Contents - LAC Biosafety

Table 8.2. Comparison between the numbers of insect species associated with native and
exotic tree plantations a
Tree species
Score b
for number of insect
species in
Native
plantations
Exotic
plantations
Acacia mangium 1 8 Greater
Eucalyptus spp. 11 c
40 Greater
Falcataria moluccana - 5 -
Gmelina arborea 10 2 Lesser
Hevea brasiliensis 6 3 Lesser
Leucaena leucocephala 1 4 Greater
Pinus caribaea 3 3 Equal
Swietenia macrophylla 1 2 Greater
Tectona grandis 23 2 Lesser
a Data from Nair (2001a)
8.3 Pest problems of indigenous vs. exotic species 143
Whether exotic
plantation has greater
or lesser no. of
associated
insect species
b
Scores are used instead of actual numbers as the number of associated insects is only
approximate. One score is assigned to one to ten species. Thus, for example, score ten indicates
91–100 species and score 40 indicates 390–400 species
c
Excluding those in the temperate region
2. Newly adapted insects
As mentioned earlier (Section 8.2.3) some indigenous insects adapt and
become serious pests of exotic tree species over time. Examples are the bagworm
Pteroma plagiophleps on Falcataria moluccana in India, wingless grasshoppers on
pines in Africa, the myrid bug Helopeltis spp. on Acacia mangium in Southeast Asia
and on Eucalyptus in India, the noctuid Spirama retorta on Acacia mangium in
Malaysia etc. They become adapted in a short period because of their shorter
generation time than trees, and trees in plantations, unlike those in natural
stands, have no chance of developing resistance mechanisms through natural
selection.
3. Specialized insects preadapted to closely related plant species
The examples of Hypsipyla robusta on mahogany and the shoot moths Dioryctria
spp. and Petrova spp. on pines in Southeast Asia (see Chapter 10) show that
an introduced tree species may encounter insects already adapted to closely
related tree species in the location of introduction. This leads to quick attack of
the exotic by these specialized oligophagous insects because the same or
a closely related host selection mechanism developed over evolutionary time