Insect-pests - Biology East Borneo

42 General Conclusions
and stem rot in Acacia mangium. Other potential
threats are less obvious. Eucalyptus spp.and Acacia
mangium have been grown in Indonesia over fairly
large area for a longer period than other species in this
group. Many pest problems develop over a long period,
facilitated by favourable conditions provided by
extensive monocultures. Therefore, the risk of pests
and diseases for most species in this group is
unpredictable. Experience with large monocultures of
the same species in other countries can provide some
pointers to the potential problems, although this is not
fully dependable. For example, Pinus merkusii is
plagued by shoot borer in Philippines, Thailand,
Vietnam, India and Northern Sumatra, but not in Java.
We have the advantage of such experience in the case
of Eucalyptus spp., Gmelina arborea and to some
extent, Acacia mangium.
Eucalypts have been generally pest free, except for
subterranean termites attacking the tap root during the
establishment stage. Diseases create problems in
nursery, but are manageable. Eucalyptus spp. are
susceptible to foliar diseases caused by fungi in humid
environments, but selection of resistant species and
provenances has circumvented this problem. In
Indonesia, some species e.g. E. urophylla, have been
found to be susceptible to root rot but others e.g.
E. pellita, are less so. These problems have led to the
present trend in Indonesia is to replace Eucalyptus
spp. with Acacia.spp.
Gmelina arborea, except for minor problems with a
stem borer, is currently pest free in Indonesia, as in
many other countries where it has been planted as an
exotic, but the situation needs monitoring, as it suffers
from serious pests in its native range. Some companies
are now enlarging the area under Gmelina arborea, in
place of Acacia mangium, although it requires more
fertile sites than the acacia.
Acacia mangium suffered a heart-rot problem which
threatened to proliferate in Malaysia, but it is being
kept in check by enlarging the genetic base of planting
stock and by planting the heart rot resistant hybrid,
A. mangium x A. auriculiformis. It has been
suggested that the heart and root rot problems are
the result of mismatching of the species with the
sites, with the absence of a seasonal dry spell
facilitating the development of the diseases (Arentz
1996; Lee and Arentz 1997). The situation therefore
needs monitoring. Although there are no serious
insect pest problems at present, the situation also
needs attention, in view of the potential threat of
the mosquito bug, Helopeltis spp. becoming adapted
as more of the Indonesian landscape is planted with
A. mangium. There is also the treat of unpredictable
caterpillar outbreaks, as indicated by some instances
in Malaysia.
In the case of other new species, most of which are
indigenous, there are no serious pest or disease
problems, at present. For some of them, limited
experience in other countries or the chemical profile
of the species (e.g. Azadirachta spp.) suggests that
there is little risk (Table 5.2) but for others the risk is
unpredictable.
Future most important pests and diseases
If one insect pest is to be named as the most dangerous
to Indonesian forest plantations in future, the choice
will undoubtedly fall on the sengon borer, Xystrocera
festiva. Its population is likely to increase further as
more area is brought under P. falcataria all across
Indonesia due to its promotion by industrial and
agroforestry plantation initiatives. Xystrocera festiva has
a number of alternative hosts, in the family
Leguminosae, including Albizia spp. and Acacia spp.
Although A. mangium is not a favoured host of
X. festiva, its expansion may also help to increase
X. festiva population. This borer seems to be welladapted
to Indonesia as it is replaced in neighbouring
countries by X. globosa, a species also present in low
numbers in Indonesia.
Another insect likely to build up in future is Helopeltis.
Many closely related species of Helopeltis are
important pests of horticultural plantations in the
tropics and populations have been increasing in young
A. mangium plantations, particularly in Sumatra (see
Section 4.1). It has a history of outbreaks in cashew,
tea and neem in India and in tea and Eucalyptus spp.
in Sumatra. Care must be exercised to prevent the
build up of Helopeltis species.
Good quality timber will remain in demand despite of
the present emphasis on pulpwood species.
Improvements in machinery and utilisation methods
will enable smaller dimension timbers to be used