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Ovipositioning Behaviour of Aedes aegypti in
Different Concentrations of Latex of Calotropis
procera: Studies on Refractory Behaviour and its Sustenance across Gonotrophic
Cycles
Manju Singhi, Vinod
Joshi#, R.C. Sharma and Keerti Sharma
Desert Medicine Research Centre
(Indian Council of Medical Research), New Pali Road,
Jodhpur 342 005, India
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Abstract
Dengue fever associated with dengue haemorrhagic
fever is gaining endemicity in India. Due to lack of any chemotherapy against
this arboviral infection, the control of the
disease depends largely on preventive measures against Aedes
mosquito vectors. A wild shrub, Calotropis procera, commonly growing in the desert areas of
Rajasthan has shown a remarkable effect as a larvicide
against Aedes aegypti.
However, different water concentrations of this biocide have also brought
forward very important observations on the ovipositioning
behaviour of Aedes aegypti. At 0.7% concentration of latex, the ovipositiong was avoided by the gravid female
mosquitoes and this behaviour continued till
three gonotrophic cycles. However, at lower
concentrations (0.2% and 0.1%) of the larvicidal
latex, the refractory behaviour of ovipositioning could not be retained up to the third gonotrophic cycle. The concentration of latex such as
0.7% and 0.2% were observed as ovicidal also and
this effect continued across all the gonotrophic
cycles. The behavioural observations reported in
the present study may serve as significant information on choosing bio-larvicides for vector control against dengue.
Keywords: Dengue, Aedes aegypti, Calotropis procera, ovipositioning,
refractory behaviour, gonotrophic
cycle.
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Introduction
Dengue fever and dengue haemorrhagic fever (DF/DHF)
is gaining endemicity in many states in India[1].
In the absence of chemotherapy and vaccines, vector control, largely based on
larval control, is the only option available. In Rajasthan, a number of
epidemics of dengue associated with DHF
have been reported[2-4].
It has also been established that dengue virus undergoes transovarial
transmission across generations of Aedes aegypti under natural as well as experimental conditions[5,6]. Latex
of Calotropis procera,
a milky weed plant growing all across the desert areas in the state has shown
promising larvicidal properties in a series of
laboratory
experiments carried out against Aedes aegypti (unpublished data, Desert Medicine Research
Centre). In an attempt to assess the value of this agent as an oviposition attractant for use in ovitraps,
it was discovered that the agent showed oviposition
refractoriness instead. The present communication incorporates the findings
of these studies.
Materials and methods
Six experimental sets were designed for the study. Cages of 30 cm3
size, with wooden frame and iron mesh with muslin cloth on one side, were
used as units of present experiments. In each cage, 16 gravid females of Aedes aegypti
were released. The concentration of latex in water, which showed the highest
(0.7%), moderate (0.2%) and no mortality (0.1%) effect in the larvicidal efficacy experiments, were prepared and put in
beakers in the cages A, B, C, D and E while cage F was kept without latex
solution and contained only water. A beaker containing plain water was also
placed in each of the six cages to serve as control. While in cage D all the
experimental concentrations were placed along with the control, in cage E all
the concentrations were placed without choice of a control. In the sixth cage
(cage F) two beakers with plain water were placed without any experimental
lethal concentration. The eggs laid by female Aedes
aegypti were counted after 48 hours in each of
the experimental cages. Second and third blood meals were provided to
facilitate G2 and G3 (gonotrophic
cycles) to mosquitoes.
The experiments were conducted at
25-30 °C (room)
temperature and at relative humidity of about 60-70%. The eggs of each
generation were counted under a dissecting microscope in the control as well
as experimental sets. All the eggs were immersed in plain water to observe
whether exposure to larvicide had any ovicidal effect on them.
Results and discussion
Table 1 shows relative observations on different concentrations of latex of Calotropis procera
on the ovipositioning behaviour
of Aedes aegypti.
In experimental cage A, while in the control set, 65 eggs were laid, in the
corresponding lethal concentration of 0.7% of the same cage, no eggs were
laid. In this cage all the mosquitoes showed persistent refractiveness
of ovipositioning across all gonotrophic
cycles from G1-G3. In cage B where 0.2% concentration
was offered along with control, refractive behaviour
up to two cycles only was observed while in the third cycle (G3),
127 eggs were laid. Similar observations were made in cage C. It was
interesting to note that in cage D where all the experimental concentrations
were offered to mosquitoes along with choice of control, even in G1 cycle
no refractiveness was shown. In experiment E where
choice of control (plain water) was not offered, maximum preference for ovipositioning was shown in lowest concentration (0.1%).
The different preference among different concentrations continued across all
the gonotrophic cycle (Table 1).
Table 1. Ovipositing preference of Aedes
aegypti in different larvicidal
concentrations of latex of Calotropis procera
|
Cage
|
Latex
concentration (%) in experimental cages
|
|
A
|
B
|
C
|
D
|
E
|
F
|
|
Eggs laid within 48 hrs
|
C
|
0.7
|
C
|
0.2
|
C
|
0.1
|
C
|
0.1
|
0.2
|
0.7
|
0.1
|
0.2
|
0.7
|
C
|
C
|
|
G1
|
65
|
0
|
156
|
0
|
180
|
0
|
109
|
18
|
18
|
6
|
101
|
51
|
11
|
102
|
110
|
|
G2
|
93
|
0
|
156
|
1
|
149
|
0
|
100
|
0
|
0
|
0
|
33
|
10
|
7
|
96
|
83
|
|
G3
|
275
|
0
|
230
|
127
|
275
|
52
|
270
|
98
|
31
|
0
|
215
|
150
|
20
|
233
|
197
|
C: Control
G1-G3: Gonotropic cycles
Table 2. Effect of latex on
the percentage viability of eggs of Aedes aegypti
|
Cage
|
Experimental
cages
|
|
A
|
B
|
C
|
D
|
E
|
F
|
|
Latex (%)
|
C
|
0.7
|
C
|
0.2
|
C
|
0.1
|
C
|
0.1
|
0.2
|
0.7
|
0.1
|
0.2
|
0.7
|
C
|
C
|
|
Eggs immersed
|
65
|
0
|
156
|
0
|
180
|
0
|
109
|
18
|
18
|
6
|
101
|
51
|
11
|
102
|
110
|
|
Eggs hatched
|
55
|
0
|
78
|
0
|
126
|
0
|
78
|
0
|
0
|
0
|
6
|
2
|
0
|
96
|
103
|
|
% eggs hatched
|
84.6
|
0.0
|
50
|
0.0
|
70
|
0.0
|
73
|
0.0
|
0.0
|
0.0
|
5.9
|
4.0
|
0.0
|
93.1
|
93.6
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C: Control
Table 2 shows the results of the experiment carried out to study the
viability of eggs that were exposed to different larvicidal
concentrations and to control groups. The eggs laid in experiments A, B, C
and D showed no hatching, while in experimental cage E where no control had
been kept, the eggs laid up to a concentration 0.2% showed some viability and
in cage F where only controls were kept the laid eggs showed 93% viability.
Dengue fever associated with DHF has become a problem of public health
importance. Available evidence shows that the virus undergoes vertical
transmission across generations of mosquitoes[6].
Larval control, therefore, is the most effective approach to restrict the
vector and virus sustenance in
nature. The wildly grown plant, Calotropis procera, has shown very encouraging results in its
different lethal concentrations. However, if this plant species is to be used
as a material of choice, its other aspects such as the ovipositioning
behaviour of gravid females towards the larvicide can also become known.
The significance of the reported observations is that the refractiveness
developed by the species is sustained across two gonotrophic
cycles when one larvicidal concentration was
offered with a corresponding control (Table 1). However, when all the
concentrations viz. 0.1 %, 0.2 % and 0.7% were offered with one control, such
avoidance was not shown. Similarly, in
the experimental set E where all concentrations of latex were made available
without any control, the maximum egg laying was preferred in 0.1 % of latex.
The data suggest fine chemo-sensation in Aedes aegypti where the ovipositioning
female could distinguish to choose the least larvicidal
concentration for its egg laying.
The observation showed that if in the various types of domestic containers
where water is stored a non-lethal concentration of latex (0.1%) is used, the
refractiveness of ovipositioning
will not be there and the
domestic mosquito fauna in such premises will lay the eggs but they will lose
their viability to hatch into larvae (Table 2).
The role of Calotropis procera as
a larvicide has been reported by other workers from
India[7]. However, contrary results have been reported by some workers[8] where
insect growth regulators actually enhanced ovipositioning.
The observations reported by us not only present the results of the ovipositioning behaviour induced
by a bio-larvicide, but also add the information on
relative preference in ovipositioning and its
sustenance across gonotrophic cycles when
different larvicidal
concentrations were offered.
Acknowledgements
The authors gratefully acknowledge the guidance and inspiration received from
Mr N.L. Kalra, Member, Scientific Advisory
Committee of the Desert Medicine Research Centre, Jodhpur,
India.
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