Vector Control Using Semiochemicals - Biological and Environmental Control of Disease Vectors - page 96

Semiochemicals are detected by cells
located within the olfactory appendages, which
generally comprise the antennae, but for some
insects, e.g. mosquitoes, also include the
maxillary palps and proboscis. These olfactory
appendages house olfactory receptor neurones
(ORNs) that detect semiochemicals. The ORNs
are found at the base of tiny hairs called sensilla
that are found on the surface of the olfactory
appendages. Odour molecules enter through
pores on the sensilla and are transported across
the lymph by odorant binding proteins (OBPs) to
the olfactory receptors on the dendrites of the
ORNs (Mciver, 1982; Zhou et al. , 2004). The
entire olfactory system of ers a high level of
sensitivity and specifi city, particularly with
pheromones. With the correct confi guration of
individual or blends of chemicals, the central
nervous system is stimulated. This, in turn,
induces a behavioural response in the insect
(Hansson, 2002; Zhou et al. , 2004). By under-
standing the mechanisms involved in the
detection of semiochemicals, new and innovative
control tools could be developed in the future
that are designed to interfere with this process, at
the peripheral or molecular level. For example,
pheromones of vectors could be actively detected
by sensors tuned into these compounds to pro-
vide early warning of vector activity. Molecular
technologies may also allow the manipulation of
insects to interfere with or switch of the sensory
apparatus causing the insect to be unable to fi nd
a host (Carey et al. , 2010).
Traditionally, however, semiochemicals are
used in less sophisticated ways. For example,
odour cues that stimulate upwind movement
by orientation to the wind (odour-mediated
anemotaxis) of er chemicals that could be used
as attractants. Alternatively, semiochemicals
that signify an unsuitable host, in the case of
mosquitoes and tsetse fl ies (Gikonyo et al. , 2002;
Logan et al. , 2008), could be exploited as
repellents. Indeed, there are many potential uses
for semiochemicals, but they must fi rst be
characterized and evaluated for use in the fi eld.
To exploit a semiochemical ef ectively, there
are four basic steps in chemical ecology, which
must be completed.

in the fi rst instance and involve a systematic
assessment of individual compounds and blends
to determine the optimum concentrations,
mixtures and ratios.
2. The desired behaviour should be determined
whereby the lure is compared with the natural
source to establish the relative level of ei cacy.
3. The optimum release rate, formulation and
trap design must be considered for use in the
fi eld.
4. The completed lure and trap must be evalu-
ated in a fi eld trial.

Although sophisticated chemical ecology tech-
niques, such as electroantennograms, single
sensillum recordings, mass spectrometry and
nuclear magnetic resonance are now being
utilized in many labs to study semiochemicals,
there is still much work to be done. For example,
scientists have not convincingly fully replicated
natural host odours for mosquitoes, as discussed
later in this chapter. Despite the dii culties
associated with the identifi cation and logistics,
useful semiochemicals, which stimulate a
signifi cant behavioural response, have been
identifi ed successfully and have been shown to
impact signifi cantly on vector control. This is
particularly apparent for host-derived semio-
chemicals that are commonly used in traps.

6.2.1 Host-derived semiochemicals

The most common use of host odour-baited
traps in vector control is for research purposes or
monitoring populations of vector insects by
providing early warning of insect activity to
local authorities for rapid and targeted control.
Traps can also be used in direct control to remove
individuals from the environment. For the traps
to be ef ective, a good understanding of host-
seeking behaviour, the arthropod-host
interaction and the environment is essential.

Exploiting host-seeking behaviour

During host location, haematophagous insects
utilize olfactory cues from the breath or body of
vertebrate hosts, which results in a series of
behaviours leading to the successful location
of a suitable host, and therefore blood meal, or
the avoidance of an unsuitable host (Klowden,

1. The chemical(s) must be characterized and
potency determined through behavioural
assays. These assays are often laboratory based