Plants with Pest Control Properties Against Urban Pests - Urban Insect Pests: Sustainable Management Strategies - page 217

decades. Over-reliance exclusively on
chemicals for pest control is now being
reviewed and is under constant scrutiny.
Consequently new pesticides based on
natural products are being discovered and
developed to replace the compounds lost as
a result of new registration requirements
(Copping and Duke, 2007). Apart from being
perceived as environmentally benign,
natural-product-based pesticides also pro-
vide the benefi t of shorter half-lives, result-
ing from the fact that they do not possess
unnatural ring structures and contain
relatively few halogen substituents (Dayan
et al ., 2009). This environmental signature
of plant products has generated a wide-
spread interest across the world.

larval stages of fl oodwater mosquitoes.
In addition, methoprene has been in use
for the indoor control of fl eas. Methoprene
has also received an approval from
the World Health Organization (WHO)
for use in drinking water to control
mosquitoes. Another compound extracted
from Ageratum seeds, 6,7-dimethoxy-2,2-
dimethyl chromene, also known as
precocene II, was reported to have anti-
juvenile-hormone activity in Periplaneta
(Burt et al. , 1979).
Historically, insects have been the most
signifi cant herbivores in the evolution of
land plants. It is thought by researchers that
most plant defences are directed against
insects rather than against vertebrate
herbivores, such as birds and mammals.
Chemicals known as secondary metabolites
are synthesized by plants to protect against
herbivory and these secondary metabolites
can generally be classifi ed into three general
categories, namely alkaloids, terpenoids
and phenolics. The important role of
secondary plant substances in plant defence
was described very well by Dethier (1954)
and Fraenkel (1959). This area of plant
resistance has become an important area of
research in agriculture and plant breeding
because it can serve as a safe and low-cost
alternative to the use of pesticides. It is also
recognized, however, that plant defensive
chemistry has probably evolved to dis-
courage herbivory, rather than to kill the
herbivore outright (Isman and Akhtar,
2007). This is clearly evident from the fact
that very few plant-derived compounds are
as lethal as commercially existing synthetic
ones.
A survey of the available literature shows
a vast number of plants possess insecticidal
properties. A multitude of insecticidal
chemical constituents with different struc-
tural types have been identifi ed and are in
the process of being identifi ed. Among them
amides, physostigmine, toosendanin, limo-
noids, coumarins, benzofurans, acetogenins,
quinones, pyrroles, chromenes, terthienyls
and polyacetylenes are recognized as potent
compounds with insecticidal properties.
Many more with a unique mode of action
are in the process of being discovered.

Alkaloids, phenols, terpenes and other
compounds

The science of modern natural product use
against insect pests possibly started with
the discovery by Sláma and Williams (1965)
of the 'paper factor', an extract from
American Balsam fi r, the main pulp tree
used in the US paper industry. The extract
acted as an insect growth regulator. Prior to
this, humans commonly looked for repel-
lents and antifeedant properties in plants.
Either of the two characteristics could be
used for repellence, not for killing the insect
pest. The active component from the Balsam
fi r was subsequently characterized as a
methylester of todomatuic acid, which is
now known as Juvabione. Unfortunately
this isolated Juvabione was found to be a
very specifi c juvenile hormone (JH) mimic
for insects belonging to the family
Pyrrhocoridae but not for any other insects.
The discovery, however, triggered a search
for new insect growth regulators that could
be used as an alternative to synthetic
chemicals.
This new-found direction led to the
synthesis of a few thousand compounds
showing growth regulatory activity. Two
such compounds, methoprene and hydro-
prene, are in commercial use. Methoprene
was introduced as a mosquito growth
regulator for use against second to fourth