The larvicidal activity of various solvent (ethanol, ethyl acetate and n-hexane) extracts of
Persea americana seed and Chromolaena odorata leaves against Aedes vittatus mosquitowas analysed. The most potent solvent (n-hexane) extracts of both plants were fractionated using column chromatography and most effective fractions isolated and identified using Gas Chromatography Mass Spectrometry and Fourier Transform Infrared techniques. Phytochemical screening revealed the presence of steroids, cardiac glycosides and terpenoids in all the extracts. The larvicidal bioassay of Persea americana seed gave LC50 values of 0.827ppm, 1.799ppm and 2.732ppm for n-hexane, ethanol and ethyl acetate extracts respectively, while, Chromolaena odorata leaf extract had LC50 values of 1.835ppm, 3.314ppm, and 5.163ppm for n-hexane, ethanol and ethyl acetate respectively. Column chromatographic fractionation of most potent n-hexane (crude) extracts of both plants, showed increased activity in some of the fractions of Persea americana (nHPa6) and Chromolaena odorata (nHCo6) which showed higher mortality, with LC50values of0.486ppm and 1.308ppm respectively. GC/MS analysis of components in nHPa6 and nHCo6 showed oleic acid as the most abundant, in fractions of both plants. The FTIR analyses of nHPa6 and nHCo6 showed absorption bands of the functional groups present, which included; alcohol, alkane, alkene, alkyl halide, aldehyde, carboxylic acid and carbonyl ester, thus, supporting the GCMS result. The n-hexane, ethanol and ethyl acetate extracts of P.americana seed and C. odorata leaves have shown good larvicidal activity and shouldtherefore be further exploited for the control of mosquito larvae.
CHAPTER ONE 1.0 INTRODUCTION
Insect-transmitted diseases remain a major cause of morbidity and mortality worldwide. Mosquito species belonging to genera; Anopheles, Aedes and Culex, are vectors (Redwane et al., 2002) for the transmission of malaria, dengue fever, yellow fever, filariasis,schistosomiasis and Japanese encephalitis (JE), transmitting diseases to more than 700 million people annually (Oyewole et al., 2010; Govindarajan, 2009). Mosquitoes also cause allergic responses in humans which include local skin irritation and systemic reactions such as angioedema. Aedes spp are generally regarded as a vector responsible for transmission of yellow fever and dengue fever, which is endemic to Southeast Asia, the Pacific island area, Africa, Central and South America.
The World Health Organization (W.H.O. 2012) has recommended vector control as an important component of the global strategy for preventing insect-transmitted diseases. The most commonly employed method for the control of mosquito-borne diseases involve the use of chemical-based insecticide, though it is not without numerous challenges, such as human and environmental toxicity, resistance, affordability and availability (Ghoshet al., 2012).
Extracts from plants has been good sources of phytochemicals as mosquito egg and larval control agents, since they constitute an abundant source of bioactive compounds that are easily biodegradable into non-toxic products. In fact, many researchers have reported on the effectiveness of plant extracts or essential oils against mosquito larvae. They act as larvicides, insect growth regulators, repellents, and oviposition attractants (Pushpanathan, 2008; Samidurai et al., 2009; Mathivanant et al., 2010).
Persea Americana is an ever green tree belonging to Lauraceae family and its fruits arecommonly known as avocado pear or alligator pear. The plant originates from Central America but it has shown easy adaptation to other tropical regions, thus widely cultivated in tropical and subtropical regions. The various parts (leaves, fruits and seed) of this plant have numerous uses from edible pulp as source of nutrients to the seed preparation as remedy (Arukwe et al., 2012).