INTRODUCTION
From the foregoing, several natural occurring
compounds have been extracted, standardized and applied from plantand utilized for therapeutic purpose including
medicine and pesticides (Goselle et al., 2017).
The activities of plantsare due to
their broad-spectrum toxicity to pathogenic microbes, parasites and pest (Angaye et al., 2017a; 2017b). The preference
and exploration of this natural occurring plant-derived
products by several researchers is largely due to their safer
administration over synthetic agents. On the other hand, in terms of administration
the issue of dose is a major concern. Essentially, the quest for safer drugs
from natural occurring and edible medicinal products, from plants has become of
global interest.
Malaria
is a disease caused by the blood-sucking female Anopheles mosquitoes.
Generally, mosquitoes have a complex life cycle from the egg, larva, pupa and
matured adult insect. Typically, their metamorphosis ranges from the juvenile
aquatic to the adult terrestrial adult phases (Musa et al., 2015),
provided environmental conditions are favorable (Muturi
et al., 2012). The larval stage is an essential stage in the development
and survival of the mosquito. The favorable environmental conditions that are
required for the larva usually are usually sustained to adulthood (Muturi et al., 2012; Musa et al., 2015). While interspecies interactions occur amongst larvae, and their
geographical distribution (Reiskind and Lounibos, 2009; Musa et al., 2015), one of themajor transmitters of malaria in Africa the Anopheles gambiae (Angaye et al.,
2017a; 2017b).
Cymbopogon citratus commonly known as Lemongrass
is a perennial and rapid growing medicinal plant found in the tropical regions
and Asian countries like; India, Malaysia, Indonesia and Sri Lanka (Musa et
al., 2015). The leaf has an aromatic lemon fragrance and essential
medicinal oil when compressed as herb. The therapeutic applications have of the
plant have been documented as for its insect repellencies, especially
mosquitoes (Edmon, 2013), decoctions have also been used
for a wide range of medicinal application. Globally, the plant has a rich
history of applications in folkloric medicine, food ingredient and cosmetics (Nambiar and Matela, 2010). Recent
studies have also shown the larvicidal activities of
the oil (Musa et al., 2015), as well as the leaf decoction, infusion and
maceration (Goselle et al., 2017). As such it
will be necessary to consider the larvicidal
efficacies of the crude and ethanolic extracts of the
leaf.
MATERIALS AND METHOD
Plant Sample collection and Extraction
Samples of lemongrass were obtained from the
market, and then transported to the Laboratory where it was subjected to the
various solvent extraction techniques.During the crude extraction, the fresh leaves of the plant was pounded
with mortar and pestle and the juice squeezed out using muslin cloth. For the methanolic extraction, three hundred grams of the pounded
leaves was macerated in 500ml of ethanol (ethanol, BHD Chemical Ltd. Poole
England) for 72hours). Furthermore, after decanting the
through a muslin cloth to a clean empty beaker. The filtrate was
concentrated in a rotary evaporator at 60oC. Afterward the obtained
residue with the active ingredients was preserved for the bioassay.
Mosquito Larva culture
Spoilt vehicular tires filled were obtained, half-filled
with water and placed in conspicuous breeding site around bushes where stagnant
water was found. The breeding protocols of Goselle et
al., (2017) were adopted with slight modifications. The mosquito larvae
were collected in plastic container with water melon as feed and covered with
fiber mesh (Musa et al., 2015).The Identification of the Anopheles species
was carried out with the aid of a microscope and comparison with previously
identified species.
Experimental setup
Experimental setup
for the larvicidal bioassay using solvent extracts of
Lemongrass against the Anopheles larvae was carried out using standard
procedures (World Health
Organization, 2005).There was
slight modification as 1 ppm pyrethrum pesticide was used as the positive control,
while water used for breeding the larva was used as the negative control.
Concentrations of the plant extracts were setup in triplicates ranging from 10
– 100 ppm was used for the screening in a descending order. Mortality rates was
assessed after 24 hours of exposure,
Statistical analysis
The mean mortality
rates and standard deviations were determined at each concentration, in order
to estimate the median lethal concentration (LC50). A probit graph was determined using the 2016 version of
Microsoft excel with an error margin of 5% error.
RESULT AND DISCUSSION
The larvicidal
bioassay demonstrating the activities of the crude and ethanolic
extracts of Cymbopogon citratus is presented in Figure 1. The positive control treatment induced total
mortality at concentration of 10 ppm, while no mortality was observed in the
negative control. Meanwhile, larvicidal bioassay for
the crude extract showed mortality rates ranging from 26.66 – 100%.
The minimal adverse effect level of the crude extract was reported at
concentration of 10 ppm having a mortality rate of 26.66%. Notwithstanding,
the total mortality (i.e 100%) at 60 ppm (Table 4.1).
As presented in Figure 1, the larvicidal activity of
the crude extract was demonstrated with an LC50 value of 54.23 ppm (Figure
4.1).
Comparatively, the larvicidal
screening of the ethanol extract of the plant was more active with mortality
rates ranging from 10.00 – 100% (Figure 1). The minimal adverse effect level was observed at concentration of
10.00 ppm with mortality rate of 26.66%. However, the minimal total mortality
was demonstrated at concentration of 80 ppm. In addition, the ethanol extract
was active with an LC50 value of 14.60 ppm (Figure 4.1).
The larvicidal efficacies of the crude and ethanol extracts of C.
citratus leaves showed that the plant is highly
effective having total mortalities at such low concentrations. The mortality
rates were even faster at higher concentrations. Similar trend has been
observed for other studies using C. citratus oil
for the insecticidal and larvicidal bioassay of
Anopheles mosquitoes (Musa et al., 2015), as well as the Decoction,
Infusion and Maceration against Culex mosquitoes
(Goselle et al., 2017).Compared to the findings of this current study, Sosan
et al. (2013) recorded lower activities for the insecticidal efficacy of
Cymbopogon citratus
against Aedes. aegypti with total mortalities at concentrations
of 120, 200, and 300 ppm. The larvicidal activity of the plant have been largely attributed to the
diverse phytochemicals like; phenol, flavonoids, saponins
and alkaloids existing in plant (Ravikumar et al.,
2010).
CONCLUSION
Fortunately, in this study lemongrass was
found to be a safe bioactive larvicidal agent that
can be further developed as
exploited for the control of malaria. Although both extracts demonstrated
activities, but the ethanol extract treatment was significantly more active
than the crude extract treatment. Base on the findings of the study both
extracts of the plant are hereby recommended for large scale study,
comprehensive characterization of phytochemicals, actual dose and field trials.
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