ABSTRACT
The mosquito species diversity and fitness of rice fields in Minna, Niger State Nigeria were studied. Four selected Rice fields located in Fadikpe, Bosso, Maitumbi and Chanchaga areas of Minna, were investigated for the Ecology of Rice field Mosquitoes, as a pre- requisite for developing effective control measures. Mosquito Larvae and Water samples were concurrently collected in the sampled Habitats between August and October 2012, while Soil samples were collected in November same year. Immature mosquitoes recovered were reared to adults and identified in the Laboratory using Standard Morphological keys. Ten Mosquito species were encountered in the following order of decreasing abundance: Culex pipiens pipiens 651(21.9%)> Culex quinquefasciatus 421(14.2%)> Anopheles gambiae 400 (13.5%)> Anopheles funestus 334 (11.2%)> Culex  restuans 297 (10.0%)> Anopheles maculipalpis 187(6.3%)> Anopheles quardrimaculatus 178(6.0%)> Aedes dorsalis 173 (5.8%)> Aedes aegypti 169 (5.7%)> Aedes vexans 162 (5.5%), with species density significantly higher in rainy season than dry season. The results of water quality analyses indicated different degrees of variations in physicochemical parameters of the Rice fields in different locations. Statistically, Temperature, pH, Turbidity, Alkalinity, Hardness, Phosphate, Ammonia, Carbon dioxide, Chloride, Dissolved Oxygen and Biochemical Oxygen Demand were not significantly different (P>0.05) among Rice fields. However, Conductivity, Sodium, Potassium, Nitrate varied significantly (P<0.05). The results of dry season Soil analyses  revealed the following  mean values of physicochemical parameters as follows; Nitrogen=0.14%, Phosphorus=5.6mg/kg, Sodium=600mg/kg, Potassium=442.5mg/kg, Conductivity=224µS/cm, pH=7.61, Organic matter=1.59%, Sand=62.1%, Clay=30.1% Silt=7.8%. Statistically, the soil physicochemical parameters varied significantly (P<0.05) among Rice fields. The fluctuating asymmetry of Rice field mosquitoes revealed a high vectorial fitness. Significant Correlations between Mosquito abundance and physicochemical parameters of breeding Habitats were recorded for all species in both rainy and dry seasons. The Mosquito species encountered in this study are of Public Health importance, and the findings of the study should guide implementation of adequate vector control in Rice fields.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background of the study
According to fossil evidences, it was estimated that mosquitoes originated in the early tertiary period, some 70 million years ago or even earlier (Al-Sariy, 2007). They are distributed throughout the world. However, the majority are found in the tropics and subtropics. The warmer temperature in the tropics allows them to be more active and the rainfall provides them with aquatic sites for larval and pupal stages (Service, 2004)
Mosquitoes are significant pests not only to humans but also to domestic livestock with potential fatal out come. Although control measures are been taken but mosquito borne- diseases still thrive in many countries and cause millions of deaths (Tren and Bate, 2001). Mosquitoes are insect arthropods belonging to the Order Diptera and Family Culicidae with 3 sub families, namely; Anophelinae, Culicinae, and Toxorhynchitinae. About 43 genera and over 3,500 species of mosquitoes have already been described (Harbach, 2011)
Mosquitoes exploit all Kinds of lentic aquatic habitats for breeding, but Rice fields are amongst the most productive sites for mosquitoes. Rice agro- ecosystem perfectly fits the ecological requirement of mosquito vectors and specifically suitable for the pioneer species, members of the Anopheles gambiae complex (Al-Sariy, 2007). Water quality of aquatic habitats is an important determinant of female mosquito oviposition and successful larval development (Joseph, 2007). Larval abundance reflects oviposition preference of female. Changes in physicochemical characteristics of Rice field water may create conditions that are either favourable or unfavourable to breeding success. This can have implication for the transmission of vector-borne diseases.
Mosquitoes sometimes go into their own form of off season aestivation as the rainy season winds down. Depending upon the species and sometimes climate, mosquitoes can successfully survive the unfavourable dry period in the egg, larval or adult stage. The resumption of direct development in the resting or diapausing eggs is commonly influenced by a variety of environmental factors such as the rise in temperature and oxygen levels or exposure to light (Ricci, 2001; Gyllostrom and Hansson, 2004; Vanderkhove, Decbrick,Bredock,Condeporcuna, Jeppesen and deMecster, 2005). In some cases, specific abiotic factors such as seasonal changes in physicochemical characteristics and biochemistry of the host soil can stimulate the resumption of direct development.
The Vectorial fitness of mosquitoes is greatly influenced by its breeding ecology (Olayemi, 2008). Rice Management practices certainly have an influence on larval development and adult body size of mosquitoes. Body size is a pivotal trait for mosquitoes, because it has been related to survival, blood feeding behavior, reproductive success and vectorial fitness (Agnew, Hide, Sidoboc, and Michalakis, 2002). Wing size reflects body size, which is related to survival and reproductive success (Carron, 2007). Adult body size is often correlated with higher vectorial fitness. Larger body size confers better fitness (Carron, 2007)
The diverse mosquito species occurring in African Rice agro- ecosystems have been scarcely studied, despite the strong link between irrigated rice cultivation and mosquito- borne diseases (Muturi, 2007). This no doubt left farmers with little or no knowledge of the potential health hazards’ associated with mosquitoes arising from flooded rice fields. However, the knowledge of mosquito species, occurrence and distribution is an essential component of vector ecology and a guiding principle to the formulation and implementation of integrated vector management program (Ephantus, 2008).
As in other parts of the country, Rice fields in Minna constitutes potential breeding site for various mosquitos’ species (Olayemi, 2008). Rainfall and high temperature in the city area favours the bionomics of mosquitoes. Although, vector control is a major component of the Global Malaria Control Strategy (GMCS) and still remains the most effective measure to prevent Malaria transmission (Oruganje, Alaribe, Oduola, Adeogun, and Awoola, 2011). Successful application of vector control measures in a given location requires the understanding of the bionomics of the mosquito species responsible for the disease transmission, including the correct and precise identification of the target species and its distribution. Unfortunately the correct and precise identification and contribution of rice fields to the mosquito diversity and its physicochemical characteristics are yet to be determined from Minna in the North Central region of Nigeria.
1.2 Justification of the Study.
Irrigation is a key factor to enhance crop production, but it often results in negative health outcomes and consequential to the increased frequency and transmission dynamics of water-related vector-borne diseases. The widely practiced system of growing rice in mostly stagnant water, inevitably results in expansion of mosquito larval habitats, specifically suitable for Anopheles species. This is true in many parts of Nigeria, especially Minna in the North Central Zone as Malaria is strongly associated with water logging, poor maintenance of irrigation systems and rice cultivation. Similarly, the use of herbicides and insecticides to protect the crops from weeds and rice pests as well as the use of fertilizers to improve yield, inevitably results in changes in surface water quality and physicochemical parameters of breeding habitats, which affects the survival and development of Mosquitoes. However, studies on mosquito vectors in Nigeria, are not only few but limited to vector control measures which are further confounded by the presence of different ecological zones that support the breeding of diverse mosquito species. Unfortunately, there have been little or no entomologic studies to assess the impact of rice cultivation on mosquito production and disease transmission. This study seeks to fill the gap in the knowledge of mosquito bionomics, species composition and density of breeding and aestivating mosquitoes in the wetland rice agro ecosystem of Minna in north central Nigeria.
1.3 Aim of the Study
The aim of this study is to elucidate the ecology of Rice fields Mosquitoes, as a pre- requisite for developing effective control strategies.
1.4 Specific Objectives of the Study
a. To determine species composition, relative abundance and spartial distribution of mosquitoes breeding in rice fields in Minna during the rainy season.
b. To determine species composition, relative abundance and spatial distribution of mosquitoes aestivating in rice fields in Minna during the dry season.
c. To determine the body size and wing symmetry of both breeding and aestivating mosquitoes in rice fields in Minna.
d. To determine the physicochemical characteristics of rice field mosquito larval habitats in the area.
e. To determine the physicochemical properties of soil samples from rice field mosquito habitats.
1.5 Significance of the Study
The diverse mosquito species breeding in Nigerian rice fields are scarcely studied despite the strong link between rice cultivation and mosquito-borne diseases. The outcome of this research will provide public health stakeholders with adequate knowledge of the potential health hazards associated with mosquitoes arising from flooded Rice fields and reduce the high health and socioeconomic burdens of mosquito borne-diseases. Understanding of mosquito species occurrence and fitness is an essential component of vector ecology and a guiding principle to the formulation and implementation of integrated vector control strategies.
1.6 Scope and Limitations of the Study
The study is limited to rice fields’ mosquito larval breeding habitats during the rainy and dry seasons. Larval collection and water quality analyses were conducted only during the rainy season in the various sampling locations. Hatching and rearing of aestivating mosquitoes and the analyses of soil physicochemical properties were conducted during the dry season only. Species composition and relative abundance were determined at adult stage.
This material content is developed to serve as a GUIDE for students to conduct academic research
MOSQUITO SPECIES COMPOSITION, ABUNDANCE AND PHYSICOCHEMICAL FACTORS OF RICE FIELDS IN MINNA, NIGER STATE, NIGERIA
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