Contents - LAC Biosafety

Dirzo (1982) reported that in a study in Mexican tropical rain forest up to 60%
of the seedling populations of trees were damaged by herbivorous insects
although only less than 25% of leaf tissue was lost in the affected seedlings.
In the pristine subtropical mixed conifer forest in Baja, Mexico, with a mean
tree density of 160 trees ha 1 with no species dominating, Maloney and
Rizzo (2002) reported widespread incidence of the bark beetle Scolytus ventralis
(fir engraver) on white fir (Abies concolor). Other pests encountered included the
bark beetles Dendroctonus jeffreyi, D. valens and Ips spp. and the sawfly Neodiprion
spp. on Jeffrey pine (Pinus jeffreyi); D. ponderosae and Ips spp. on sugar pine
(P. lambertiana) and D. ponderosae on lodgepole pine (P. contorta). Apart from fire
which affects all trees, fungal diseases and insects were the primary cause of
mortality of older trees in these forests. Most bark beetle infestation occurred on
dead trees, with much less incidence in living trees. For example, the borer
Scolytus ventralis was found on 87% of dead but only on 10% of live white fir,
and Dendroctonus jeffreyi was found on 71% of dead and less than 2% of live
Jeffrey pine.
It can be seen from the above that low-level pest incidence is common in
mixed tropical forests.
4.2.2 Pest outbreaks
In spite of the general belief to the contrary, there are many examples of
pest outbreaks in natural forests in the tropics. Based on observations in Barro
Colorado Island, Panama, Wolda and Foster (1978, p. 454) even stated ‘‘it seems
that outbreaks of insects in a good tropical forest are by no means rarer than
they are in a temperate forest.’’ Examples, with brief details where available
(as noted earlier, many examples are of anecdotal nature), are given below,
arranged by insect order.
Lepidoptera
Eulepidotis spp. (Noctuidae) in Panama and Brazil
4.2 Empirical findings 83
Eulepidotis superior (Noctuidae) is an insect whose larvae feed on the
young leaves of Quararibea asterolepis (Bombacaceae), a canopy tree species
common in the tropical moist forest on Barro Colorado Island, Panama.
An outbreak of E. superior on Q. asterolepis was observed in a 50-ha plot at the
above site, in late May to early June 1985 (Wong et al., 1990; Pogue and Aiello,
1999). During the outbreak, thousands of caterpillars descended from defoliated
crowns on silken threads. Among the Quararibea trees in the 50 ha plot, about
20% suffered near total defoliation, 5% suffered no defoliation and the rest were
in between. E. superior is a tender-leaf specialist; variation in leaf phenology at
the time of the outbreak explained the variations in defoliation level among