Contents - LAC Biosafety

92 Insect pests in natural forests
but not on all. Also, only some species of phytophagous insects, not all, may
become pests. Outbreaks are apparently less frequent and less severe in natural
forests. In addition, in the absence of routine damage survey in natural forests, it
is very rare that an infestation receives our attention, even when it occurs,
because in mixed forests with a large number of tree species pest damage is not
conspicuous. For example, compare the visibility of shoot borer (Hypsipyla
grandella) attack in mahogany trees which are distributed at a density of about
one mature tree per hectare in the Brazilian natural forest with its visibility in
a young monoculture plantation. On the other hand, infestations are more
visible in the temperate forests dominated by single tree species, as well as in
some types of subtropical forests of similar nature, like eucalypts in Australia
and sal (Shorea robusta) in India.
It is obvious that all forest pests had their origin in natural forests and
are still present there. But due to a variety of natural control factors, both
biological and physical, the populations of most insects remain small in natural
forests. Thus the natural forest, far from being free of pests, is a reservoir of
pests. However pest outbreaks are rare and their impact is therefore minimal.
Our economics-based definition of pests is not adequate for natural forest
situations. ‘Pest’ is primarily an agriculture and plantation-related concept. In
the continuum of insect damage scenarios ranging from minor feeding to largescale
outbreaks in the natural forest, it is difficult to determine what constitutes
a pest situation, particularly when it is granted that minor insect damage may
even have a stimulatory effect on plant productivity (Mattson and Addy, 1975).
Population outbreak is the result of uncontrolled increase in the population. The
factors which control the dynamics of insect populations and the circumstances
under which outbreaks develop are discussed in Chapter 7, and the possible
reasons for greater pest incidence in plantations are discussed in Chapter 8.