Journal of Mosquito Research 2015, Vol.5, No.18, 1-8
5
It has been shown that the extraction of active
biochemicals from plants depend upon the polarity of
the solvents used. Polar solvents will extract polar
molecules and non-polar solvents extract non-polar
molecules. This was achieved by using solvent
systems ranging from hexane/ petroleum ether, the
most non polar (polarity index of 0.1 that mainly
extracts essential oil) to that of water, the most polar
(polarity index of 10.2) that extracts biochemicals
with higher molecular weight
viz
., proteins, glycans,
etc. It has been found that in most of the studies,
solvent with minimum polarity have been used
viz
.,
hexane or petroleum ether (Ghosh et al., 2012).
Arivoli and Samuel (2011) in their preliminary
investigation tested the different solvent extracts
(hexane, diethyl ether, dichloromethane and ethyl
acetate) of
Murraya
koenigii
leaves against
Aedes
aegypti
,
Anopheles
stephensi
and
Culex quinquefasciatus
for larvicidal activity and found that the hexane
extract was the most active. The results of the present
study corroborate with the report of Arivoli and
Samuel (2011) by confirming the presence of active
substances to be present in the ‘D’ fractionated group
of hexane extract, indicated by the lowest LC
50
value
reported. The findings of the present study are in line
with the high potential of non-polar (dichloromethane,
chloroform and hexane) extracts demonstrated against
mosquito larvae (Krishnappa et al., 2012).
Ghosh et al. (2012) reported that hexane is the most
non polar that mainly extracts essential oil. In the
present study, hexane fractions were toxic against
Aedes aegypti
,
Anopheles stephensi
and
Culex
quinquefasciatus
. This could be due to the presence
of essential oil, terpenoids, phenolic compounds and
alkaloids within the fraction. It was proven by Mann
and Kaufman (2012) that as typical lipophiles, the
essential oil passes through the cell wall and cytoplasmic
membrane, disrupts the structure of different layers of
polysaccharides, fatty acids and phospholipids and
permeabilizes them. They also interfere with the basic
metabolic, biochemical, physiological and behavioural
functions of insects (Tripathi
et al
.
, 2009), disruption
of the molecular events of morphogenesis, alteration
in the behaviour and memory of cholinergic system
and inhibition of acetylecholinesterase (AChE). Of
these, the most important activity is the inhibition of
acetylcholine- esterase
activity as it is a key enzyme
responsible for terminating the nerve impulse
transmission through synaptic pathway (Rattan, 2010).
Also, terpenoids are known to possess insecticidal
properties (acute toxicity) (Mann and Kaufman, 2012).
Phenolic compounds including tannins and flavonoids
are known to possess insecticidal properties and act as
mitochondrial poisons for insect vectors (Mann and
Kaufman, 2012). Alkaloids are nitrogenous compounds
that show insecticidal properties at low concentration
and the mode of action on insect vectors varies with
the structure of their molecules, but many are reported
to affect acetylcholinestrase or sodium channels
(Rattan, 2010).
Hexane, dichloromethane and acetone fractions of
Spondias mombin
leaf extracts when tested against the
larvae of
Aedes
aegypti
,
Anopheles
stephensi
and
Culex
quinquefasciatus
, it was found that hexane
fractions were found to possess more larvicidal
activity that the other fractions reported by low LC
50
values of 22.54, 92.20 and 326.53 ppm against
Aedes
aegypti
,
Anopheles
stephensi
and
Culex quinquefasciatus
respectively (Eze et al., 2014). Lame et al. (2015)
pointed out that among the four fractions (hexane,
chloroform, ethyl acetate and methanol) of
Annona
senegalensis
leaves tested against the larvae of
Anopheles
gambiae
, the hexane fractions were effective, exhibiting
LC
50
value of 298.8 ppm.
Mudalungu et al. (2013) tested the fractions (C1-C3)
of
Fagaropsis angolensis
chloroform leaf extract
against
Anopheles
gambiae
and found fractions C3
and C2 to exhibit high larvicidal activity with LC
50
values of 144.4 and 147.6 ppm respectively after 24
hours of exposure. Nyamoita et al. (2013) reported the
acetone root bark extract fractions of
Vitex
payos
(F8,
F11 and F12) to possess larvicidal activity against the
larvae of
Anopheles
gambiae
after 24 hours of
exposure and found the fraction F8 to exhibit the
highest larvicidal activity whose LC
50
value was 7.0
ppm followed by fraction F11 and F12 with 13.4 and
19.6 ppm respectively. Nzelibe and Chintem (2013)
tested the
Ocimum
gratissimum
leaf n-hexane fractions
(F1-F6) against
Culex
quinquefasciatus
larvae, and
found the fraction F1 to be effective with a LC
50
value
of 1.49 ppm on 24 hours of exposure. Chintem et al.
(2014) stated that
Datura
stramonium
leaf methanol
extract fractions (DSEE-F1 to DSEE-F7) when tested
against the third instar larvae of
Culex
quinquefasciatus
,
