Contents - LAC Biosafety

of the air, pollutants, electromagnetic radiation, chemical composition of the
soil etc. affect the life of insects either directly or indirectly in various ways.
As mentioned above, physical factors can influence insect populations either
directly or through their effects on host plants or natural enemies. When the
effect is indirect, for example when it adversely affects host foliage quality, it can
lead to a delayed response from the insect population.
7.3.2 Biotic factors and logistic growth
The biotic factors influencing a given species include other individuals
of the same species as well as other species of animals and plants.
Interactions among members of the same species, i.e. intraspecific interactions,
may have beneficial as well as inhibitory effects on population growth,
depending on the population density. Moderately high density favours mate
finding and offsets the impact of natural enemies. The beneficial effect of
cooperation reaches its peak in social insects where there is division of labour
among castes. High population density can also be beneficial when it breaks
down the host tree defences, as seen in the cases of the sal borer Hoplocerambyx
spinicornis attacking Shorea robusta and bark beetles attacking pines, or helps to
overwhelm the parsitoids and predators as during teak defoliator outbreaks (see
Chapter 10). On the other hand, high population density can also lead to
competition among individuals for limited resources of food and shelter.
This competition inhibits population increase. It retards the birth rate and/or
enhances the death and emigration rates through various mechanisms. The net
per capita growth rate, g, decreases progressively as population size increases,
until it reaches a constant value, resulting in a logistic population growth curve
(Fig. 7.2). The population growth curve levels off when the carrying capacity (K )of
the environment is reached. K represents the maximum population size that
can be supported by a given environment. For example, a patch of grassland has
a maximum number of grazing deer that can be supported, depending on the
regenerating capacity of the grass, although there are many other factors that
prevent populations from reaching the carrying capacity of the environment.
Population growth under these conditions is described by the equation
N= t ¼ g½1 N=KŠ N;
7.3 Factors affecting population change 123
where N is the initial density of the population, t is time, g is the net per capita
finite growth rate, and K is the carrying capacity of the environment.
Among interspecific interactions, the most important are the insect–host tree
interaction and the insect–natural enemy interaction.
Host quality in terms of nutrients, secondary plant chemicals, physical
and chemical deterrents etc. have important implications for pest population