The Use of Plants in Vector Control - Biological and Environmental Control of Disease Vectors - page 45

the median lethal concentration values ranging
from 0.84 to 2.90 ppm (Larson et al ., 2010).
This is a promising new direction for mosquito
larval control with analogues of compounds
isolated from plants at very low concentrations
that are insui cient to exhibit vertebrate toxicity.
However, like many phytochemicals, these
compounds are not photostable and will require
careful formulation such as slow release
formulations combined with UV-blocking
additives as is already in place for the commonly
used larvicide methoprene.

activities fall into six categories: (i) antifeedency;
(ii) growth regulation; (iii) fecundity sup-
pression; (iv) sterilization; (v) oviposition
repellency or attraction; and (vi) changes in
biological fi tness (Miller and Chamberlain,
1989; Schmutterer, 1990; Sharma et al ., 1993a;
Monzon et al ., 1994; Su and Mulla, 1998a,b,
1999b; Mulla and Su, 1999; Kaur et al ., 2003).
Neem contains a large number of triterpenoids,
the best known of which is the liminoid
azadirachtin (AZA), which is concentrated in
the seeds, leaves and bark of the tree. The
concentration of AZA is positively correlated
with its ef ects on insects (Isman et al ., 1990).
From its many applications of neem, the most
promising use in vector control is against
immature mosquito stages. Neem is a growth
disrupter or insect growth regulator (IGR), i.e. it
af ects the growth of mosquito larvae without
killing them. Azadirachtin blocks the synthesis
and release of moulting hormones from the
prothoracic gland leading to incomplete ecdysis
in immature insects and sterility in female
insects (Mitchell et al ., 1997). The ef ects of
neem on mosquito larvae have been shown to
result in growth disruption, prolongation of the
larval period, larval deformities and increased
mortality (Chavan et al ., 1979; Zebitz, 1986)
(Fig. 4.1).
Neem oil, the main product obtained from
the neem tree, is used in medication, cosmetics
and soaps, as well as insecticides. Neem oil is
pressed out of the kernels and fruits, leaving a
by-product called 'neem cake', which is widely
accepted by farmers as cattle fodder and an
organic paddy fertilizer that increases harvest
yields in South-east Asia. The application of
neem cake fertilizer also reduces mosquito larval
densities (Reuben et al ., 1990). The neem cake
settles on the bottom of rice paddies, where
mosquito larvae feed upon it, and decomposes,
thereby releasing its active ingredients (Rao,
1987b). Consequently, the larvae that feed on
the surface are less af ected, an important
consideration for anophelines that are surface
feeders (Reuben et al ., 1990). In addition, the
drop in larval numbers is limited to 2 months
following the application of neem cake fertilizers
(Rao, 1987a; Rao et al ., 1992). Recent laboratory
studies of dif erent types of commercially
available neem cake demonstrated their ei cacy
against the dengue vector St . albopicta (Nicoletti

Neem ( Azadirachta indica )
One of the most widely researched plants used
against insects is the neem tree, Azadirachta
indica (Schmutterer, 2002). It has been used for
a multitude of purposes for thousands of years
in its native India where it has the af ectionate
name 'the village pharmacy', and is applied both
in India and throughout the world against
arthropod pests (Sears, 1996; Konradsen et al .,
1997; Palsson and Jaenson, 1999; Forster and
Moser, 2000; Karunamoorthi et al ., 2009).
Neem extracts have insecticidal activities against
a variety of disease vectors, ranging from
mosquitoes to ticks, head lice, bed bugs,
cockroaches, mites and sandfl ies (Sharma and
Dhiman, 1993; Singh et al ., 1996; Schmahl et
al ., 2010).
The neem industry has been growing
steadily, led by India with approximately 22
million neem trees (the largest concentration of
neem in the world), although extensive neem
plantations also exist in China and Brazil. Over
the past 5 years, 20 million neem trees have
been grown in Yunnan and other southern
provinces of China. In Africa, neem trees have
been planted for reforestation, in refugee
rehabilitation centres for shade, and to control
desertifi cation in the Sahel (Saxena, 1999).
Neem can grow in a diverse range of conditions
within the tropics and has scope in reforestation,
agroforestry and land rehabilitation.
The insecticidal ef ects of neem were noted
by Heinrich Schmutterer, who witnessed desert
locusts in Sudan defoliating all fl ora apart
from neem trees (Schmutterer, 1990), and have
been documented towards more than 400
insects of agricultural and medical import-
ance (Schmutterer, 2002). Neem's insecticidal