A review of dipterocarps - Center for International Forestry Research

Pests and Diseases of Dipterocarpaceae 116
Pests
Seeds
Dipterocarp seeds are produced irregularly and sparsely
in some species, and fruit production varies in quantity
and quality from year to year. Mass fruiting appears to
favour seed predators, but it can also be a strategy to
escape complete seed destruction (Janzen 1974). Seed
predation can be very high, and the crop can be
completely wiped out. Curran and Leighton (1991)
reported that the 1986 crop was entirely destroyed
(100,000 seeds/ha) in the lowland forest of West
Kalimantan. The major losses are caused by insect pests.
Natawiria et al. (1986) observed weevils (Curculionidae)
damaged 40-90% of the seeds of Shorea pauciflora, S.
ovalis, S. Iaevis, S. smithiana and Dipterocarpus
cornutus. Daljeet-Singh (1974) reported that weevils
were responsible for more than 80% of the total seed
damage in all case studies except Shorea macrophylla,
in which the most important pests were the Colytidae. In
1991, 70% of Dryobalanops aromatica seeds were
damaged by weevils in Malaysia (Elouard, unpublished).
While insects are the major seed pests, there is
destruction by birds and mammals. Wild pigs (Sus scrofa),
squirrels (Callosciurus prevostii and C. notatus) and
monkeys (Presbytis rubicunda) caused damage to the
crops of some species (Kobayashi 1974, Natawiria et
al. 1986, Curran and Leighton 1991). Kobayashi (1974)
observed that 80% of the mature seed crop of Hopea
nervosa was damaged by squirrels. Parrots (Psittacula
sp.) have been observed feeding on dipterocarp seeds
(Natawiria et al. 1986). However, monkeys and squirrels
prefer to eat other available fruit and seeds (Curran and
Leighton 1991). Dipterocarp resin contains a high
percentage of alkaloides and can be repellant to
mammals. Neobalanocarpus heimii seeds are hardly
eaten by mammals, but losses are due to the destruction
of a part of the seed tasted by the rodents and then
rejected (Elouard et al. 1996).
Over 80 species of seed pests have been described
on various dipterocarp seeds, with both pre- and postdispersal
insect pests. The former attack the fruits on
the tree before dispersal, whereas the latter attack fruits
on the ground. The pre-dispersal fruit pests are weevils
(Curculionidae) and Lepidoptera, and the post-dispersal
ones are Lepidoptera (Toy 1988). It is rarely possible to
distinguish between pre- and post-pests of Lepidoptera.
The mode of attack of the weevils and Lepidoptera on
dipterocarps is described by Daljeet-Singh (1974). The
weevils come at the early development of the fruits,
pierce the pericarp and deposit a single egg. The larvae
feed on the cotyledons throughout the period of growth.
The pupal chamber is made of larval frass. Usually, the
fruit drops to the ground before pupation and the adult
weevils remain within the fruit for a few days before
emerging. They are sexually immature at emergence. The
Lepidopteran predators lay their eggs on the dipterocarp
fruits. On hatching, the larvae bore into the fruit, feed on
the cotyledons and pupate. Prior to pupation, the larva
attacks the pericarp leaving only a thin covering that the
newly emerging adult can break.
Toy (1988) observed in Malaysia that species of
Nanophyes (Curculionidae) were generic specialists and
some species appeared to be even sub-generic
specialists. The existence of insect pests which have a
‘familial specialisation’ raises questions on the function
of mass-flowering as a pest satiation strategy (Janzen
1974). The survival of these insects between fruiting
events are ascribed to three hypotheses: i) they have either
alternative hosts in non-dipterocarp families; ii)
dormancy; or iii) maintain more or less continuous
generation of pests developing on sporadically flowering
trees (Toy 1988). In a study of Nanophyes shoreae
survival in Shorea macroptera, Toy observed that a
maximum 1.8% of insects survived during sporadic
events, thus dispersal of the insect is not probable. He
suggested generalist feeding of adults is the key to their
persistence between fruiting events.
Seedlings and Saplings
Few records exist on pests of seedlings and saplings in
nurseries, though some reports are available for natural
forests. Insects are the main source of damage as leaf
feeders, borers, suckers and in gall formation. The other
pests recorded are wild boars, rodents and nematodes.
There are few reports of leaf damage to seedlings
and saplings (Becker 1983, Tho and Norhara 1983) and
the defence properties of essential oils in mature leaves
were discussed by Becker (1981). Galls causing leaf
damage were reported on dipterocarp species in
Singapore, Malaysia and India (Anthony 1972, 1977,
Mathur and Balwant Singh 1959), mortality and setback