Female Anopheles mosquitoes are the vectors of human malaria. The use of chemical insecticides for vector control has hampered with environmental pollution and insect. This suggests the need for the development of more potent and environment-friendly insecticides for effective control of malaria. This research investigated the larvicidal, pupicidal and adulticidal activities of Chromolaena odorata and Vernonia amygdalina leaf extracts against, An. gambiae in the laboratory at ambient temperature of 28 ± 2 °C and 75 ± 5% relative humidity. Different concentrations of 20 mg/L, 40 mg/L, 80 mg/L, 120 mg/L and 160 mg/L were prepared and these aqueous solutions were used for the experiments. Larval, pupal and adult mortality of An. gambiae were tested after 24 hours of exposure. Results showed that there were significant differences (P < 0.05) in toxicity level of the two plant extracts on An. gambiae larvae, pupae and adults. Vernonia amygdalina extract was the most toxic to An. gambiae larvae at all tested concentrations of 20 mg/L, 40 mg/L, 80 mg/L, 120 mg/L and 160 mg/L causing 47.5%, 82.5%, 100%, 100% and 100% mortality after 24 hours of treatment, respectively. Chromolaena odorata extract caused 32.5%, 60%, 82.5%, 92.5% and 100% mortality of An. gambiae larvae after 24 hours of treatment at concentrations 20 mg/L, 40 mg/L, 80 mg/L, 120 mg/L and 160 mg/L, respectively. Vernonia amygdalina extract was the most lethal to An. gambiae pupae and adults which caused 55% mortality of adult An. gambiae at concentration 160 ml/L. The concentration of C. odorata and V. amygdalina leaves extracts required to evoke 50% death of An. gambiae adult were 296.20 mg/L and 147.98 mg/L respectively. The LC90 of C. odorata extract was 3107.55 mg/L while V. amygdalina extract was 2221.05 mg/L for mosquito adults. The plant extracts were not as effective against adults compared to larva and pupa of An. gambiae. This study showed that C. odorata and V. amygdalina were toxic to malaria vector with V. amygdalina being more potent. This suggest that V. amygdalina extracts could serve as an alternative method to synthetic chemical control of malaria vectors.
1.1 Background of the study
Several diseases are transmitted by arthropod vectors of which mosquito is one of them. Mosquitoes are responsible for the transmission of diseases such as malaria, lymphatic filariasis, and dengue fever among others most especially in the tropical regions of the world. Out of these diseases transmitted by mosquitoes, malaria is the most devastating in terms of the number of incidence, prevalence, morbidity and mortality. It is transmitted by the mosquitoes of the genus Anopheles and it is caused by the protozoa of the Plasmodium genus. Examples of Anopheles species include Anopheles gambiae, Anopheles culicifacies, Anopheles stephensi, Anopheles fluviatilis, Anopheles minimus, Anopheles dirus and Anopheles sundaicus among others with Anopheles gambiae being the prominent one among them all. Five species of Plasmodium namely Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale and Plasmodium knowlesi are responsible for human malaria but P. falciparum and P. vivax are most notorious in terms of the number of cases they are responsible for. They were an estimated 219 million cases of malaria with 435,000 of them resulting in death in 2017. In order to stop or reduce the transmission of malaria parasites, there is need to control the vector population. Several methods such as habitat modification, the use of synthetic chemical insecticides and the use of biological means such as employing natural predators, parasites and parasitoids to control the populations of mosquitoes. The major method that have been employed in the control of mosquitoes is the use of synthethic insecticides such as chlorodane, aldrin, Dieldrin, dichlorodiphenyl trichoroethane (DDT) among others. However, these chemical have been shown to have negative effect in the dynamics of the earth’s ecosystem resulting in resistance of mosquito species to the chemicals, environmental pollution and toxicity to human and other non-targeted organisms. Hence, there have been clamor by various quarters to develop a safe means to combat the proliferation of insect vector species. This has led to the focus on developing insecticides from botanical sources that are safe to use in the environment due to the fact that they are easily degradable and less toxic to humans and non-targeted organisms.
Several plant materials such as Alstonia boonei, Curuma longa, Cymbopogon winterianus, Ocimum americanum, Nepeta cataria, Viola odorata, Melaleuca quinquenervia, Azadirachta indica, Citrus medica, Nicotiana tabacum, Anacardium occidentale, Aframomum melegueta, Garcinia kola, Citrus sinensis and Murraya koenigii have been used to control mosquitoes. Chromolaena odorata is a fast-growing perennial herb which belongs to the family Asteraceae. The plants are known for its medicinal value in many areas as well as pests control purposes. Chromolaena odorata contains carcinogenic pyrrolizidine alkaloids. It is not habitat specific and nature of soil, it grows generally in the wild (abandoned land). The plantis acknowledged for its high proliferation and one of the world worst tropical weeds. Vernonia amygdalina belong to the family Asteraceae and grows in the tropical regions of Africa. The plant is a shrub, evergreen in colour and can grow between 2-5 m in height with petiolate leaf of about 6 mm in diameter. The plant is bitter due to the characteristic odour and bitter taste of the leaves owing to the presence of anti-nutritional factors such as alkaloids, saponins, tannins and glycosides.