Vector Control Using Semiochemicals - Biological and Environmental Control of Disease Vectors - page 99

512) and control (Mosquito Magnet Pro). The
traps were modifi ed to release dif erent ratios of
(R)- and (S)-1-octen-3-ol enantiomers in
combination with CO 2 . Traps containing a
greater proportion of the (R)-enantiomer
caught signifi cantly more host-seeking female
Culicoides than traps containing a greater
proportion (S) enantiomer or the racemic
mixture. The number of Culicoides caught in a
trap using the most ef ective treatment, 500 ml
min −1 CO 2 combined with 4.1 mg h −1 (R)-1-
octen-3-ol, was then compared with the number
of Culicoides caught in a 'drop trap' using live
sheep hosts. The results indicated that Culicoides
species complements are similar between the
synthetic lure and the live host. The authors
concluded that traps containing the (R)-
enantiomer could be a useful tool for monitoring
Culicoides populations and have the potential to
represent a host better than the standardized
light-trapping method (which does not catch
day-fl ying midges) or the labour-intensive
collections from live animals currently used
(Harrup et al. , 2012).
These studies demonstrate that there is not
one 'fi ts all' lure for haematophagous insects,
and highlight the importance of understanding
each vector species, its host preferences and the
fi ne detail of the chemistry relating to the
olfactory processes that mediate host location.

For An. gambiae mosquitoes, synthetic
blends of human odour have been formulated
and have demonstrated strong behavioural
responses (Smallegange et al. , 2010b). However,
many of these blends do not compete favourably
when tested against a natural host. For example,
a blend identifi ed by Smallegange et al. (2005),
comprising ammonia, lactic acid and aliphatic
carboxylic acids, attracted mosquitoes, but was
less ef ective than natural odours either from a
hand or from skin odour residues on a nylon
stocking. This suggests that either other
important components are missing, or the
concentration or ratios of the blend components
are incorrect (Smallegange et al. , 2005, 2010b;
Verhulst et al. , 2011). Some recent progress has
been made where odour blends for An. gambiae
achieved trap catches close to those obtained
using natural human skin odorants. In one
study, an odour blend consisting of CO 2 ,
ammonia and carboxylic acids was tested in a
large-cage semi-fi eld enclosure using attractant-
baited MM-X traps placed at a distance of 20 m
apart and in fi eld experiments where traps were
placed inside experimental huts, and compared
with ten adult male volunteers (Okumu et al. ,
2010). They found that the blend attracted from
three to fi ve times more mosquitoes than
humans when tested in separate huts, but was
equally or less attractive than humans when
compared within the same huts (Okumu et al. ,
2010).
The standardized blend identifi ed by
Smallgange et al . (2005, 2010a) comprising
ammonia, (S)-lactic acid and tetradecanoic
acid has been improved more recently
(Mukabana et al. , 2012). The ef ect of adding
additional human-derived components to the
standardized blend including isovaleric acid,
4,5 dimethylthiazole, 2-methyl-1-butanol and
3-methyl-1-butanol was investigated in various
dif erent combinations and concentrations in a
semi-fi eld facility and two villages in western
Kenya. They found that 3-methyl-1-butanol
signifi cantly increased the attraction of the
standardized blend under semi-fi eld and village
conditions (Mukabana et al. , 2012). This new
blend has not yet been tested against natural
odour, however, it is encouraging to note that
several malaria vectors ( An. gambiae sensu
stricto, An. arabiensis and An. funestus ) and
vectors of Bancroftian fi lariasis and arboviruses

Other host-derived semiochemicals
Many other semiochemicals that are produced
by vertebrate hosts are involved in host-location
and there is an overlap in the kairomones to
which dif erent species respond (as already
demonstrated above). Much of this work has
focused on An. gambiae mosquitoes. For example,
compounds such as ammonia and an array of
aliphatic carboxylic acids, known human skin
and sweat components, play a role in host-
seeking of many mosquito species (Smallegange
et al. , 2005). It is thought that skin bacteria play
a role in the production of many of these body
odour components by converting non-volatile
'sweat' compounds into volatile compounds.
Volatiles produced by human skin bacteria
grown in culture are attractive to An. gambiae
mosquitoes, for example, human eccrine sweat
is more attractive to An. gambiae after incubation
with skin bacteria (Smallegange et al. , 2011).