1 Background

Mosquitoes have become the most important single group of insects well-known for their public health importance, since they act as the vector for many tropical and subtropical diseases. (Egunyomi et al., 2010). Of the 42 genera of this family; the genus Aedes, Culex, Anopheles and Mansonia are responsible for the transmission of the most common and important diseases causing significant mortality and morbidity in man and animals (Akinkurolere and Zhang, 2007). Mosquitoes are estimated to transmit disease to more than 700 million people annually in Africa, South America, Central America, Mexico and much of Asia with millions of resulting deaths (Akinkurolere et al., 2011). Malaria is the most prevalent of mosquito borne diseases; being endemic in about 109 countries, infecting 190-330 million people and causing about 1 million deaths every year. The species responsible for the transmission of the causative pathogen of malaria is the Anopheles; the control of which has been mainly effected by use of conventional insecticides but these have caused their own problems, such as adverse effects on the environment and the encouragement of pesticide resistance in some mosquitoes (Su and Mulla, 1998). Nigeria is known for high prevalence of malaria and it remains one of the leading causes of childhood cum maternal morbidity and mortality, low productivity and reduced school attendance in the country (Aribodor et al., 2007). The burden of malaria has however been increasing due to development of resistance against anti-malaria drugs and insecticides, complex social structures and rapid environmental changes that have intensified in the last decade (Eliningaya et al., 2008). The increase in the development of mosquito resistance is the primary concern of all control programmes (Pimsamarn et al., 2009). Owing to the fact that application of synthetic insecticide has envenomed the surroundings as well as non-target organisms, these problems stimulated a search for safer alternative anti-mosquito control. This has necessitated the use of other alternatives; and in this case, phytochemicals obtained from plants with proven efficacy against mosquitoes. These phytochemicals have been documented to be environmentally safe, cost effective, biodegradable and benign to man (Egunyomi et al., 2010). Hence in recent years, use of environment friendly and biodegradable natural insecticides of plant origin has received renewed attention as agents for vector control and it has been found that herbal extracts are one safer alternative method of control, especially the extracts of certain medicinal plants (Suwanee et al., 2006). Generally most of these phytochemicals can be grouped into five major chemical categories which are (1) Alkaloids (2) Terpenoids (3) Phenolics (4) Growth regulators and (5) Proteinase inhibitors (Marta and Sarah, 2011). An early report on the use of plant extracts against mosquito larvae was that of Campbell et al. (1933) where it was found that plant alkaloids like nicotine, anabasine, methyl anabasine and lumpinin extracted from the Russian weed; Anabasis aphylla, killed the larvae of Culex sp. Monzon et al. (1994) also reported that some medicinal plants containing natural toxins were effective against mosquito larvae. Accordingly, the efficacies of plant extracts have been extensively researched on (Kovendan et al., 2014; Akinkurolere et al., 2011; Effiom et al., 2012; Tennyson et al., 2012).

2 Materials and Method

2.1 Extraction of oils from plant materials

The extraction of plant materials were done based on the procedures of Hidayatulfathi et al. (2004) with slight modifications. 100g of each plant (sun dried) was blended and filtered using a sieve. The powder was macerated with 500mL of n-hexane, petroleum ether and ethanol solution and left to stand at room temperature for 72 hours. The mixture was filtered through a muslin cloth and glass funnel. The filtrate was then concentrated using the Rotary evaporator in order to get the plant extracts. The extract was collected using a plastic bottle and this was exposed to the air for 24 hours in order to remove the vapour and traces of the extraction solvents that might remain. It was then stored in the refrigerator at 4°C until needed for tests.

2.2 Breeding of mosquitoes

Water, sand and dried leaves were collected in plastic bowl and placed in a shady area of a residential building. After a few days mosquito larvae were observed to be emerging from eggs which have been laid by the adult mosquitoes and the set up was transferred to the laboratory. In the laboratory, the larvae were transferred into plastic containers with water; these were in turn transferred into a cage (30cm x 30cmx 20cm). The cage had net with minute mesh sizes that will not allow the passage of adult mosquitoes. It had a sliding screens and inlets by the side through which the experimenter could put a hand in order to operate within the cage yet preventing the escape of the mosquitoes. The larvae in the beaker were fed with yeast and after a few days, adult mosquitoes were observed to be emerging. Sucrose solution was provided for the male mosquitoes to feed on while a live rat (Rattus norvegicus) was provided for the females to obtain blood meal from.

2.3 Bioassay of extracts

Different concentrations; 0.1%, 0.2%, 0.4%, 0.8% and 1.0% each of the plant extracts were prepared in beakers for the larvicidal and pupaecidal effect. 10 Larvae and pupae of A. gambiae were then introduced into the assays and were afterwards observed every 10 minutes for 1 hour for mortality effect of each extract. The lethal effect of the extracts was also observed after 24hours. The experiment was replicated four times for each concentration of each plant extract. The fumigant effect of the plant extracts on adult A. gambiae was tested by depositing 0.1mL, 0.2mL, 0.4mL, 0.8mL and 1.0mL of each extract on 2cm x 2cm Whatman’s no.1 filter paper strips (Hidayatulfathi et al., 2004). These strips of filter paper were then introduced into the test tubes containing the mosquitoes to be fumigated. The lethal action of each extract was observed and recorded every 10 minutes for 1 hour. Each of the treatment was replicated four times per concentration of each plant extract.

Although all the extracts effected larvae mortality at all concentrations after 24 hours, there was significant difference at P<0.05 in the percentage mortality of Datura stramonium compared with those of Piper guineense and Nicotiana tabacum. Piper guineense achieved 100% larvae mortality at all concentrations after 24hours but its effect was only significantly different from that of Nicotiana tabacum at 0.1% and 0.2% concentrations. There were no significant differences at P<0.05 in their effects at the remaining concentrations(Table 2).

Table 2 Table of larvae mortality per concentration of plant extract in 24 hours

Table 5 shows the fumigant effect of the extracts on the adult A. gambiae. D. stramonium and P. guineense were both ineffective at all concentrations while N. tabacum only produced tangible results at 0.8% and 1.0% concentrations within the 1 hour observation period. Effectiveness of extract can be assumed to be concentration dependent as higher mortality was recorded at 1.0% concentration than at 0.8% concentration of the N. tabacum treatment.

Table 5 Table of adult mortality per concentration of plant extract in 1 hour

4 Discussion

According to Saxena et al. (1993), exposure of larvae of Anopheles stephensi to plant alkaloids resulted in a significant loss in fecundity, fertility and caused mortality in the adult species of the mosquitoes. In the same vein, the ethanol, petroleum ether and n-hexane extracts of the leaves of D. stramonium, seeds of P. guineense and leaves of N. tabacum respectively showed toxicity to the pupal and larval stages of the mosquito. However it was only the extract of N. tabacum that exhibited adulticidal activity against the mosquito at the treatment concentrations studied.