Control with Arthropods - Biological and Environmental Control of Disease Vectors - page 11

 Arthropod ecology: certain characteristics of

the control agent ecology may not be suitable
for biological control. For instance, to be
ef ective, arthropods need not only to have a
narrow host range, but also the ability to
survive at low prey density. For sustainable
control, they also need a high (and relatively
fast) reproductive potential in the environ-
ment into which they are released. Control
agents that can disperse freely as adults (i.e.
by fl ying) need to preferentially lay eggs in
the same environment as the disease vector.
Similarly, consecutive generations of the
disease vector population need to be repeat-
edly brought into contact with the biological
control agent. Since most adult disease
vectors can disperse by fl ying, the larval
stages are usually targeted. Thus, the pres-
ence of the control agent should not repel the
adult vector from laying eggs in that environ-
ment.
 Logistical dii culties: there are inherent dii -
culties in mass producing, storing and
handling large enough numbers of arthro-
pod predators for successful biological
control, for example many of them are
cannibalistic and require live prey for food.

transmission, which will be discussed in more
detail in this chapter. Most of the chapter will
deal with dengue control, since this has been the
major success story, but work involving malaria
and fi lariasis vector control will also be outlined.
For more information on the theory behind
biological control, see the open access chapter by
Rodriguez-Perez et al . (2012).

2.2 Dengue Control

Dengue is the world's most rapidly expanding
mosquito-borne disease with 2.5 billion people
currently at risk (WHO, 2009). Dengue viruses
are transmitted by mosquitoes in the Stegomyia
genus (previously named Aedes ), which also
transmit diseases such as yellow fever and
fi lariasis. Since no vaccine or ef ective medication
exists, the mainstay of dengue control is vector
control.
Several characteristics of Stegomyia biology
must be considered before applying dengue
control techniques. First, Stegomyia females lay
their eggs in man-made water-holding struc-
tures, and the mosquitoes' ability to fi nd even
the smallest individual bottle caps means that a
concerted ef ort must be deployed in order to
have an ef ective impact on local Stegomyia
populations. These habitats are usually peri-
domestic and relatively easy to locate. Second,
Stegomyia eggs are able to survive desiccation,
which precludes emptying man-made habitats
of water to remove larval habitats. Finally,
Stegomyia adults are active during the day. Due
to their diurnal activity, they cannot be
controlled by the same tools used against the
nocturnal vectors of malaria and fi lariasis, such
as insecticide-treated bed nets. Thus, dengue
control is mainly aimed at controlling the
Stegomyia larval stages as they are usually in
peri-domestic and readily identifi able habitats.
The nature of these habitats lends itself to the
use of biological control tools. There is a wide
variety of invertebrate predators that can be
used to control Stegomyia numbers in the wild
(Jenkins, 1964; Mogi, 2007; Quiroz-Martinez
and Rodriguez-Castro, 2007; Shaalan and
Canyon, 2009), but there are two examples that
have dominated the literature: copepods and
Toxorhynchites mosquitoes.

It is not easy to manipulate the natural
ecosystem balance and maintain that change
so that the vector population is reduced more
than it normally would be by predation. The
control agents themselves could also become
pests once the natural balance is disrupted, or
apex predators can wipe out the biological
control agent population. Finally, many
arthropod predators are not compatible with
the use of chemical insecticides because they
themselves would be killed. This would cause a
problem as, until recently, chemical insecticides
have been the mainstay of disease control
programmes.
There are many diseases that are trans-
mitted by invertebrate vectors. However, because
most of the work has been carried out with
mosquito vectors, this chapter will focus on the
use of arthropods in dengue, malaria and
lymphatic fi lariasis control. There are several
well-documented instances where arthropods
have been successfully used to control disease
vector populations and to suppress disease