Biology Reference
In-Depth Information
them in some areas of Ethiopia, Chad, South
Sudan and Mali because, despite the global
eradication programme, these countries are still
reporting cases of the Guinea worm
Dracunculus
medinensis
(WHO, 2012).
Copepods
are inter-
mediate hosts of this helminth parasite and
humans can become infected when drinking
water containing Guinea worm-infected cope-
pods.
Although this parasite is not lethal to
humans, it causes severe pain and disability and
introduction of intermediate hosts will be
considered unethical.
Several studies have reported that
Meso-
cyclops
copepods have been used to eradicate
dengue transmission from many areas of
Vietnam (Kay and Nam, 2005; Kay
et al
., 2010),
and they are being successfully used to suppress
Stegomyia
populations around the globe.
Predatory copepods are found all over the world,
they can be easily handled and transported and
they can establish populations in the required
areas. They are a good example of arthropods
that are available for operational use as
biological control agents against disease vectors.
In addition, they have been incorporated into
successful IVM programmes, and there is great
potential for them to become integrated into
further IVM dengue control programmes
Aspects of copepod biology, and their use as
biological control agents against mosquito
larvae, have been reviewed previously (Marten
and Reid, 2007).
average of 12.3
St
.
aegypti
larvae in 24 h in
discarded car tyres (Trpis, 1973). Because the
life cycle of
Toxorhynchites
is almost three times
longer than the prey mosquitoes' life cycle, it is
estimated that a single
Toxorhynchites
larva will
consume up to 5000 early stage larvae or 300
late stage larvae in the course of its development
(Focks, 2007). In addition,
Toxorhynchites
are
able to survive long periods without food,
making them a good candidate for the biological
control of mosquitoes; predators whose popu-
lations crash with the prey population will not
of er sustainable control.
Vector control using
Toxorhynchites
can be
more problematic than using copepods because
the adult
Toxorhynchites
disperse by fl ying. The
benefi ts of such dispersal are that
Toxorhynchites
mosquitoes can control disease vectors in hard-
to-fi nd habitats.
Toxorhynchites
females only
oviposit a small number of eggs in many dif erent
larval habitats to avoid larval cannibalism. The
downside of the adult dispersal is that they are
only ef ective against vector species with which
they share ovipositional preferences. In general,
Toxorhynchites
mosquitoes prefer to oviposit in
shady areas in both man-made (tyres, water
containers, fl ower pots) and natural habitats
(tree holes, cut bamboo), although this changes
with species. Therefore, as with copepods,
Toxorhynchites
are mostly deployed to control
Stegomyia
mosquitoes, with which they share
oviposition sites and larval habitats, rather than
other disease vectors. Either
Toxorhynchites
eggs
or predacious larvae can be placed into every
single
Stegomyia-
suitable container in an
environment, or gravid adult females are
released into the environment so that they then
oviposit in certain breeding sites. The latter is
less time consuming but relies on ovipositional
overlap between the predator and prey mosquito
species.
In all of the continents where dengue poses
a serious risk, some
Toxorhynchites
species have
been reported to naturally overlap with prey
Stegomyia
species. In Asia,
Tx. splendens
were
found to naturally co-exist with and reduce
numbers of
St
.
albopicta
larvae in Malaysia
(Nyamah
et al
., 2011).
Tx splendens
could be a
good candidate for dengue vector control in this
area because they preferentially consume
Stegomyia
rather than
Culex
larvae (Nyamah
et al
., 2011). Similarly, in Bangkok, Thailand, a
2.2.2
Toxorhynchites
Another promising arthropod that is being used
to control disease vectors is the
Toxorhynchites
mosquito.
Toxorhynchites
belongs to the
Toxorhynchitinae subfamily and there are
around 70 species groups within three
subgenera (Focks, 2007).
Toxorhynchites
adults
are not involved in disease transmission cycles
because they do not suck blood. In contrast, they
obtain all the nutrients required for the
production of eggs from their larval predatory
behaviour.
Toxorhynchites
larvae eat aquatic
invertebrates with a preference for mosquito
larvae if they are present in the same water body
(both interspecifi c and cannibalism) (Schreiber,
2007). A fi eld study in Tanzania found that one
fourth instar
Tx
.
brevipalpis
larva consumed an
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