Biology Reference
In-Depth Information
this volume), and plant- and chemically derived
semiochemicals (see Lorenz et al ., and Logan et
al ., Chapters 4 and 6, this volume, respectively)
to control and suppress disease vector
populations. Plant extracts often form the
basis of the development of new insecticides,
against both vector larvae and adults (Koch
et al ., 2005).
1.2.2 Genetic manipulation of disease
vectors
Other techniques to control populations of
disease vectors, which we do not describe in
more detail in this topic, are genetic approaches
to vector control. Here we describe three recent
examples that utilize dif erent molecular
techniques in the fi ght against disease vectors.
A transgenic method closely related to
Sterile Insect Technique (SIT) is the release of
insects carrying a dominant lethal gene (RIDL),
where Stegomyia males are modifi ed to carry a
lethal gene for female mosquitoes, causing the
death of the dengue vectors unless they are
reared on specifi c dietary supplements (Thomas
et al ., 2000). In the Cayman Islands, it was
demonstrated that releasing genetically modifi ed
males of St . aegypti for a 4-week period resulted
in about 56% mating success with wild females,
however with large uncertainties around this
estimate (fi eld competitiveness compared to
wild-type males: 0.56; 95% CI: 0.032-1.97)
(Harris et al ., 2011). Additional essential
parameters that determine the competitive
fi tness of mosquitoes, such as longevity and
dispersal of the released transgenic males, were
not recorded, making the overall applicability of
the RIDL technique in realistic settings dii cult
to judge. Recently, a genetic strain of the
maternally inherited bacterium Wolbachia ,
which naturally infects 60% of insect species,
reduces the lifespan of adult Stegomyia
mosquitoes (McMeniman et al ., 2009) and
blocks transmission of dengue (Walker et al .,
2011), was shown to successfully invade two
natural St . aegypti populations in Australia
(Hof mann et al ., 2011). Therefore, this novel
approach could reduce the capability of St .
aegypti to transmit dengue without the need to
eradicate the whole mosquito population.
Finally, Anopheles stephensi mosquitoes, the main
malaria vectors in south Asia and the Middle
East, were modifi ed to over-express Akt, a protein
that regulates insulin signalling. Transgenic
mosquitoes were 60-99% less likely to be
infected with malaria parasites and had an
approximately 20% shorter lifespan than
unmodifi ed mosquitoes, thus reducing the time
that mosquitoes can be infective to humans
(Corby-Harris et al ., 2010). These approaches
are just some examples amongst many currently
1.2.1 Control options against vector
larvae and adults
Traditionally, biological agents such as lar-
vivorous fi sh, Bti/Bs and arthropods were
employed to target larval breeding sites, thus
making them mainly useful against vector
species with few and easily identifi able breeding
sites. Stegomyia (formerly Aedes ) aegypti female
mosquitoes, vectors of dengue and other
arboviruses, lay their eggs in peri-domestic
habitats, which are relatively easy to locate and
can thus be controlled ef ectively with larval
control agents. More recently, there has been
increased research into biological control agents
that can also target adult vectors, for example the
entomopathogenic fungi Beauveria bassiana and
Metarhizium anisopliae (see Stevenson, Chapter 5,
this volume). Their use as adulticides, particularly
against anopheline malaria vectors, is based on
the epidemiological principles of malaria (Smith
et al ., 2007). Late-life acting (LLA) biopesticides
or insecticides will kill mosquitoes before they
can transmit the malaria parasite but after they
have reproduced, thus decreasing the selection
pressure to evolve resistance against the control
agent (Read et al ., 2009). Plant- and chemically
derived semiochemicals can be utilized in
another sustainable way, which is predicted to
select for lower resistance, by manipulating
natural olfactory-driven behaviours such as
mate- or host-seeking, breeding site selection or
avoidance of natural predation (Gibson and Torr,
1999; Logan and Birkett, 2007). The deployment
of odour-baited traps and insecticide-treated
targets reduced tsetse fl y populations by more
than 99.9% in two key trials in Zimbabwe (Vale et
al ., 1986, 1988). Many insect repellents are
based on plant-derived compounds, such as PMD
(para-menthane-3,8-diol), which reduced the
risk of contracting malaria in Bolivia by 80%
(Hill et al ., 2007).
 
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