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Journal of Mosquito Research, 2013, Vol.3, No.5, 33
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39
Figure 10 Mean length of individual infective juveniles of
H.
bacteriophora
emerged from a
C. quinquefasciatus
cadaver as
a function of changing initial nematode density
4 Discussion
Mosquito insecticide-resistance at larval stages is extr-
emely high compared to adult stage. For this reason,
another biological, safe, and effective control method
is favored over the chemical one. In the present study,
the nematode impact in biological control of mosque-
itoes was ranked according to various factors affecting
the infection process. The exposure period assay ind-
icated, indirectly, how quickly insects were infected
by the nematodes. The one-on-one and dose response
assays measured the overall infection process in regards
to nematode species and doses which affects the
control levels as well as the nematode characteristics
as fecundity.
In the present study, one-on-one assay indicated that
H. bacteriophora
is more virulent to mosquito larvae
than
H. indica
and the recorded mortality values give
a promising spot light on the possibility of using both
species to control mosquito larvae. Our results were in
agreement with the early work of Poinar & Kaul
(1982), where they investigated the parasitism of
C.
pipiens
by the nematode,
H. bacteriophora
and they
recorded high levels of host mortality.
On the other hand, both tested
Steinernema
species
failed to establish in the
C. quinquefasciatus
mosquito
larvae and were melanized. Bronskill (1962) recorded
melanization of Neoaplictanid rhabditoid nematode
(DD136) by
A. aegypti
larvae.
In the exposure period assay, both
Heterorhabditis
species entered the host with nearly the same rate
during the first 3 hr. Increasing exposure time separated
the efficiencies of both species as fast invader (
H. bac-
teriophora
) and slower invader (
H. indica
). By the end
of 12 hr exposure period,
H. bacteriophora
was proved
to be still more active than those of
H. indica
and this
might be the reason for the higher
H. bacteriophora
numbers that entered the larvae (20 ij) in comparison
with (14 ij) in case of
H. indica
. These two parameters
(invasion rate and nematode numbers) are closely related,
and are considered characteristic for specific nematode
species. On the other hand, Abd El Rahman & Hussein
(2007) recorded that
H. indica
invaded
G. mellonella
larvae in higher numbers than
H. bacteriophora
only
at lower initial nematode concentration while
H. bact-
eriophora
entered in higher numbers than
H. indica
at
high nematode concentrations.
Concerning the effect of exposure period on host
mortality,
H. bacteriophora
was found to be a faster
killer for
C. quinquefasciatus
(with high invasion rate)
than
H. indica
. This was clear from the LT90 values.
In this context,
H. bacteriophora
needed 43.12 hr to
kill 90% of the host population which is nearly half of
the time needed by
H. indica
to attain the same host
mortality values. These results support the previous
ones concerning nematode entrance as a measure of
invasion rate. However, low penetration level and
slow invasion rate may not necessarily mean that a
nematode has lower efficacy. In the present work,
H.
indica
raised the host mortality percentages very
efficiently reaching 80%, which is a good indication
of virulence towards this host species. This contrast
may be attributed to variation in infection strategies.
Since the infective juveniles of heterorhabditid nema-
todes develop into hermaphrodites, a single invader
can potentially reproduce. For this reason, low number
of nematodes entering to their hosts will be sufficient
to establish the next generation. In contrast, steinerne-
matids are amphimictic and mating is necessary to
reproduce, thus an invasion of high numbers of indiv-
iduals increases the probability of mating and further
reproduction (Koltai et. al., 1995).
The data obtained in the exposure period assay suggest
that this assay may be used to compare different species
or production batches of nematodes with different
penetration ability. However, the biological impact of