Contents - LAC Biosafety

feeding with no significant impact to occasional large-scale outbreaks resulting
in massive tree mortality may occur in natural tropical forests. However, the
frequency and severity of pest damage is greater in plantations as summarised in
Chapter 5 and described in detail in Chapter 10. Chapter 4 also showed that the
most common insect outbreaks in natural forests occurred in high-density
stands approaching monoculture.
A detailed analysis of the plantation effect on pest incidence in tropical tree
species was made by Nair (2001a). He compared the pest incidence in natural
forests and plantations of several species for which relevant published literature
was available—Eucalyptus spp., Gmelina arborea, Hevea brasiliensis, Swietenia
macrophylla and Tectona grandis, and found that all of them suffered greater pest
damage in plantations. In a meta-analysis of 54 individual studies reported in
the literature, Jactel et al. (2005) also concluded that, overall, forest monocultures
are more prone to pest infestation than more diverse forests. Thus the
greater pest incidence in plantations is an undisputed scientific fact.
Two main hypotheses have been proposed to explain the lower pest incidence
in natural forests – the ‘enemies hypothesis’ and the ‘resource concentration
hypothesis’ (Root, 1973; Carson et al., 2004). Recently, Nair (unpublished)
proposed a third hypothesis called the ‘pest evolution hypothesis’.
8.2.1 Enemies hypothesis
According to the enemies hypothesis, the lower pest incidence in the
mixed-species stand is due to greater action of the pests’ natural enemies. This is
thought to be facilitated by the diverse plant community providing (1) alternative
prey or hosts on which the natural enemies can sustain themselves and build up
during periods when the pest is not present in the habitat, (2) a better supply of
food such as pollen, nectar and honeydew for the natural enemies that enhances
their fecundity and longevity and therefore overall effectiveness and (3) greater
variation in microhabitats and microclimate that provides a larger variety of
shelters for natural enemies. The increased natural enemy effectiveness
therefore is thought to prevent pest build-up in the natural forest.
8.2.2 Resource concentration hypothesis
8.2 Do plantations suffer greater pest damage? 135
According to the resource concentration hypothesis (Root, 1973), also
called host concentration hypothesis (Carson et al., 2004), monoculture favours
pest build-up by providing (1) a larger absolute supply of food resources,
(2) greater ease in host location due to the physical proximity of the host trees
and absence of interfering non-host volatiles and (3) reduced dispersal of the
pests from the host patch. Arresting the dispersal, i.e. curbing the tendency of
the herbivores that arrive on a clump of host plants to leave the area, appears to