Contents - LAC Biosafety

abundant prey and recruit other members of the group in their hunting trips.
Numerical response results when the natural enemy population increases
through increased reproduction because of greater prey (food) availability.
Numerical response can also be brought about when greater numbers of the
predator or parasitoid migrate to an area where the prey density is high. Higher
population density of insects may also favour outbreak of fungal, bacterial and
viral diseases because of the ease of transmission of the pathogen due to host
proximity. Most pests have a large group of natural enemies, some of which
exert decisive influence on population growth. However, their effectiveness
depends on a number of complex biotic interrelationships. For example, annual
outbreaks of the defoliator Hyblaea puera occur in teak plantations in India in
spite of the presence of not fewer than 44 species of parasitoids, 108 predators
and 7 pathogens (see Chapter 10).
Apart from these, there are countless ways in which biotic relationships
involving multitudes of plants and animals affect the life of a given insect
species. The potential number and magnitude of interactions involving a large
number of abiotic and biotic factors affecting a herbivore is enormous and
difficult to enumerate. Imagine the web of relationships involving the 44 species
of parasitoids, 108 species of predators and 7 species of pathogens of H. puera,
with their alternative preys, and over 45 plant hosts (see e.g. Fig. 10.37 in
Chapter 10). Add to this the differential impact of physical factors on the life
process of these organisms, all of which can influence their impact on the life
of the pest under consideration. In spite of such complexity of ecological
interrelationships, we often find that many of the complex interrelationships
are unimportant in the life system of a species. A few key variables or key factors
usually have a major role in driving the population dynamics of an insect
species. They play such an important role that they can be used to predict
the population growth. Such abstraction is unavoidable and has often proved
sufficient to predict the outcome. It is like using a mathematical prediction
equation (e.g. the girth and height of a tree can be used to arrive at the
commercial volume of a tree). Thus the study of population dynamics involves
the use of some simplifying assumptions.
7.4 Principles governing population dynamics
7.4 Principles governing population dynamics 125
Under natural conditions, herbivorous insects are ubiquitous components
of terrestrial ecosystems and they usually remain in small numbers. Pest
outbreaks are exceptions. Herbivorous insects form a link in the energy cycle of
the ecosystem as one of the consumers of primary production, and in turn serve
as food for secondary consumers, ultimately contributing to the cycling of