[Edited_by_A._Ciancio,_C.N.R.,_Bari,_Italy_and_K.(Bookos.org)

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FARHA-REHMAN ET AL.
whereas on the other hand invite its predators. This mechanism becomes operational
only on herbivore attack. The reaction of herbivores oral secretion and plant elicitors
activates certain host genes. Thus, plants rely on a system of multiple chemical
switches that control the partitioning of biomass to herbivores and predators, at least
up to three trophic levels. Moreover, plants have either no or limited “memory” of
previous insect attack, with some exceptions. Thus, an equilibrium in the transfer of
biomass from hosts to herbivore-predators is maintained naturally, throughout the
plants life cycle.
Among pests response to plants arsenals, it is worth to recall how herbivorous
insects evolved adaptive mechanism to search suitable healthy plants for oviposition.
As shown, nocturnal pests, i.e. Heliothis veriscence avoid injured plants during day
time for oviposition, to save their offsprings from day time predators (De Moraes
et al., 2001).
Herbivory caused by grasshoppers accelerated nutrients cycling and plant
production and abundance (Belovsky & Slade, 2000). It is also reported that larvae
of Manduca sexta modified, before feeding on damage leaves, the host normal
defensive metabolites by reducing systemic JA in roots, and subsequently the
nicotine content in the whole plant (McCloud & Baldwin, 1997).
Herbivores and predators also rely on receptor molecules, starting from the
activation of plants defense genes on an insect attack. Plant defences to herbivory
can be simulated by JA, since in some plants the defense mechanism was brought
under operation through JA and artificial injury together. However, more
experiments are required to induce chemical defense and use this mechanism of
chemical signalling for pests control, in a way that may be much more effective and
environment friendly than using pesticides. Biological control may prove indeed to
be more economic not only for crop losses, but also for the ecosystem maintenance.
It must be kept in mind that elimination of one herbivore species may prove to be
more detrimental for the ecosystem if it happens to be a keystone species. Any
biological control strategy should seek at an equilibrium among plant biomass,
herbivores as well as predator populations, to be maintained
Plants adaptive mechanism evolved to ward off herbivores is both structural and
functional (constitutive and inducible). Host plants synthesize volatile chemicals as
part of their defense strategies against excessive herbivory. Artificial injury induced
by leaf clipping in some species did not produce the blend of volatiles which are
repulsive for herbivore or provide clues to the predators, since they possess specific
genes activating the synthesis of volatiles only on an insect attack and up regulated
by the chemicals contained in the herbivores mouth.
The volatiles synthesized in plant tissues after insects attack were useful to ward
off insect directly. Also, since the volatiles provide clues to the predator about
the preys (herbivores) presence and also signal the neighbouring undamaged plants about
the attack of the herbivores, the course of co-evolution acquired a higher level of
complexity when certain insect species cleverly adopted to these volatiles. Some
herbivores, furthermore, were capable to modify the blends of volatiles and thus
used modified plant arsenals (volatiles) to defend themselves from the predator.
Due to co-evolution, most plants had limited memory for insect attack but not in
some perennial trees (mainly Australian eucalypts), in which the plant volatiles were