The Use of Plants in Vector Control - Biological and Environmental Control of Disease Vectors - page 50

(Sharma and Ansari, 1994). Neem oil can also
be vaporized into the air using electrical mats to
spatially repel mosquitoes with results
comparable to the ei cacy of allethrin (Sharma
et al ., 1993b). However, the use of electrical
mats in developing countries is not an
appropriate technology, because most rural
households do not have electricity. Although the
use of neem in mosquito coils is yet to be tested,
numerous studies have tried neem oil as a topical
repellent. A fi eld study in Bolivia applying 2%
neem oil measured 56.8% protection for 4 h in
the fi eld (Moore et al ., 2002). Other studies have
reported much higher repellency (Sharma et al .,
1993a; Singh et al ., 1996; Caraballo, 2000),
however methodological dif erences make it
dii cult to compare results. The Environmental
Protection Agency has not approved neem oil for
use as a topical insect repellent as it confers
limited protection against mosquito bites, and
therefore it is not recommended for individuals
who require highly ef ective protection such as
those travelling or living in countries where
vector-borne diseases are endemic (Goodyer et
al ., 2010).

results (Barnard, 1999; Choochote et al ., 2007),
confi rming that these compounds need to be
incorporated in specialized repellent formu-
lations to prolong their repellent ef ect, for
example by adding a large molecule like vanillin
to reduce the release rate of the volatile repellent
compounds (Kongkaew et al ., 2011) or by using
microencapsulation (Specos et al ., 2010). There
is potential for essential oils as topical repellents
if industries invest in better formulations that
retard the release rates of the volatile compounds
in the essential oils. Essential oils should never
be used undiluted as this may result in
toxicological ef ects from simple skin irritations
to life-threatening anaphylactic shocks (Maia
and Moore, 2011). The use of essential oil
repellents for disease prevention is not advisable
unless there is no alternative repellent available.

CITRONELLA Essential oils from plants of the
genus Cymbopogon are the most widely used
botanical insect repellent. The best-known
member of this genus is lemongrass, commonly
used in the food and cosmetic industry due to its
fresh fragrance. Citronella is an essential oil
cocktail, mainly composed of citronellal, citro-
nellol, geraniol, citral, -pinene and limonene.
Citronella is used in various formulations from
topical repellents to scented candles acting as
spatial repellents. The use of scented candles is
mostly popular in the developed world where
vector-borne disease burden is minimal. Studies
have compared the ei cacy of citronella candles
to repel mosquitoes and sandfl ies, showing that
they of er poor protection only within a 1 m
radius from the candle source (Muller et al .,
2008a,b). Many communities plant lemongrass
near human dwellings to repel mosquitoes,
though it is still unclear to what extent this
practice is benefi cial. Hanging a series of plants
near the eaves of houses in a semi-fi eld system
as well as in the fi eld showed that mosquito
house entry can be signifi cantly reduced with
this method (Seyoum et al ., 2002a, 2003).
Unfortunately, lemongrass was not among the
plants investigated in these studies, so further
research is necessary to evaluate the traditional
practice of planting lemongrass. If applied
topically, 5-10% citronella is a very ef ective
repellent, achieving repellency levels equivalent
to those of DEET. However, the repellent
compounds are rapidly volatilized due to their

Essential oils
Essential oils are concentrated hydrophobic
liquids obtained from the distillation of plant
components such as leaves, bark, roots, fl owers
or seeds that contain volatile aromatic
compounds. Their use is commonplace in the
cosmetic, perfume, detergent and food industries.
Consumers are more familiar with these
compounds and easily accept them as safe,
although this is often not the case (Strickman et
al ., 2009). Some essential oils present excellent
repellent properties, although their longevity is
too short to feasibly provide ef ective protection
without formulation to slow their evaporation
(Curtis et al ., 1987). Trongtokit et al . (2005)
tested 38 dif erent plant essential oils as topical
repellents against St . aegypti in the laboratory
and demonstrated that essential oils only
prevented mosquito bites if used undiluted and
no longer than for 2 h. The most ef ective oils
tested were citronella ( Cymbopogon nardus ) (see
below), patchouli ( Pogostemon cablin ), clove oil
( Syzygium aromaticum ) and makaen, a Thai
citrus tree ( Zanthophylum limonella ). Other
studies testing essential oils showed similar