Contents - LAC Biosafety

10.16 Swietenia species (Meliaceae) 307
without proof, that lateral shade offers a mechanical obstacle to moths in search
of suitable plants for oviposition, that the slower growth of mahogany under
shade makes it less attractive to the gravid moths etc. Newton et al. (1993) reviewed
several instances of mixed planting of mahogany in Puerto Rico, Surinam, Brazil,
Belize and Guatemala, either as enrichment planting in secondary natural forests
or when planted in the open in admixture with other species, some with suitable
controls. They concluded ‘growing mahoganies in mixtures with other species
seems to have afforded some degree of protection’ (p. 308). They also pointed to
several other mixed planting trials in Honduras and Costa Rica where shade or
cover did not reduce Hypsipyla attack. Matsumoto et al. (1997) and Matsumoto and
Kotulai (2000) reported that some plantations of S. macrophylla in Malaysia and
Indonesia surrounded by Acacia mangium were not attacked by H. robusta.
In a study of mahogany seedlings established in clearings within natural forests
in Mexico, Snook and Negreros-Castillo (2004) found that in plots cleared of
competing vegetation from around the seedlings, 44% of the seedlings were
attacked by H. grandella, compared with 12% in uncleared plots. From the available
literature, we can draw the conclusion that planting mahogany in mixture with
other species does not guarantee successful Hypsipyla control, although it often
does, but underplanting of mahogany in managed natural forests reduces the
incidence of attack, for reasons not fully understood.
Timely pruning of affected shoots to destroy the larva is another silvicultural
method advocated and found effective (Cornelius, 2001).
Genetic resistance It has been reported that H. grandella moths are
attracted to Toona ciliata and oviposit on it, but the larvae die when they begin
to feed, suggesting the presence of some toxic substance (CABI, 2005). This
substance is not toxic to H. robusta which readily attacks T. ciliata in Asia and
Africa where both are native. These observations suggest that there is scope for
breeding for resistance to H. grandella (Newton et al., 1993).
Chemical control Hypsipyla larvae concealed within shoots are inaccessible
to insecticidal sprays and the infestations usually occur in the rainy period when
the sprays get easily washed off the plant. Due to these reasons, conventional
insecticidal application has not proved effective. To be effective, spraying needs
to be carried out repeatedly to target the exposed young larvae and this is
neither economically worthwhile nor ecologically acceptable. However, systemic
insecticides like carbofuran applied to soil at the time of planting were found
effective in field trials in Costa Rica (Newton et al., 1993). Mohanadas (2000) also
reported the effectiveness of phosphamidon and dimethoate against H. robusta
in field trials in India. However, the duration of effectiveness of systemic