Contents - LAC Biosafety

306 Insect pests in plantations: case studies
H. ferrealis (Hampson) is present in tropical America, but it exclusively attacks the
fruit of Carapa guianensis (Newton et al., 1993). Wide variations have been
reported in the biology and behaviour of H. robusta in different geographic
locations and, as mentioned above, more than one closely related species may
be involved.
Natural enemies More than 50 species of parasitoids of H. robusta have
been recorded in India alone, although the rates of parasitism were low (mostly
41%). They include 17 braconids, 13 ichneumonids, 12 chalcidoids, 2 each
of trichogrammatids and tachinids, and 1 each of elasmid, eulophid and
eurytomid (Newton et al., 1993). Fewer parasitoids are on record for H. grandella.
Although the causative agent was not identified, disease levels of 4–16%
of sampled larvae were recorded from H. robusta in surveys in India (Newton et al.,
1993). Misra (1993) recorded the fungal pathogen Beauveria bassiana on H. robusta
in India. In H. grandella a fungus, Cordyceps sp., was recorded (Newton et al., 1993).
Control It has been generally observed that mahogany saplings growing
under partial shade in mixed natural forests are less prone to Hypsipyla attack
than those growing in the open, although the reasons are not clear.
Experimental studies have produced variable results. Mahogany, however, is
not immune to attack of Hypsipyla in the natural forest. For example, Yamazaki
et al. (1990) observed that in the Peruvian Amazon, the population of H. grandella
increased rapidly in the rainy season when food availability increased with the
growth of new sprouts. In southeast Mexico, the insect attacked the fastest
growing seedlings in the logged-over natural forest (Dickinson and Whigham,
1999). In spite of considerable research, no practical control measure has
emerged and the shoot borer continues to be the main factor limiting the
cultivation of mahogany. In natural forests, mahogany occurs in very low
density. In Mexican forests, its average density is 1–2 mature trees per ha, and
the range may vary from 1 tree per ha in Brazil to 20–60 trees per ha in Bolivia
(Mayhew and Newton, 1998). It is possible that the comparatively low incidence
of Hypsipyla attack in natural forests is attributable to both shade effect and the
action of many natural enemies. Efforts made to control Hypsipyla in plantations
are briefly discussed below.
Silvicultural control Several authors have recommended the planting of
mahogany under the shade of an overhead canopy of evergreens or with lateral
shade given by planting in mixture with a faster growing species (Beeson, 1941).
Species suggested for mixing range from Senna siamea (syn. Cassia siamea), Cassia
timoriensis and Leucaena leucocephala in Indonesia to maize in Honduras. The
benefits of such measures have seldom been critically evaluated. It is argued,