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Journal of Mosquito Research, 2013, Vol.3, No.5, 33
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length of both
H. bacteriophora
and
H. indica
with
increasing infection density. In this respect, the larger
host supports more the development of higher numbers
of juveniles without competitions and constrains that
were found in small hosts, where crowding effect
appears as a great factor affecting juvenile growth and
hence juvenile length.
The tradeoff between quantity and quality of emerging
infective juveniles (in respect to length) has been rep-
orted early for other parasites (Kino, 1984). Because
taller infective juveniles harbor more nutrients, they
can be expected to survive for a longer period than
shorter nematodes. These juveniles were expected to
be more active and hence have higher searching capacity
than shorter, less active ones. In contrast, producing
large numbers of small, short-lived infective juveniles
may decrease the probability of rapidly locating a new
host. The explanation of these results may rely on the
lipid content of the nematode juveniles. Lewis et al.
(1995) studied the relationship between the metabolic
rate, energy reserves, and foraging behavior in three
species of entomopathogenic nematodes;
S. carpocapsae
,
S. glaseri
, and
H. bacteriophora
, each species is char-
acterized by differing in juvenile length. Their studies
showed that lipids, the major components of nematode
energy reserves, were stored in larger quantities in
longer juveniles than in shorter ones. These lipids were
declined at species-specific rates.
The density-dependent factors may play an important
role in entomopathogenic nematode fecundity. The
density dependent effect may be important in regulating
nematode populations either by acting directly through
affecting the numbers of infective juveniles produced
from each cadaver, or indirectly, by changing the infe-
ctive juvenile longevity. In laboratory culturing and
biological control applications where recycling and
persistence is advantageous, the impact of infection
density may be of critical importance in maximizing
nematode efficacy.
The dose response bioassays has been used many
times previously and probit analysis has been used to
analyze the data to calculate LC
50
values. However,
when a parasite is highly virulent, the applicability of
probit analysis is questionable, since a single steinern-
ematid or heterorhabditid nematode is often capable of
killing an insect (Ricci et al., 1996).
Although similar ranking was observed in the present
bioassays, the ability to separate the species statistically
varied among assays. One-on-one assay effectively
separated
H. bacteriophora
and
H. Indica
from each
other and from
S. carpocapsae
and
S. feltiae
. The later
two species could not be separated from each other by
this assay. This assay was conducted in multi-well
plates, so, nematodes and insects were kept in close
contact and the influence of foraging strategies was
limited. Differences in nematode ability to penetrate
into the insect and complete its life cycle served as the
main factor distinguishing between species. Also, the
dose response assay could not separate both heterorha-
bditid species from each other except in the low and
high doses of 100 and 200 ij/insect after 48 hr. These
treatments were the best for separating both species.
The LC
50
or LC
90
values also separated both species.
The present work dealt with demonstrating the variat-
ions of entomopathogenic nematode species perform-
ance in different bioassays. The differences in the
activity of nematodes in the exposure period assay
made a spot light on the potential of measuring some
behavioral responses as specific criteria for nematode
virulence. The presented data support the fact that,
since nematodes vary in their behavior, one bioassay
cannot be used as a unique measure of virulence for
all species (Caroli et al., 1996).
In general, particular bioassays may be used for other
purposes: for the selection of a specific population for
use against an insect, a variable assay measures which
are more laborious but simulate natural environmental
conditions or invasion by nematode (e.g. nematode
entrance) should be considered. In cases where produ-
ction batches of the same nematode strain are compared,
a simple rapid assay is needed (e.g. One-on-one or
exposure period assay). The obtained results may add
much to our information concerning the use of nemat-
ode-bacteria system to control
C. quinquefasciatus
larvae. Its significance is that it is the first attempt in
Egypt to get benefits of augmenting host-specific,
lethal bacteria within the nematode to the aquatic
larvae to reduce the mosquito population before adult