Journal of Mosquito Research 2015, Vol.5, No.15, 1-15
3
insecticide that would be lethal to majority of
individuals in a population. Insecticide tolerance/
resistance is pre-adaptive, that is mutations resulting
in survival (resistance genes) do not arise as a
consequence of insecticide usage, but spontaneously
at a frequency dependent on the natural mutation rate
(Callaghan, 1991). Under insecticide selection pressure
more individuals with resistant genes will survive than
those without these mutations. The development of
resistance in the field may be influenced by various
factors. The primary routes of resistance in all insects
are alteration in the insecticide target sites or the rate
at which it is detoxified. Three enzyme systems
namely, Glutathione-S-transferases, Esterases and
Monooxygenases are involved in detoxification of
four major class of insecticides namely organochlorides,
organophosphates, carbamates and pyrethroids. These
enzymes act by rapidly metabolizing the insecticides
to nontoxic products or by binding and very slowly
turning over the insecticide (sequestration) (Hemingway
& Karunarathne, 1998). It is generally acknowledged
that, Esterases play an important role in degrading the
organophosphates. Recent studies have shown that
pyrethroid resistance is also associated with increased
nonspecific Esterase activity in mosquitoes (Ganesh et
al., 2002; Urmila et al., 2001).
Development of resistance by vectors has been a
major problem in controlling communicable diseases
transmitted especially by mosquitoes. Monitoring the
resistance development and the mechanisms associated
with it is important at each place as it may involve
different and complex pattern. In this regard, one of
the significant advancement in the mosquito genetics
has been the investigation of enzyme variability by
electrophoretic technique. The technique involves the
electrophoretic separation and specific staining of
enzyme bands and allows the examination of variation
in protein production by genes. Further, understanding
the more applied but analogous problems involved in
insecticide resistance management depend on the
ability to measure allelic frequency in different
populations (Brent, 1986). Isozyme polymorphism as
evident from gel electrophoresis could be used as
‘biochemical markers’ in studying the genetics of
insecticide resistance in the absence of any visible
markers (Chakraborti et al., 1993). Carboxylesterases,
Phosphatases and Dehydrogenases are the group of
enzymes that take part in the detoxification process.
Polymorphism is a notable characteristic of insect
Esterases. The former is involved in the detoxification
of organophosphates and pyrethroids (Devonshire et
al., 1992). Depending on their activity, Carboxylesterases
are designated as A-Esterases (Est-A) and B-Esterases
(Est-B). Further, Monooxygenase or the mixed function
oxidases (MFO) are the enzymes that give protection
against a variety of insecticides in arthropods.
Metabolic detoxification is associated with changes
in Monooxygenase activity, producing pyrethroid-specific
resistance (Berge et al., 1998). This complex involves
a reductase and one or more cytochrome P-450s and
requires NADP as cofactor (Devonshire et al., 1992).
Glucose-6-Phosphate Dehydrogenase (G6PD) generates
this cofactor for monooxygenases. Hence an increase
in the activity of MFO’s will be reflected in the
activity of G6PD (Kumar et al., 1991). An increase in
the activity of this enzyme is the most versatile
mechanisms of resistance in insects. Oxidation
mediated by Monooxygenase is considered to be the
major pathway for pyrethroid detoxification though to
a limited extent esterase hydrolysis is also possible
(Ganesh et al., 2002).
Recent field study in Orissa by Swetapadma et al.,
2014 has shown that extensive use of insecticides and
development in agricultural practices had an impact
which led to the changes in mosquito fauna observed
from the Orissa coastal area and Chilika lake inspite
of ecological changes, industrial development and
other natural calamities. Therefore, identification and
surveillance of insecticide resistance/tolerance should
be an important component of any vector control
program at local level. Given the high cost associated
with developing, testing and producing new insecticidal
compounds, it is imperative to understand how
resistance develops in a target species and to design
control strategies that eliminates or minimize resistance
development.
In light of these informations, the present investigation
was taken up to assay and compare the susceptibility
status of
Ae. aegypti
larvae collected from different
areas of Mysore city and surrounding places against
synthetic pyrethroids and to analyze the isozyme
variations of Est-A, Est-B, G6PD, Acid Phosphatase
(AcPH) and Alkaline Phosphatase (APH) in order to
work out the allelic frequencies in the populations of
