Contents - LAC Biosafety

76 Ecology of insects in the forest environment
are of primary importance in precipitating herbivore outbreaks and this kind
of regulation of a plant community is often referred to as bottom-up control
(Chase et al., 2000). Alternatively, the ‘Host Concentration Theory’ proposes
that specialist insect herbivores will exert strong regulatory effects on plant
communities whenever their hosts form large, persistent dense stands
(Carson and Root, 2000; Long et al., 2003; Carson et al., 2004). Host concentration
is believed to promote pest build-up and outbreak by providing a larger absolute
supply of food, greater ease in host location due to the physical proximity of the
host trees as well as the absence of interfering non-host chemicals and reduced
dispersal of pests out of the dense host patch. This kind of regulation of
plant community from above (i.e. a higher trophic level) is called top-down
control. The host concentration theory is discussed further in Chapter 8, in
connection with pest incidence in monocultures versus mixed stands.
Insects may sometimes influence tree evolution. Insect herbivory can affect
many aspects of tree performance – growth, form, seed production, seed
germination, competitive ability and survival. These effects often exert a
negative or positive influence on the success of individual plants or groups
of plants which exhibit genetically controlled deviations from the rest of the
conspecific population, and can drive evolutionary change in the plants.
To illustrate this, consider a simplified example of interaction between a
plant and an insect. Assume that a teak tree develops, through mutation,
a heritable capacity to produce on bark injury a chemical (inducible defence)
that is lethal to newly hatched larvae of the beehole borer, Xyleutes ceramicus
(see pest profile under teak, in Chapter 10). When the larva attempts to bore into
the bark of the tree, the chemical is released and the larva is killed. This chemical
will protect such a tree from the pest and increase its survivorship in comparison
with other teak trees. Therefore, in course of time, the proportion of individuals
which carry this novel borer defence mechanism will increase by the process of
natural selection. This process can go on and lead to evolutionary change in the
host. Sometimes, an evolutionary ‘arms race’ will result, with the insect
developing new strains that can detoxify the harmful chemical. It is logical to
assume that herbivorous insects may influence the population dynamics and
evolution of plants in the manner described above, but it is difficult to come up
with conclusive evidence because of the complexity of interactions involving a
multitude of physical and biotic factors and the many other unknown functions
a mutation may serve. Perhaps such effects may operate more effectively
on plants which have a shorter life cycle, while on trees they may be limited to
instances where particular insect species have the propensity to cause premature
death of trees. In general, it may be the insects that adapt and evolve according
to tree characteristics because of the very short generation time of insects