Contents - LAC Biosafety

2.3 The concept of pests 51
Insects, as a group, are capable of feeding on almost all parts of a tree – the
leaves, flowers, fruit, shoot, bark, sapwood, heartwood and the roots. Usually
each tree species has a characteristic spectrum of associated insects comprising
plant feeders, fungus eaters, detritivores, predators, parasitoids and even simply
casual visitors. Phytophagous insects do not adversely affect the tree when the
insect numbers are small, which is usually the case. Apparently a tree can
dispense with some portion of its biomass without adverse effects on its growth.
To show the diversity of insects associated with a living tree, insects found on
teak in India and the adjacent countries are listed in Table 2.4. Altogether there
are 174 species. The list would be longer if species found on teak in all countries
were included. The vast majority are leaf feeders, accounting for 137 species,
followed by 16 sap feeders, 14 shoot/stem feeders, 5 root feeders and 3 seed
feeders. Only a smaller number of species may be present in a given locality at a
given time. Most species cause only slight or occasional damage and their impact
on the tree is negligible. However, a few species are serious pests on teak. These
include the leaf-feeding caterpillars Hyblaea puera and Eutectona machaeralis
(or Paliga machaeralis in some countries) and the wood-boring caterpillar Xyleutes
ceramicus (or Alcterogystia cadambae in India). An additional few like the hepialid
sapling stem borers and the scarabaeid seedling root feeders are pests of lesser
importance.
As with teak, a large but variable number of insect species is usually
associated with each tree species. Sometimes, some of them increase in numbers
enormously, creating a pest situation. Although some insects can cause
economic damage even when present in small numbers (e.g. a worm in an
apple or a borer in wood), generally it is a large increase in the number
of individuals of a species that creates a pest situation. This may happen due
to one or more of several causes which are discussed in Chapter 7. Usually,
only very few of the insect species associated with a tree species will develop
pest status as in the case of teak.
A pest is defined as an organism which causes economic damage or other
negative impact on human well-being. It therefore reflects a human viewpoint.
For example, termites are pests when they feed on the root of eucalypt saplings
in plantations and kill them, or when they destroy valuable papers or woodwork
in a building, but they are beneficial when they feed on wooden refuse in our
backyard or on fallen logs in the natural forest. In the strict sense, only those
insects which cause economic loss should be called pests, but in practice all
insects that feed on a plant are called pests as the economic impact of many
insects has not been determined. Again, an insect species may be a pest at one
time but not at another. Thus it is improper to call an insect a pest; an insect is a
pest only in some circumstances. Therefore, we can only talk of a pest situation;