Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

88 Biological Control of Insect Pests: Southeast Asian Prospects
Damage
The status of C. sordidus as one of the most important pests of bananas is
often reported (Swaine 1971; Purseglove 1972) and, indeed, many adults
and larvae are often present. It is reported to be now the most important pest
of bananas in Africa (Nahif et al. 1994; Ortiz et al. 1995). However, in order
to place these reports in context, it is necessary to outline the stages of
growth of the plant. Bananas are propagated vegetatively by planting
rhizomes (corms), which give rise to shoots after a few weeks. As the plant
grows, a pseudostem is formed from the sheaths of the leaves which
continue to develop internally until a flowering shoot emerges from the top
of the pseudostem at a height of 2 to 4 m depending upon the variety. When
each bunch of bananas is cut, the pseudostem bearing that bunch is also cut
down. At the same time a healthy sucker growing from the same base is
selected to succeed and other suckers removed. C. sordidus larvae tunnel in
the rhizome and the base of the pseudostem, but do not attack the roots. This
tunneling may kill young plants and greatly increases the susceptibility of
mature plants to wind damage. Adults cause little damage and feed mainly
on rotting banana tissue. Injury by larvae to the rhizome can interfere with
root initiation and sap flow within the plant and grossly infested plants may
bear small bunches of undersized fruit (Wright 1976). Much of the damage
attributed to C. sordidus is probably caused by rhizome rot or nematodes
(Ostmark 1974). In East Africa the combined attack of nematodes and of
banana weevil borer is considered to be the main reason for the serious
decline in productivity of bananas (De Langhe 1988), but the precise role of
the weevil is still to be established. Suckers infested with nematodes were
found to be more than four times more likely to be attacked by C. sordidus
than suckers without nematodes (Speijer et al. 1993). Although the banana
weevil borer has occasionally been responsible for severe losses of newlyplanted
rhizomes, extensive experiments in Central America agree with
some reports from Australia (Smith 1993; Wallace 1937) that weevil
damage is not as important as frequently claimed since the larvae have a
strong preference for rhizomes of harvested plants over healthy rhizomes
(Ostmark 1974). There are reports that some banana cultivars are
comparatively resistant to attack by C. sordidus (Mesquita et al. 1984;
Mesquita and Caldas 1987), but the mechanism of such resistance
(repellency, toxicity, greater tolerance to damage, etc.) is not known.
In Australia, Braithwaite (1963) concluded that the importance of
C. sordidus infestation is aggravated by poor culture, but that benefit could
be derived from almost complete control with insecticides (Braithwaite
1958). In the same region Loebel (1975) concluded that heavy weevil