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Journal of Mosquito Research, 2013, Vol.3, No.5, 33
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this assay and its relationship to nematode activity in
the field is yet to be determined.
The dose response assay could be another way to
determine nematode virulence. The increase in nematode
infective juvenile numbers/ host larva resulted in an
increase in mortality of 3
rd
and 4
th
instars of mosquito
larvae after using both
Heterorhabditis
nematode
species. These results were in agreement with the
effect of host size recorded by (Poinar & Kaul, 1982).
From the factors that governed the degree of infection
is the stage of the host. Parasitism in general was
highest in fourth instar larvae. This is due to the fact
that larger hosts could more readily ingest nematodes
without damaging them. In contrast, second-instar
larvae rarely ingested whole nematodes, more often
crushing them with their mandibular teeth because of
their smaller oral aperture. Once the nematode cuticle
was broken, the parasite perished.
Our results and early work of Dadd (1971) and Poinar
& Kaul (1982) were following the same pattern of
response to changing nematode concentration, but their
work showed the inability of the rhabditoid nematode-
bacteria system to complete their life cycle in
C. pipiens
mosquitoes. However, the mortality records were in
agreement with our records concerning the host instar
effect. They observed that although 4
th
instar larvae
may ingest hundreds of nematodes within few hours,
very few individual successfully established themselves
in the hemocoel and that those worms remaining
within the peritrophic membrane commence to disin-
tergrate within a few hours, often leaving no recogni-
zable remains after a day. But the benefit of our nem-
atode strains is that they were able to complete their
life cycle in their host larvae till infective juvenile
emergence. The same pattern of dose response was
found in their experiment.
These results were in agreement with those of Capinera
et al., (1988) who found a positive correlation between
numbers of infective juvenile used and the number of
nematodes which successfully invaded the host. Mean-
while, Abd El Rahman & Hussein (2007) did not found
a significant effect of increasing nematode concentr-
ation from 250 to 500 ij/host neither on mortality of
L.
decemlineata
larvae nor on the nematode invasion rate.
They also did not find a significant difference in 3
rd
and 4
th
instar larval mortality upon exposure to
H.
bacteriophora
infective juveniles for 24 to 72 hr.
Considering nematode fecundity after changing initial
nematode concentration per host, the results showed
that
H. bacteriophora
was able to tolerate high densities
within the host than
H. indica
but both species respon-
ded with the same pattern to increasing nematode con-
centration. Metabolic rate, processing of host tissues
by symbiotic bacteria, and the physiological parameters
required for growth differ between nematode species.
Differences between both heterorhabditid species may
result from the differences in host utilization. Also,
the reason for the differences in the effect of viable
bacterial cells of
P. luminiscens
(
H. bacteriophora
symbionts) may be that these bacteria could survive
more the host immune response and present toxic
components that killed the host while the bacterial
symbionts of
H. indica
were susceptible to host imm-
une system (Selvan & Blackshaw, 1990).
The effect of initial infection density was noticed
clearly in the quality of the produced nematode adults
as well as the produced juveniles which in turns had
an important influence on the population dynamics of
parasites. Hermaphrodite length is often used as an
indirect measure of nematode fecundity (Selvan et al.,
1993). The decline in length which is an indication of
reduced fecundity at low density may be probably due
to either the decreased level of bacterial inoculums
causing slower break down of host tissues, or conver-
sely, insufficient bacterial culling by the nematode
resulting in unrestrained bacterial growth. In contrast,
at high densities, host utilization by nematode and
bacteria is more rapid and may result in inadequate
nutrition resulting from competition for limited nutrients
within the host. Because
H. bacteriophora
was able to
survive at high densities, this could explain how it
successfully reproduced at all of the studied densities.
The effect of the initial infection density on juvenile
length was clearly obvious, where the longest infective
juveniles were produced at lowest densities, not at the
densities producing the largest number of infective
juveniles. Abd El Rahman & Hussein (2007) recorded
a negative correlation between infective juvenile