ABSTRACT
The evaluation of the larvicidal potency of the plant seeds might lead to the development of safe and environmentally friendly insecticides. The oils of the seeds of Adansoniadigitata, Parkiabiglobosaand Perseaamericanawereevaluated for their chemical composition and theability to kill mosquito larvae of Aedesaegypti, Aedesvittatusand Culexquinquefasciatus under laboratory and outdoor conditions.The seeds of the three plants were extracted from 100g powdered kernel with 300mL petroleum ether using soxhlet extractor for 5-6hr. The solvent was evaporated under vacuum leaving behind oil seed.The extracted oils were quantified gravimetricallyto determine the presence of chemical constituents of larvicidal importance.Bioassay oflate third larval instar was tested using six concentrations of 0.01, 0.02, 0.04, 0.08, 0.16 and 0.32mL ofeach seed oil extract making up to 100mL with distilled water. A batch of 25 mosquito larvae were tested in three replicates for each concentration along with the respective control of distill water devoidof extracts. The chemical composition of the seed oil extracts revealed the presence of acid, free fatty acid, iodine, peroxide and saponification values in different quantities. Acid and free fatty acidvalues did not differ significantly (p > 0.05) between the extracts of A. digitata (5.04 and 2.50mgKOH/g) and P. biglobosa(5.05 and 2.50mgKOH/g) except forP. ameriacana (19.63 and 9.90mgKOH/g) (p < 0.05) where significance was recorded. Iodine and peroxide differ significantly (p < 0.05) across all the three seed oil extracts. Saponins was very high in all the seed oil extracts compared to other phytochemical compositions (acid, free fatty and peroxide values), but was only significant (p < 0.05) in P. americana (199.33mL/Kg). The cidal effect of the seed oil extracts was dose-dependent for A. digitata, P. americana and P. biglobosa. The median lethal concentration (LC50)valuesof Adansoniadigitata seed oiltested against three mosquito species for laboratory and outdoor conditions were respectivelyfound to be as follows:Ae. aegypti0.1179mL/L and 0.4886mL/L;Ae. vittatus0.0998mL/L and 0.00055mL/L; and Cx. quinquefasciatus0.0519mL/L and 0.00397mL/L.For Parkiabiglobosa, theLC50values determined for Ae. Aegyptiwere 0.1084mL/L and 0.3327mL/L;Ae. vittatus, 0.1891ml/L and 0.0010ml/L; and Cx. quinquefasciatus 0.1058mL/L and 0.0134mL/L for laboratory and outdoor conditions respectively. Perseaamericanahad the following LC50 values: Ae. aegypti 0.3026mL/L and 0.0561mL/L;Ae. vittatus0.1118mL/L and 7.0x10-6mL/L and Cx. Quinquefasciatus 0.0858mL/L and 0.1862mL/L for laboratory and outdoor conditions,repectively.These results suggest that the seed oilsof A. digitata, P. biglobosaand P. americanacan be harnessed for the development of bio-larvicide for the control of the mosquitoesspecies studied and invariably the diseases they transmit.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background of the Study
Mosquitoes represent a significant threat to human health because of their ability as vector pathogens which cause diseases which afflict millions of people worldwide (WHO, 2010). Several species belonging to theAedes, Anopheles and Culex genera are vectors of the pathogens of various diseases like yellow fever, dengue fever, dengue hemorrhagic fever, malaria,yellow fever, Japanese encephalitis, West Nile virus and filariasis (Rahuman, 2009; Borah et al., 2010; Samuel 2010).Mosquitoes have the potential to feed on more than one individual during a single gonotropic cycle and this enhances their vectorial roles (Mackenzie et al., 2004). Proper control of mosquitoes lies in personal protection and public awareness but the most economical method involves eradicating breeding sites and controlling these pests through application of environmental friendly larvicides (Corbel et al., 2004).
Among the thirteen genera of the family Culicidae, the genus Aedes is considered dangerous because of its significant public health threat all over the world (Rajesh et al., 2013).Aedes vittatusand Aedes aegyptiare potential vector speciesof chickungunya, dengue fever and yellow fever virus (Service, 1970; Aliet al.,2014).They can also transmit yellow fever virus from monkey to monkey in the laboratory (Bang et al., 1981) and have been suspected as vectors in the Nubamountain epidemic in Sudan (Ali et al., 2014). Aedes species are also important vectors of encephalitis and many other arboviruses, and in a few restricted areas, they are also vectors of Wuchereria bancrofti and Brugiamalayi (Kandaswamy et al., 2012).
Anopheles speciesand Culex quinquefasciatus are responsible for the transmission of several species ofparasite that cause diseases such as malaria and filariasis respectively.Anopheles speciestransmit malaria that kills more people than HIV/AIDS with about 50% of the population experiencing atleast one episode of malaria each year (Greenwood et al., 2005).
Culex quinquefasciatus is one of the most widespread mosquitoes in the world. It is found throughout most of pan and subtropical Americas (Weinstein et al., 1997; Carveret al., 2009; Andreadiset al., 2010), the Neotropics, Afrotropics (White, 1975; Diaz-Badilloet al., 2011), Indomalayan, Australian (Lee et al., 1989; David et al., 2012) and Eastern Asian regions of the world (Bram, 1967; Rios-Ibarra et al., 2010). It is also present in the United Kingdom and parts of the Middle East such as Pakistan, Iran among others.It is an important vector of periodic filariasis in parts of the world (Belkin, 1968; Rios-Ibarraet al., 2010) and is known to carry and transmit Wuchereria bancrofti to some degree in many regions of the globe.Of the estimated global 128 million lymphatic filarial cases, 91 per cent are caused by Wuchereria bancrofti Cobbold (Singh, 1967;Andreadis et al., 2010).
Currently, the main tool for mosquito control is the use of diverse synthetic chemicals as larvicides (Govindarajan and Rajeswary, 2014). The drastic effects of synthetic insecticides in the environment have received wide public apprehension (St. Lager et al., 1996; Morin andComrie, 2010). Indeed, synthetic insecticide misuse in agriculture and public health programmes has caused many problems like insecticide resistance, resurgence of pest species, environmental pollution, toxic hazards to humans and other non-target organisms (Sarwar et al., 2009). To alleviate these problems, major emphasis has now been shifted on the use of naturalderived
plant products as larvicides, which can serve as alternate to synthetic insecticides (Junwei et al., 2006).
One of the most effective alternative approaches under the biological based programme is to explore the floral biodiversity and enter the field of using safer insecticides of botanical origin as a simple and sustainable method of mosquito control (Shallan et al., 2005). In addition, unlike conventional insecticides, which are based on a single active ingredient, plant-derived insecticides comprise botanical blends of chemical compounds which act concertedly on both behavioural and physiological processes (Gokulakrishnan et al., 2013). Thus, there is very little chance of pests developing resistance to such substances. Identifying bio-insecticides that are efficient, as well as being suitable and adaptive to ecological conditions, is imperative for continued effective vector control management. Plant essential oilshave widespread insecticidal properties and will obviously work as a new weapon in the arsenal of insecticides and in future may act as suitable alternative product to fight against mosquito borne diseases (Morais et al., 2007;Gokulakrishnan et al., 2013).
Botanicals are basically secondary metabolites that serve as part of the defence mechanism of plants to withstand the continuous selection pressure from herbivores, predators and other environmental factors. Several groups of phytochemicals such as alkaloids, steroids, terpenoids, saponins, essential oils and phenolics from different plants have beenreported for their insecticidal activities (Shallan et al., 2005). Insecticidal activitiesof plant extracts vary not only according to plant species, mosquito species, geographical varieties and parts used, but also due to the extraction methodology adopted and the polarity of the solvents used during extraction(Anupamet al., 2012). A wide selection of plants from herbs, shrubs and large trees
have been used for extraction of toxins using different methods for mosquito control(Anupamet al., 2012).
Phytochemicals have been extracted either from the whole body of little herbs or from various parts like fruits, seeds, leaves, stems, barks, roots, of larger plants or trees. More than 2000 plant species have been known to produce chemical factors and metabolites of value in pest control programmes. Members of the plant families- Solanaceae, Asteraceae, Cladophoraceae, Labiatae, Meliaceae, Oocystaceae and Rutaceae have various types of larvicidal, adulticidal or repellent activities against different species of mosquito (Shallan et al., 2005).Some plants such as Solanum villosum have shown very strong insecticidal effect (Isman, 1999).
In the survey of literature on insecticidal properties of essential seed oil from the year 2004 onwards indicates that essential oils from about 90 plant genera belonging to 38 plant families were reported to have toxic properties against mosquito larvae (Mann and Kaufman, 2012) and pupae (Gokulakrishnanet al., 2013). In general, plant essential oils are important natural alternatives to insecticides (Gbolade et al., 2000). Many plant extracts and essential oils possess larvicidal activity against various mosquito species (Govindarajan et al., 2013; Danga, et al., 2014). Many plants contain chemicals which are helpful for the control of insects and are useful for field applications in mosquito control programmes (Kalyanasundaram and Das, 1985; Isman, 1999).
Adansonia digitata Linn. (Malvaceae) is commonly known as baobab, is a majestic tree revered in Africa for its medicinal and nutritional value(Buchmann, et al., 2010). The plant parts are used
to treat degrees of diseases such as diarrhoea, malaria and microbial infections. It has been reported that it has an excellent anti-oxidant due to the vitamin C content which is 6-10 times higher than the vitamin C content of oranges(Kamatou et al., 2011). Baobab has diverse biological properties such as antimicrobial, antiviral, anti-oxidant and anti-inflammatory activities. Phytochemical studies revealed the presence of flavonoids, saponins, phytosterols, amino acids, fatty acids, vitamins and minerals (Kamatou et al., 2011).
Parkia biglobosa Jacq., commonly known as the locust bean tree, is a perennial deciduous tree of the Fabaceae family (Teklehaimanot, 2004). It is found in a wide range of environments in Africa and is primarily grown for its pods that contain both a sweet pulp and valuable seeds. Where the tree is grown, the crushing and fermenting of these seeds constitutes an important economic activity. Various parts of the locust bean tree are used for medicinal purposes (Abioye et al., 2013), such as treating hypertension (Karou et al., 2013), malaria (Traoré et al., 2013) and healing wounds (Adetutu et al., 2013).
Persea americanaMill., commonly known as pear or avocado tree belongs to the Lauraceae family has an edible fruit. It originated from Central America but is easily adaptable in tropical regions (Leite et al., 2009). The avocado fruit has an olive-green peel and thick pale yellow pulp that is rich in fatty acids such as linoleic, oleic, palmitic, stearic, capric and myristic acids (Dreher and Davenport, 2013). This fruit is normally used for human consumption, but it has also been used as a medicinal plant in Mexico and elsewhere in the world (Dreherand Davenport, 2013).The avocado seed represents 13–18% of the fruit, and it is a by-product that is generally not utilized. Normally, the seed is discarded during the processing of the pulp. The seed waste may present a severe ecological problem (Ortiz et al., 2004). However, at the same time, it may be of interest to industry as a source of bioactive compounds. Chemicallyit is composed of phytosterols, triterpenes, fatty acids and two new glucosides of abscisic acid (Ramos et al., 2004).
Several biological activities of the avocado seed have been reported such as antioxidant, antihypertensive, fungicidal, hypolipidemic and recently amoebicidal and giardicidal
activities (Rodriguez-Carpenaet al., 2011). From the perspective of the use of avocado seeds as sources of phytotherapeutic agents, they have been traditionally used to treat mycoses and parasitic infections. In addition, avocado seeds are known to have local anaesthetic effects that decrease muscle pain (Ozolua et al., 2009).
1.2 Statement of the Research Problem
Several parts such as leaves, bark and stem of Adansonia digitata (baobab),Parkia biglobosa(locust bean) andPersea americana(avocado pear) have been investigated for their larvicidal action against mosquito larvae but the seed oil have not been fully explored in this respect such that their larvcidal potentials have not been investigated and therefore unknown.
1.3 Justification
By evaluating the larvicidal potency of the plant seeds might lead to the devolepment of safe and environmentally friendly insecticides.This will go a long way in controlling the vectors of the deadly diseases and eliminate the numerous ills that have been associated with synthetic pesticides such as high cost, environmental pollution, immune dysfunction, cancer,birth defects and resistance build up among others.
1.4 Aim
To evaluate the larvicidal prospects ofseed oilextratcs ofAdansonia digitata(baobab),Parkia biglobosa(locust bean) andPersea americana (avocado)against the larvae of Ae. aegypti, Ae. vittatus and Cx. quinquefasciatus under laboratory and outdoor conditions.
1.5 Objectives
i. To determine the chemical composition of the seed oil extracts of baobab, locust bean and avocado.
ii. To determine the efficacy of the seed oil extracts of baobab, locust bean and avocado against the larvae of Ae. aegypti, Ae. vittatus and Cx. quinquefasciatus in both laboratory and outdoor settings.
1.6 Hypotheses
i. There is no significant difference in the chemical composition of baobab, locust bean and avocado seed oil extracts.
ii. The seed oil extracts of baobab, locust bean and avocado have no significance effects on the larvae of Aedes aegypti, Aedes vittatus and Culex quinquefasciatusin the laboratory and outdoor settings.
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