The Use of Plants in Vector Control - Biological and Environmental Control of Disease Vectors - page 49

to have a standard testing methodology that is
widely accepted and followed, e.g. WHOPES
(2009). It is most important to compare the
repellent product weight for weight against 20%
DEET in ethanol as a gold standard, on humans
wherever possible, as this refl ects the end user.
This means that data from multiple tests and
authors can be compared. It is also essential to
describe the mosquito species used in the test,
and that the species is representative of the place
where the repellent is designed to be used, e.g. St .
aegypti for dengue protection in the urban
tropics. Testing conditions should also be
representative since repellent longevity is highly
dependent on temperature and humidity that
af ects the rate of evaporation of repellent
molecules. Furthermore, correct reporting of
data is useful, generally accepted as the ef ective
concentration to prevent 99% of bites (EC 99 )
and the duration of ei cacy until a single bite
followed by a confi rmatory bite is received (time
to fi rst bite), or until repellent ei cacy had
decayed to 80%. All data should be reported as
the mean or median number of mosquito
landings with corresponding estimate of
reliability, i.e. the standard error or the
interquartile range, respectively, in addition to
any statistical data or percentage protection.

rate of the volatile repellent molecules is slower
than that of most essential oils, attributable to
PMD's lower vapour pressure. The repellent ef ect
of PMD and 20% DEET was tested in a blinded
fi eld-study in southern California (Carroll and
Loye, 2006). Twenty-four volunteers performed
human landing catches: 20 volunteers applied
PMD, two volunteers DEET at 20% and two were
control. The study was performed for 6 h, during
which more than 1000 host-seeking mosquitoes
of the species Ochlerotatus melanimon and
Aedimorphus vexans attempted to bite the
volunteers in the control group, opposed to two
attempted bites per person for the DEET group
and less than one attempted bite per person for
the PMD group. In South America, in an area
where the local malaria vector An . darlingi bites
in the early evening before the population retires
to the protection of their bed nets, a randomized
control trial demonstrated that the use of PMD
signifi cantly reduced the risk of contracting
malaria among users by 80% (Hill et al ., 2007).
PMD is the only plant-based repellent to be
recommended for vector-borne disease pre-
vention by the Center for Disease Control (CDC)
(Zielinski-Gutierrez et al ., 2010) and is considered
to pose no risk to human health (EPA, 2009). It is
important to note that while PMD is approved by
the EPA for use as an insect repellent, the
unmodifi ed essential oil of lemon eucalyptus is
not approved due to its low ei cacy and potential
skin sensitizing ef ects.

PMD
Para-menthane-3,8-diol (PMD) is produced
through the acid modifi cation of the essential oil
present in leaves of the lemon eucalyptus tree
Corymbia citriodora . Although the lemon
eucalyptus originates from Australia, it has been
naturalized throughout the tropics and the
essential oil of the tree is produced mainly in
Brazil and China. The repellent properties of
PMD were fi rst discovered in the 1960s in China
during a bio-prospecting campaign aimed at
screening Chinese medicinal plants for repellent
properties. The waxy liquid remaining after
hydrodistillation of the leaves showed excellent
repellency (Curtis, 1990). PMD is an increasingly
popular topical repellent in the current markets,
as consumers are choosing natural products
over traditional synthetic alternatives such as
DEET or icaridin. Its popularity is also a result of
its remarkable ei cacy, with studies consistently
demonstrating protection equivalent to that of
DEET (Carroll and Loye, 2006). The evaporation

Neem
In addition to its larvicidal and growth
regulatory potential (Section 4.1.1), neem is
also an insect repellent and is traditionally used
in many parts of the world to repel host-seeking
mosquitoes by smouldering or burning its leaves.
The spatial repellency provided by the direct
burning of neem leaves was measured in Guinea
Bissau amounting to 76% protection against
wild host-seeking mosquitoes (Palsson and
Jaenson, 1999). The protection given by thermal
expulsion of the volatile compounds present in
the leaves against An . gambiae resulted in 24.5%
less mosquito entry in experimental huts in the
semi-fi eld in Kenya (Seyoum et al ., 2002b). In
India, adding 1% neem oil to a kerosene lamp
resulted in 94.2% protection from Anopheles spp.
and 80% protection from Culex spp. in the fi eld