Contents - LAC Biosafety

80 Insect pests in natural forests
4.2 Empirical findings
Before we consider pest incidence in natural forests, we may recall the
discussion in Chapter 2 on the concept of pests. We defined pests as organisms
that cause economic damage or adversely affect human welfare. In the natural
forest, it is often difficult to judge whether an insect causes economic damage or
not. This is due to several reasons. First, pest incidence in natural tropical forests
has not received enough research attention. Second, it is not easy to carry out
economic analysis of a pest situation in natural forests because of a large
number of variables and uncertainties, and therefore this has seldom been done.
Third, all gradations of insect incidence may occur in natural forests, from
mere presence of phytophagous insects in small numbers, to occasional local
eruptions, to widespread outbreaks. Therefore there is uncertainty as to what
conditions might qualify for calling an insect a pest in the natural forest. We will
discuss this question further after examining the empirical findings.
Systematic investigations on pest incidence in natural forests are rare and
most available information is of an anecdotal nature, i.e. based on unplanned,
incidental observations. It is convenient to discuss these empirical findings
under two headings, general pest incidence and pest outbreaks, although this is
an arbitrary separation of the continuum ranging from minor insect feeding
to large-scale outbreaks.
4.2.1 General pest incidence
In a specific pest incidence study in natural forests in Kerala, India, Nair
et al. (1986a) observed 20 tree species in moist deciduous forests and 18 tree
species in evergreen forests, at monthly intervals over a two-year period (Fig. 4.1).
All the 38 tree species suffered some insect damage. The most common damage
was leaf feeding, noticed on all tree species at some time. Sap-sucking,
gall-forming and wood-boring insects were also recorded on some species.
The annual defoliation percentage ranged from 0.1–6.7 for the different tree
species. The mean monthly defoliation value did not exceed 21% for moist
deciduous species and 17% for evergreen species, although individual trees of
some species suffered more than 50% defoliation at times. For many species,
the mean monthly defoliation never exceeded 5% (Fig. 4.1b). In general,
evergreen tree species suffered less damage than moist deciduous species. One
species in which greater than 50% defoliation was noted in some individual
trees was Tectona grandis (teak) in the moist deciduous forest. This defoliation was
caused by the caterpillar Hyblaea puera, which is a well-known outbreak species
in plantations of teak (see Chapter 10). In the above study, not all the species that
caused damage were collected and identified due to difficulty in gathering