry season than in the wet season. This observation is similar to the report of Anosike et al. (2007) and Okwa et al. (2018) in which abundance of mosquitoes were greater in the dry season than in the wet season. This observation may be due to the abundance of rainfall in the wet season which has been known to be detrimental to the survival of mosquitoes in the environment. Many larvae and potential breeding sites are washed off by run-off, thus causing a reduction in the population of the mosquitoes in the wet season. In the dry season, there are usually more stagnant waters in the environment, which is considered favourable and suitable for the breeding of mosquitoes.
It is also exciting to note that the three identified genera co-breeded in the same habitats as the three genera were recorded in all the microhabitats in the three selected study sites. There were variations in the abundance and distribution in the microhabitats which could be attributed to the differences in environmental conditions or microclimatic variations as well as availability of suitable preferred breeding sites.
This study has provided vital information on the faunistic composition, abundance and distribution of mosquito larvae within Obafemi Awolowo University, Ile -Ife Campus. A combination of factors such as indiscriminate disposal of domestic containers, leakages, unkept gutters and drainage channels provided enormous suitable breeding sites for the mosquitoes. Elimination of such mosquito breeding sites can be an effective and sufficient means of abating the prevalence of mosquitoes in the environment.

AKNOWLEDGEMENTS
We are grateful to the authority of Obafemi Awolowo University, Ile -Ife for granting us access to the various locations where the mosquito larvae collections were done.