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Table of Contents
Year : 2022  |  Volume : 59  |  Issue : 3  |  Page : 265-274

Ecology of Aedes vittatus (Diptera: Culicidae) in rock pools across agroecosystem in Northern Savanna, Nigeria

1 Department of Zoology, Federal University of Agriculture, Makurdi, Benue State, Nigeria
2 Department of Zoology, Ahmadu Bello University, Zaria, Kaduna State, Nigeria
3 Department of Biotechnology, School of Sciences, Mewar International University, Masaka, Nasarawa State, Nigeria

Date of Submission10-Oct-2021
Date of Acceptance21-Feb-2022
Date of Web Publication08-Dec-2022

Correspondence Address:
Okechukwu A Obi
Department of Zoology, Federal University of Agriculture, Makurdi, Benue State
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-9062.342395

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Background & objectives: This study focuses on modulating dexterity of some ecological variables of Aedes vittatus classically breeding in rocky habitats. The study provides a useful insight into ecological variables that underpin or hinder profuse breeding of Ae. vittatus in rock pools and its probable role in disease transmission.
Methods: HANNA HI98129 pH/EC/TDS/TEMP meter was used in situ while standard protocols were used to determine other hydro-chemical variables. Aedes vittatus larvae were obtained with soup ladle and modified ladle dippers. D-frame net was used to capture macroinvertebrates while plankton net was used to obtain samples of microalgae. Tadpoles and water turtles were collected with fine mesh invertebrate net. Macrophytes were uprooted and identified at the Herbarium Unit, Department of Botany, Ahmadu Bello University, Zaria. The influence of physicochemical variables was correlated with distribution of Ae. vittatus using Principal Component Analysis. Regression and ANOVA were used to test for association between predictor variables and mosquito abundance and for the difference amongst inselbergs.
Results: Linear larval density of Ae. vittatus in rock pools which tapered across Guinea savanna were obtained from twenty-one sites with average density of 139.6 in Sudan savanna. Guinea savanna had an average larval density of 75.5 with lower subsets of moving average densities compared to Sudan savanna. One hundred and sixty-one aquatic insects belonging to four insect orders cohabited rock pools with Ae. vittatus. Toads and frogs’ tadpoles were of Bufonidae and Pyxicephalidae families while water turtles belong to Emydidae. pH, TDS (ppm), EC (μs/cm) and alkalinity (mg/l) differed significantly (p<0.05) with the abundance of Ae. vittatus in rock pools. Temperature, depth, water hardness and total suspended solid had direct influence on the distribution of Ae. vittatus in rock pools across sites. Significant positive correlation exists between aquatic insects and abundance of Ae. vittatus. Hydroperiod length, concentration of nitrate and pH were determinants that leverage profuse breeding of Ae. vittatus and survival of rock pool biota.
Interpretation & conclusion: Results revealed that the bearing influence of rock pool variables is inevitable for breeding of Ae. vittatus. A well defined measure of efficacy incorporating indigenous communities for sustained vector control on inselbergs will go a long way in decimating population of Ae. vittatus and limit the risk of spread of yellow fever hitherto areas not thriving.

Keywords: Aedes vittatus; rock pools; abundance patterns; biota composition; physicochemical variables; yellow fever

How to cite this article:
Obi OA, Adebote DA, Nock IH, Josiah JG. Ecology of Aedes vittatus (Diptera: Culicidae) in rock pools across agroecosystem in Northern Savanna, Nigeria. J Vector Borne Dis 2022;59:265-74

How to cite this URL:
Obi OA, Adebote DA, Nock IH, Josiah JG. Ecology of Aedes vittatus (Diptera: Culicidae) in rock pools across agroecosystem in Northern Savanna, Nigeria. J Vector Borne Dis [serial online] 2022 [cited 2023 Feb 2];59:265-74. Available from: http://www.jvbd.org//text.asp?2022/59/3/265/342395

  Introduction Top

Aedes vittatus is highly anthropophilic and predominantly a rock-hole breeder but there are reports of its breeding in other aquatic habitats[1][2]. Ae. vittatus is one of the competent vectors that play a vital role in the maintenance and transmission of yellow fever in Nigeria. Yellow fever is endemic in Nigeria[3][4], [33] and this country is one of the high-risk countries for yellow fever transmission[33][34]. Outbreaks have been documented, with the largest outbreaks reported between 1985 and 1991 with over 40,000 suspected cases reported[35][36]. The re-emergence of yellow fever in 2017 in Nigeria was marked by outbreaks with a total of 7894 suspected cases and 287 laboratory-confirmed cases in 55 Local Government Areas (LGAs) between September 2017 and September 2019[34],[37]. A total of 1312 suspected cases of yellow fever were reported from January to August 2021 in 367 LGAs across 36 States and the Federal Capital Territory (FCT)[33]. Aedes vittatus has also gathered public attention since its involvement in the transmission of Zika virus[5]. Their preference for wanton breeding in rock pools is influenced by ecological factors that have implications on their abundance, affecting their temporal and spatial distribution[6]. The presence and abundance of their larval stages can be controlled by the environmental and physicochemical variables of the habitat and by the occurrence of species interactions like intra or inter-specific competition, predation and mutualism[5]. Invertebrate communities of the rock pools assist in the breakdown of organic matter and cycling of nutrients and may become food for predators[7].

Predatory aquatic insects vary markedly in the different habitats that favour larval and adult mosquitoes with important effects on ecosystem processes such as primary production, detritus breakdown and nutrient mineralization and downstream spiraling[8]. The resurgence of yellow fever and probable emergence of new vector mosquitoes is caused by several factors viz., ecological changes, presence of human and potential vector population densities and the presence of suitable reservoirs[9]. In addition to these factors, Ae. vittatus eggs have been shown to survive over dry season in rocky habitats for prolonged periods[10], sustaining the transmission cycle of arboviruses in nature. Environmental factors including temperature, dissolved oxygen, conductivity and pH may affect predator and prey numbers. Therefore, it has been proposed that fluctuating abiotic conditions and interactions among species affect predators and prey differentially[11]. Moreover, ecosystem processes operating at different organization levels, and temporal and spatial scales, regulate the patterns of productivity of mosquito larval habitats in a larger landscape context[12]. Rocky outcrops assortment is open to mosquito larval ecological variables and harbour freshwater communities that are located higher on the shore between the rocky intertidal and terrestrial habitat. Community composition of the rock pools significantly associated with differences in pool size, hydroperiod and food resources. Rock pools are small assorted temporary freshwater which has great potential as field laboratories since they are often small and locally numerous, providing easy manipulation and statistical power, respectively[13]. They also usually show a gradient in several important environmental variables allowing the study of various ecological processes.

Understanding species biological limits to both biotic and abiotic variables across environmental gradients provide useful insights into how assemblages of mosquitoes are structured. Rock pools on inselbergs appear to be latent but yet significant breeding ground for Ae. vittatus and some other species of disease vectors. Despite high risk of spread of yellow fever in Nigeria, Ae. vittatus has not been given stern attention in its role as an arboviral vector coupled with suboptimal surveillance of yellow fever[33], in most states and nationwide. The inattention to inselbergs in mosquito control efforts in Nigeria could contribute to a magnitude of unusual deaths resulting from intermittent outbreaks of yellow fever[14], [34], [37]. Yellow fever outbreaks have recently been on the increase both in northern[15], and southern Nigeria[16]. These outbreaks could be exacerbated where suboptimal surveillance and immunization coverage in susceptible population may be present and thus, a risk of serious public health impact. This study is aimed at determining the ecological variables influencing profuse breeding and probable role of Ae, vittatus in disease transmission from rocky outcrops within the Nigeria’s Northern savanna in Kaduna State.

  Material & Methods Top

Study area

This study was carried out in rural and satellite towns of Kaduna State which is traversed with two disproportionate tropical grassland vegetation covers; majorly landscaped with Guinea savannah along its southern parts and Sudan savannah along its northern axis [Figure 1]. The State experiences dry and rainy seasons in a year and is endowed with numerous rocky hills that serve as a haven for picnics and hill climbing[17]. The rocky hills are indeed a tourist paradise and places of worship which were considered as sacred; thus bestowing it with a religious significance. Some are of great cultural value especially to the people of Kwoi and Nok as some inselbergs assumed the carriage role of community deity. The outstanding tropical grassland at the foot of some of the rocky hills provide permanent settlement for nomadic cattle herdsmen while the steep slope and serene environment below the hills make it a suitable environment/location for movie practitioners, drama groups and shooting of music videos[17]. This cultural landscape has great historical importance to the people of Kufena and Zaria, being the area of early settlement as evidenced by the remains of city wall around the hill, as well as some settlement which remain on the hill.
Figure 1: Map of Nigeria showing the study locations with the two major savanna regions in Kaduna State.

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Of twenty-three Local Government Areas of Kaduna State, twenty-one cities and towns which sparse over nine Local Government Areas were surveyed for breeding of Ae. vittatus in rock pools on inselbergs [Figure 1]. Across Sudan Savanna grassland, two Local Government Areas were surveyed which include Sabon-Gari Local Government Area within Hanwa and Zaria Local Government Area within Danmagaji, Kufena, Dumbi, Dutsen-Abba, Wusasa and Zango-Aya. All accessible inselbergs (including some sacred ones surveyed with permission) across Guinea Savanna were distributed into seven Local Government Areas. These include: Igabi Local Government Area within Kangimi sample sites; Kaduna North Local Government Area within Malali sample sites; Chikun Local Government Area with Baban-Sora sample sites; Kajuru Local Government Area within Kujama, Kajuru, Kufana and Tudun-Mare sample sites; Kagorko Local Government Area within Kagorko sample sites; Jaba Local Government Area within Nok, Kwoi, Chori and Samban-Gidan sample sites and Jema’a Local Government Area within Kagoma sample sites.

Sampling of Ae. vittatus and rock pool biota

At every site true and edged rock pools on each inselberg were recorded and inspected for the presence or absence of Ae. vittatus larvae, other rock pool biota and associated hydro-physicochemical variables of the habitats [Figure 2]. Each site was visited once and were surveyed between 8 am and 5 pm on each day. Depending on the size and depth of the rock pool, up to ten dips were taken from each rock pool using a modified[13] and standard ladle dipper[18] into a white plastic bowl. Matured Ae. vittatus larvae were carefully concentrated and were picked with dropping pipette into labeled specimen bottles and stored in 70% ethanol. Lower instars of Ae. vittatus were nurtured to the fourth instar with water from the larval habitats in labeled plastic bowls on a diet of baker’s yeast[18]. Plankton net was used to obtain samples of microalgae and were preserved in 4% formaldehyde. D-frame net was used to collect aquatic insects. Tadpoles and water turtles were obtained with fine mesh invertebrate net and were sorted and preserved in 70% ethanol in appropriately labeled specimen bottles[11]. Aquatic macrophytes were uprooted with hand and taken fresh to the Herbarium Unit, Department of Botany, Ahmadu Bello University, Zaria, Nigeria for identification.
Figure 2: Aerial surfaces of inselbergs with true rock pools and edged rock pool within Kaduna state. (A) true rock pool on kufena inselberg with algal bloom; (B) true rock pool on Samban-Gida inselberg; (C) true rock pool on Jere inselberg; (D) true rock pool on kangimi inselberg; (E) aerial view of Dumbi inselberg with edged (dried) rock pool; (F) aerial view of Zango-Aya inselberg showing ongoing quarrying activities.

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Hydro-physicochemical variables

Rock pool depth and surface areas were measured and estimated at random along the edges of the pools with a calibrated stick from a meter rule. pH, electrical conductivity, total dissolved solids and temperature were determined in situ with a HANNA HI 98129 pH/ EC/TDS/ Temp meter[19]. Samples of water from each rock pool were removed for determination of turbidity, total alkalinity, total suspended solids, total hardness, dissolved oxygen, biological oxygen demand, chemical oxygen demand, phosphate and nitrate at the Department of Water Resources and Environmental Engineering Department, Ahmadu Bello University, Zaria, Nigeria following standard procedures[20].

Identification of Ae. vittatus and rock pool biota

Ae. vittatus mosquitoes were morphologically identified to species taxon[21]. Aquatic insect species were observed and counted during the sampling process and was recorded for each sample habitat. Those that could not be identified to the species level in the field were preserved for identification[22]. Algae were quantified and identified to species by the methods and keys[23]. Tadpoles were identified with[24] while Water turtles were identified based on CITES identification guide[25]. Aquatic macrophytes in the rock pools were identified at the Herbarium Unit, Department of Botany, Ahmadu Bello University, Zaria, Nigeria.

Statistical analysis of data

The relationship between physicochemical variables and rock pool biota on the larval density of Ae. vittatus was determined with Pearson’s correlation. The best predictor variables associated with larval density of Ae. vittatus was tested using logistic regression. The mean differences in physicochemical variables were compared with Analysis of Variance (ANOVA). The interaction of physicochemical variables on the distribution of Ae. vittatus larvae across locations was correlated with Principal Component Analysis (PCA). The IBM SPSS statistical package (SPSS Inc., Chicago) version 22 was employed for ANOVA, Pearson’s correlation and Logistic regression analyses while Paleontological Statistical tool (Past) version 1.81 was used for PCA.

Ethical statement: Not applicable

  Results Top

A search for rock pools on inselbergs within the suburbs of Kaduna State were sought for the breeding activities of larval Ae. vittatus during the wet season for seven months between April and October 2015 [Figure 1]. Twenty-one cities and towns endowed with inselbergs which cut across the Sudan and Guinea savanna of the State were surveyed for ecological parameters of Ae. vittatus in rock pools. Linear larval density of Ae. vittatus which tapered across Guinea savanna had an average density of 139.6 in Sudan savanna [Figure 3]. Guinea savanna had an average larval density of 75.5 with lower subsets of moving average densities compared to Sudan savanna. Moving average larval density of Ae. vittatus were nearly even amongst Kagarko, Kagoma, Kajuru, Kangimi, Kujama and Kwoi inselbergs. Wusasa had the highest larval density seconded by Hanwa inselbergs in Sudan savanna while the least larval density was recorded in Dutsen-Abba inselbergs. Malali yielded the highest larval density of Ae. vittatus in Guinea savanna followed by Baban-Sora inselbergs. Tudun-Mare yielded the least larval density while Kufana inselbergs had zero density of Ae. vittatus.
Figure 3: Larval densities of Ae. vittatus in rock pools across agroecosystem in Sudan and Guinea savanna of Kaduna State, Northern Nigeria.

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A total of one hundred and sixty-one aquatic insect species belonging to four orders - Odonata (38.5%), Hemiptera (19.3%), Coleoptera (1.2%) and Diptera (41.0%) were collected from 94 inselbergs [Table 1]. Of the 4 insect orders, 98 (60.9%) aquatic insect species were found sympatric with Ae. vittatus while 63 (39.1%) species were found in rock pools without Ae. vittatus. Aeschna sp. and Libellula sp. were two odonatan species with dominant distribution with each found on 15 inselbergs. Five species of hemipteran insects were encountered on six inselbergs dominated by Buenoa margaritacea. Only two species of coleopteran insects were obtained in rock pools. Of seven species of dipteran insects, Tendipes tentans was the most preponderate followed by Chironomus sp. Both of these dipterans were sympatric with Ae. vittatus. Isogenus modesta and Isotomurus palustris were the least abundant, found only once in rock pools. Twenty-two species of aquatic macrophytes belonging to 14 families were uprooted and identified from 65 different inselbergs across sample locations. Of the 170 macrophytes identified, 110 (64.7%) were found in rock pools with Ae. vittatus while 65 (38.2%) were found in rock pools without Ae. vittatus [Table 2]. Cyperaceae was more diverse but scarcely found in pockets of rock pools on inselbergs. Cyperus difformis was the most preponderate cyperaceae species while other species of cyperaceae including Cyperus iria, Cyperus podocarpus, Mariscus ligularis and Pycreus flavescens were found only once in the study in rock pools with Ae. vittatus except Cyperus podocarpus. Members ofthe family Poaceae was the second dominant macrophyte species dominated by Oplismenus burmanni with relative preponderance in association with Ae. vittatus in rock pools. Chlorophytum laxum belonging to the family Liliaceae was the most widespread macrophyte species consisting of 50.6% of all macrophytes encountered in this study with Ae. vittatus in rock pools. Xyris straminea and Dopatrium longidens belonging to Xyridaceae and Schrophulariaceae respectively relatively had wide-spread occurrence in rock pools. Aeschynomene uniflora and Stylosanthes sp. belonging to Papilionaceae each occurred twice in rock pools. Members of the families Asteraceae, Euphorbiaceae, Illecebraceae, Moraceae, Solanaceae and Fabaceae were all represented with single species of macrophyte and each occurred once in rock pools.
Table 1: Aquatic insect species with percentage of occurrence with Ae. vittatus in rock pools on inselbergs

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Table 2: Macrophyte species that co-existed with Ae. vittatus in rock pools on inselbergs

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Eight species of algae belonging to three divisions which include Bacillariophyta, Chlorophyta and Cyanophyta were found coinhabiting rock pools with Ae. vittatus on 22 different inselbergs [Table 3]. Chlorophyta was the most widespread algal division while Cyanophyta was the least algal division. Spirogyra sp. was the dominant Chlorophyta found on eight different inselbergs while Microporafloccusa was the least Chlorophyta found only on one inselberg. Meanwhile, Cyanophyta represented with single species,Microcystis sp. occurred as the most dominant and the most associated of all algae with Ae. vittatus. Stephanodiscus rotula and Tabellaria fenestrata were two species of Bacillariophyta, each coinhabited rock pools with Ae. vittatus. Three families of vertebrates including Bufonidae and Pyxicephalidae representing tadpoles of toads and frogs respectively and Emydidae representing water turtle were found in rock pools on inselbergs. Poyn-tonophrynus beiranus was the most abundant tadpole of toads found on 6 inselbergs and were largely unassociated with Ae. vittatus in rock pools. Amietophrynus sp. and Bufo terrestris were the least tadpoles oftoads, each were found only once in rock pools. Cacosternum sp. was the dominant tadpole of frogs, found on ninr inselbergs and were also largely unassociated with Ae. vittatus in rock pools. Amietia angolensis was the least tadpole of frogs and was found only once in rock pool with Ae. vittatus. Single species of water turtle, Trachemys gaigeae was encountered on one inselberg coinhabiting rock pools with Ae. vittatus. Analysis of variance showed that pH, TDS, EC and alkalinity differed significantly (p<0.05) with the abundance of Ae. vittatus in rock pools [Table 4].
Table 3: Algal divisions and families of vertebrates that coinhabiting rock pools with Ae. vittatus

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Table 4: Physicochemical variables (Mean±SE) influencing the breeding of Ae. vittatus across sites in rock pools

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Depth, surface area, total suspended solid, turbidity, hardness, DO, COD, PO43- and NO3- did not differ (p>0.05) with the abundance of Ae. vittatus in rock pools. Logistic regression indicated that the larval density of Ae. vittatus in rock pools was influenced by water hardness (mg/l) (p=0.51; f=0.012), nitrate (mg/l) (p=0.02; f=0.017), alkalinity (mg/l) (p=0.00; f=0.012) and aquatic insects (p=0.03; f=0.000). Other ecological variables including turbidity, phosphorus, COD, DO, total suspended solid, macrophytes, vertebrates and algae had no impact on the breeding intensity of Ae. vittatus in rock pools. Correlation of physicochemical variables with PCA revealed strong negative association of depth parameter of the rock pools at Danmagaji and Dutsen-Abba inselbergs [Figure 4]. Total suspended solid negatively corelate with the distribution of Ae. vittatus in rock pools at Hanwa and Wusasa inselbergs. Water hardness had strong positive correlation with the distribution of Ae. vittatus in rock pools at Nok and Samban-Gidan inselbergs. Water temperature and chemical oxygen demand positively corelate with Ae. vittatus in rock pools at Kujama and Malali inselbergs, respectively.
Figure 4: Relationship of physicochemical variables on the distribution of Ae. vittatus in rock pools on inselbergs across sample sites. The sample sites are indicated with upper case alphabets representing: Q-Hanwa, K-Wusasa, P-Zango-Aya, X-Dut-sen-Abba, V-Danmagaji, B-Dumbi, G-Kufena, M-Kangimi, E-Malali, U-Baban-Sora, O-Kujama, D-Kajuru, F-Kufana, A-Tudun-Mare, J-Jere, N-Kagorko, S-Kwoi, C-Nok, L-Cho-ri, Z-Kagoma, W-Samban-Gida.

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  Discussion Top

Results from this study establish the relationship between the determinant variables and the rock pool biota leveraging Aedes vittatus persistence in rock pools. Breeding and their temporal distribution is of vital concern to predict its impact on disease transmission rates. In this study, breeding of Ae. vittatus in rock pools could be expedited in part by the fact that it is likely to live year-round, owing to the ability of laid eggs to withstanding cold and desiccation[26]. This inextricable process had been abetted by decades of mechanized and unmechanized excavations of the rocky hills that have created assorted suitable breeding hotspots for Ae. vittatus. The socioeconomic relevance of inselbergs through excavation of the rock could free up steady blood meal for gravid female resulting to high larval densities at Wusasa and Malali inselbergs situated at the hubs of Zaria city and Kaduna main town, respectively. Wusasa is an urban area comprising of large small businesses and trailer parks along the road with highly populated and unplanned settlements. On the contrary, Malali town comprises of government houses, estate housing, industries, renown schools and modern infrastructural facilities. Compared to most of the inselbergs on the outskirts and distant from anthropogenic activities, Kufana inselbergs had zero larval density while relatively low larval densities were also observed at Dutsen-Abba, Nok, Samban-Gida and Tudun-Mare inselbergs. Though in the absence of anthropic factors, different species of sylvatic amplifying host including sheep, goats, mokeys and bovid were cited on inselbergs found on the environs far from human populated areas. This intimates the significance of Ae. vittatus in the transmission cycle of a variety of arboviruses[6]. Nevertheless, yellow fever outbreaks in Nigeria are reported following fitting standard case definition and confirmation by laboratory techniques, hence outbreaks could be urban amplification between humans and urban mosquitoes[34]. Given the recent history of the outbreak of yellow fever in Nigeria, Ae. vittatus which is known to be highly anthropophilic coupled with suitable hosts around the inselbergs, are integral in defining the risk of epidemics of arboviral diseases.

Predicting the impact of aquatic insects on the breeding of Ae. vittatus using regression analysis showed a significant difference (p=0.03, f=0.000). Ae. vittatus were more prolific at some sites than others despite coinhabitation with intense and strict predators of mosquito larvae. This suggests that Ae. vittatus larvae could be less exposed to their natural enemies in ephemeral rock pools than permanent pools. Belostomafluminea, Buenoa margaritaoea and Nepa cinerea which are mosquito hemipteran predators could have shared different trophic level with the larvae in rock pools and as such partly constitute or may not constitute feeding competitors with Ae. vittatus larvae. This study contrasted with previous investigations on the predatory efficiency of odonatan dragonfly nymphs such as Libellula sp., Crocothemis servilia and notonec-tids[27][28]. Relatively low number of hemipteran predators in rock pools at the time of sampling could partly account for high larval abundance of Ae. vittatus. The predator’s efficiency to detect and catch Ae. vittatus larvae could be reduced by the turbid nature of the pools, although the difference on the impact of turbidity was not significant (p=1.17, f=0.247). Another prominent coleopteran predator, Hydrophyllus sp. was recorded only once throughout the study, suggesting utilization of the temporary nature of the rock pools by Ae. vittatus to breed in profusion prior to successional overlay of the rock pools with other aquatic mosquito predators.

Macrophytes were more infrequent in rock pools but pools with relative depth contained majority of the macrophytes. Although the impact of macrophytes in rock pools may be spatially dependent, rock pools with Ae. vittatus were dominantly characterized with Chlorophytum laxum and few other aquatic plants. Some of the dominant macrophytes had been identified by[29] as frequent colonizers of marshy grounds and ponds. Chlorophytum laxum was the most preferable and dependent aquatic plants reassuring Ae. vittatus abundance and distribution in rock pools through fixation of Ae. vittatus larvae to their roots by means of siphon. Other aquatic plants found in this study could also help to sustain breeding intensity of Ae. vittatus through the provision of breeding hideout from their sympatric predators. The impact of macrophytes on the breeding intensity of Ae. vittatus was not significant (p>0.05), but could affect at varying densities the abundance of aquatic insect species. Previous study had shown that many insects are dependent on the litter deposited as vegetation dies and sinks to the bottom, hence critical in determining plant species that support the greatest numbers of macroinvertebrates[7]. Sequence to this, abundance of the dominant macrophytes and other few macrophytes could inadvertently influence the total possible number of faunal communities of aquatic insects associated with Ae. vittatus in rock pools.

Amphibians that utilized the rock pools as breeding grounds were found in significantly higher densities in pools with vast surface areas even on inselbergs with high altitudes. These rock pools were devoid of Ae. vittatus larvae while other rock pools of similar structural properties tend to support fewer mosquito larvae. This suggests complete or partial avoidance of such habitats with high predation risk by gravid female mosquitoes. The abundance of Ae. vittatus did not correlate with vertebrate planktivores using Logistic regression and as such had no impact on the breeding of Ae. vittatus larvae (p=0.81; f=0.384). It was noted that vertebrate planktivores only rarely inhabit rock pools owing to its small size and temporal instability[30]. Trachemys gaigeae, the only species of water turtle obtained in this study were found in relatively deep pools at Kufena inselbergs. Adult water turtles were also found on the same inselbergs but were not collected due to the size and depth of the pool consciously made out of anthropic events. The depth of the rock pools and hydroperiod length were observed as vital ecological variables that permitted possible immigration and breeding of water turtles in the rock pools. Therefore, it can be inferred that Ae. vittatus larvae and vertebrate predators were at different ecological skyline in the rock pools. Algal mat was evident in few rock pools which had very low or without Ae. vittatus larvae. The Algal mat prevented oxygen flow to siphon and smooth diving of Ae. vittatus larvae and as such could severely reduce their population. Dida et al. (2015) also show that volatile compounds produced by microbial population in the breeding sites such as chlorophyll content and other abiotic factors can inhibit adult mosquitoes’ oviposition, coupled with habitat preferences. On the contrary, higher abundance of mosquito larvae in seen in places where phytoplankton such as diatoms, desmids, and green algae such as Spirogyra spp. are common[31]. Although there was marked variation of chlorophytes, low density of Ae. vittatus associated with alga species could further be supported with high density of toxic and noxious species of Microcystis species in some rock pools. The lethal effect of microcystin produced by this algal species could negatively affect larval production of Ae. vittatus.

At the time and period of sampling, seasonal fluctuations of physicochemical variables were not ascertained considering the differences in ephemeral nature of the rock pools. Factoring in all physicochemical variables in Principal Component Analysis, temperature and water hardness had direct influence on the distribution and proliferation of Ae. vittatus larvae in rock pools. Total suspended solid and depth of the rock pools partially influence the distribution of Ae. vittatus larvae in rock pools. Inherent attractant behaviour of ovipositing female mosquitoes could be responsible for high density breeding of Ae. vittatus observed in shallow rock pools. Relatively high temperatures optimal for mosquito breeding between 23°C to 36.5°C were recorded, but water temperature changes are influenced by time of sampling and condition of the habitat[32]. Range of pH values (7.05–12.69) were optimal for breeding of Ae. vittatus and emergence of benthic macroinvertebrates despite insignificant role of pH in the distribution of Ae. vittatus in rock pools as shown in PCA. pH could have supported the diversity of benthic macrofauna in rock pools because low pH values are associated with lower diversity of benthic macroinvertebrates and cause decreased emergence rates in them[7]. Results also showed that a limited range of dissolved oxygen and biochemical oxygen demand requirement levels preferable for Ae. vittatns abundance was detected. The low oxygen levels could be due to off-load of organic material into the rock pools that are naturally occurring and possible consequent anthropological activities that could have accumulated over time within the small circumference of the rock pools. In this study, Ae. vittatus and rock pool biota thrived in elevated level of water hardness (545.4mg/L) and nitrate (74.70mg/l). Results concurred with a study that noted elevated level of water hardness in a shared habitat of mosquitoes and their predators along Mara River in Kenya and Tanzania[11]. Relationship between Ae. vittatus and water hardness provided an insight on the suitability of the rock pools for larval growth and development.

  Conclusion Top

In conclusion, the distribution of Ae. vittatus in rock pools occurred at densities capable of provoking outbreak of diseases and its occurrence corresponds to areas with high anthropogenic activities. Evidence of the vector’s eggs surviving in hot dry rock-holes and direct relationship with water hardness could underwrite profuse breeding of Ae. vittatus larvae and subsequent maintenance of yellow fever transmission. Concentration of nitrate, temperature, total dissolved solid and the length of the rock pool hydroperiods were determinant variables that sustained rock pool communities and thus can be altered to guide intervention measures against Ae. vittatus. The aquatic insects and vertebrates were suspected to play important natural population modulatory roles against noxious mosquito species which could thus form candidate biological control agents that need protection from harm’s way in integrated chemical control of mosquitoes in rock pools. A well-defined measure of efficacy incorporating indigenous communities for sustained vector control on inselbergs will go a long way in decimating population of Ae. vittatus and limit the risk of spread of arboviruses, hitherto areas not thriving.

Conflict of interest: None

  Acknowledgements Top

The authors are grateful to field assistants for their support during sample collection within Kaduna State. We acknowledge several village heads who granted access to some hallowed inselbergs within the study areas. We thank Dr. Yahuza Tanimu, Department of Botany, Ahmadu Bello University Zaria who assisted in statistical analysis of data. We are thankful to Mr. Alika, Department of Environmental and Water Resources Engineering, Ahmadu Bello University, Zaria for laboratory analysis of water chemistry. Appreciation is expressed to Mallam Musa and other staff of Herbarium Unit, Department of Botany, Ahmadu Bello University, Zaria for identification of macrophytes.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3], [Table 4]


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