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FARHA-REHMAN ET AL.
and composition depend on the type and intensity of feeding. Different types of
herbivory affect several plant tissues, besides affecting primary production,
translocation and accumulation of photosynthates to varying degrees. Herbivory
affects a variety of ecosystem properties, primarily through differential changes in
survival, productivity and growth of plant species.
In evolutionary terms, the rise of insects represented a major selective force on
plants evolution, and led to the selection of plants by their ability to generate
defensive adaptations. Insect herbivores are mostly mandibulated that either bite or
chew vegetation. The rise of vascular plants led to the co-evolution of sap-sucking
feeders, and several other forms of herbivores such as leaf mining, gall forming and
nectar feeding insects. The study of plant defense against insect herbivory is not
only important from an evolutionary point of view, but is also useful in
understanding the extent of its impact on agriculture, human and livestock food
sources, as well as on the utility and survival of commodity plants or species of
medicinal use. As an example of insects damage due to high rates of herbivory,
grasshoppers feed on a wide range of plants and organic material, and voraciously
consume green forage approximately one-half of their body weight, every day.
In the present review emphasis is given to herbivory and plant defense
mechanisms, as well as to the chemical signalling among plants, herbivorous and
predatory insects. In fact, it is worth to determine these mechanisms before either
using pesticides or employing other biotechnological means, i.e. genetically
manipulated (GM) plants. It is known that the use of pesticides may indeed alter
some ecosystem processes or introduce a structural change in density dependent or
population regulation mechanisms, apart from the effects related to health hazard
implications and costs. Finally, GM plants have several environmental and
ecological complications, including the developement of insect resistance in a way
similar to resistance to some insecticides. Insect management is in effect a complex
and difficult task, as suggested by the insects and plants co-evolution and survival
for the past 97 million years (Labandeira, Dilcher, Davis, & Wagner, 1994).
2. PLANT DEFENSE MECHANISMS
Plants curb insect herbivory by synthesizing and releasing complex blends of
volatiles. Some of these compounds provide important host-location cues to predator
insects or parasites, that are natural enemies of insect herbivores. Synthesis and
release of these chemical signals by attacked plants are active physiological
processes, triggered by chemical elicitors or substances contained in the oral
secretion of attacking herbivores. Certain chemicals contained in the saliva of
grazing insect (herbivores) activate the synthesis and release of plant volatiles. The
process of attracting predatory insects involves the interaction of specific blends of
plant volatiles, with highly sensitive receptor molecules of the predators (De
Moraes, Mescher, & Tumlinson, 2001).
Plant volatiles represent a language through which attacked plants send signals
to healthy plants in their vicinity and also invite predators or parasites of their
herbivores. This adaptation is genetically fixed and controlled by a set of genes
present in plants. In particular, a set of five defense genes are induced when a plant