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Journal of Mosquito Research, 2013, Vol.3, No.5, 33
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37
values for
H. bacteriophora
of 43.12 hr in comparison
with 82.64 hr for
H. indica
. It is worthy to mention
that 12 hr of exposing the host larvae to infective
juveniles of
H. bacteriophora
was sufficient time
to kill 50% of the host population. Meanwhile,
H.
indica
needed not less than a day to achieve the same
control levels.
3.3 Dose response assay
In the present experiment, increasing the infective
juvenile nematode concentration of both
H. bacteriop-
hora
and
H. indica
from 100 to 300 infective juvenile/
4
th
instar host larva resulted in a significant increase in
host mortality (r=0.9, P
≤
0.01, Figure 6). The increase
in host mortality was gradual up to 200 ij/host. Further
increase in juvenile concentration (250, 300 ij/host)
resulted in a sharp increase in host mortality. The
increase in host mortality was significant only when
300 infective juvenile of both species were used per
host larva, where the maximum host mortality levels
were 96.0%, 80.0%, respectively. The LC
50
value was
121.5 ij/host for
H. bacteriophora
as compared with
141.7 ij/host for
H. indica
.
Figure 6 The percentage mortality of 4
th
insatr
C. quinquefasciatus
larvae following exposure to different concentrations of ijs of
H.
bacteriophora
(=HB) and
H. indica
(=HI)
The experiment was repeated using
H. bacteriophora
only and 2
nd
and 3
rd
instar larvae as host to detect the
effect of host instar on the control process (Figure 7).
Increasing nematode concentration from 100 to 300 ij/
larva resulted in significantly increasing the levels of
mortality in 3
rd
and 4
th
larval instars (r=0.9, P
≤
0.05).
By using 2
nd
instar mosquito larvae as hosts; no
significant change in larval mortality was recorded by
increasing the infective juvenile concentration from
100 to 300 (P
≥
0.05).
Figure 7 Percentage mortality of second, third, and fourth-
instar larvae of
C. quinquefasciatus
at different ij concentrate-
ions of
H. bacteriophora
. Mortality was recorded after 48 hr of
incubation
The effect of increasing host instar from 2
nd
to 3
rd
instar was delayed up to 200 ij/larva, where there was
no significant difference between 2
nd
and 3
rd
instar
larval mortalities at lower nematode concentrations
(P
≥
0.05). The 3
rd
and 4
th
instar larvae were more
susceptible to infection than 2
nd
instar and the
recorded host mortalities were 90.9%, 96.0% and 55%,
respectively.
Considering LC
50
, it was found that 121.5 ij/larva and
143.5 ij/larva were sufficient to kill 50% of 4
th
and 3
rd
instar larval populations, respectively. Meanwhile, to
achieve 50% decreases in population of 2
nd
instar
larvae, 231.8 ij/larva were needed.
Since both of the tested
Heterorhabditis
species com-
pleted their life cycle within the mosquito larvae, the
nematode recovery was measured for both species as
an indication of the ability of the nematode species
to persist and multiply in this host. The infective
juvenile production (recovery) for both species increa-
sed with increasing initial infective juvenile concent-
ration up to approximately 200 infective juveniles/
host (r=0.9, P
≤
0.01), where a total average of 68.3
and 97.1 infective juveniles of
H. bacteriophora
and
H. indica
, respectively, were produced (Figure 8). The
maximum number of infective juveniles produced,
was 136.6 ij/host for
H. bacteriophora
(df=14, F=37.77,
P
≤
0.01) and 134.3 ij/host for
H. indica
(df=14, F=
42.25, P
≤
0.01). Increasing nematode concentration to
more than 200 ij resulted in a decline in nematode
recovery (r=
-
0.9, P
≤
0.01, Figure 8). However, the
number of infective juveniles produced per host dropped
to levels comparable with low initial densities when
nematode initial densities were raised to 300.