The Use of Plants in Vector Control - Biological and Environmental Control of Disease Vectors - page 43

ef ective at low doses ranging from 0.01 mg l −1
(equivalent to 0.01 parts per million, ppm) for
pyriproxyfen to 1 mg l −1 (100 ppm)
for temephos (WHOPES, 2011), so in order to
be considered for larval control, botanical
insecticides or synthetic analogues of their
active ingredients must have the potential to be
used at similar doses (Table 4.1). The most
recent review on botanical larvicides was carried
out in 2005 and contains a comprehensive
listing of plant larvicides (Shaalan et al ., 2005).
This review demonstrates the current lack of
extracts that fulfi l the criteria for candidate
larvicides.
The use of plant extracts in the fi eld for
vector control programmes is constrained by the
high production costs, limiting their use in
developing countries where vector-borne
diseases are most prevalent. Only by overcoming
the challenge of artifi cially synthesizing plant
extracts can these of er a viable alternative to
chemical larvicides. Several studies have begun
to address these issues, and a number of
promising active ingredients have been
identifi ed with high potency. These include
extracts of members of the family Piperaceae ,
which also contain piperine from which the
highly ef ective synthetic insect repellent icaridin
(2-(2-hydroxyethyl)-1-piperidinecar-boxylic
acid 1-methylpropyl ester) was derived. Another
group of repellent compounds, quinones, have
also demonstrated potency as larvicides. For
example, the larvicidal ef ect of six plant-derived
para-benzoquinones was tested against larvae
of St . aegypti . Death of third instar larvae was
observed with all tested compounds with the
average lethal concentration (LC) 50 ranging
between 33 ppm and 90 ppm (De Sousa et al .,
2010). The structurally similar bromoquinones
derived from the natural compound juglone
present in walnuts ( Juglandaceae ) have demon-
strated better larvicidal activity than temephos
(Ribeiro et al ., 2009) (Table 4.1).
Cashew ( Anacardium occidentale L.) is a well-
known member of the Anacardiaceae family. The
cashew nut shell liquid (CNSL) is a unique
natural source of unsaturated long-chain
phenols obtained as a by-product of the cashew
industry. Worldwide CNSL production is around
500,000 t year −1 and is a major economic
activity in South America. Extracts from CNSL
may have potential for development into

larvicides, in particular cardol with a LC 50 of
5.55 (± 0.07) compared to temephos with 100%
mortality for St . aegypti at 3 ppm concentration
in the same test (Costa Oliveira et al ., 2011).
Another study found a far lower lethal dose
(LD) 50 of 0.048 (0.032-0.091) ppm (de
Mendonça et al ., 2005), making this extract of
interest for development into a larvicide and
warranting further study.

Mosquito sterol carrier protein inhibitors
(SCPIs)
Insects cannot synthesize cholesterol as they
lack key enzymes in the cholesterol biosynthesis
pathway and therefore rely upon it from dietary
sources (Zdobnov et al ., 2002). Cholesterol is
required for cellular membranes and bio-
synthesis of the insect moulting and sex
hormone ecdysteroid; therefore controlling
insect populations by targeting cholesterol
metabolism with new insect growth regulators
is a promising area of research. As cholesterol is
a highly hydrophobic molecule, it requires a
sterol carrier protein (SCP) to transport
cholesterol intracellularly from the lumenal to
the basal side of the midgut epithelium or from
lipid droplet to the cytoplasmic membrane in the
fat body in insects. Studies have shown that
Aedes SCP-2 (AeSCP-2), an intracellular sterol
carrier protein, is at least partially responsible
for cellular cholesterol transfer in mosquitoes
(Blitzer et al ., 2005). Inhibiting these proteins
was larvicidal to several mosquito vectors: St .
aegypti , Culex pipiens pipiens , Anopheles gambiae ,
Cx . restuans and Aedimorphus ( Aedes ) vexans at
extremely low EC 50 of 5.2-38.7 μM (Larson et
al ., 2008). Such sterol carrier protein-2
inhibitors (SCPIs) are also ef ective against
insecticide-resistant mosquitoes (Li et al ., 2009).
Recent research has identifi ed a number of
curcumin analogues (derived from curcumin, a
yellow pigment present in the spice turmeric,
Curcuma longa ) that inhibit AeSCP-2 with
similarly low ef ective concentrations (EC) 50 of
0.65-62.87 μM (Anstrom et al ., 2012).
Mangostin, from the mangosteen plant ( Garcinia
mangostana L.) that also af ects mosquito SCP-2I,
exhibited larvicidal activity against third instar
larvae of six mosquito species: St . aegypti , An .
stephensi , An . gambiae , Cx . pipiens pipiens , An .
quadrimaculatus and Cx . quinquefasciatus , with