Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

Host plants
4.5
Cosmopolites sordidus
infestation is a symptom, rather than a cause, of a declining plantation,
because 2 years of effective use of chemicals failed to improve growth or
yield in his experimental plots. In Costa Rica several insecticides were
effective in controlling C. sordidus populations, but banana yields were not
increased (Nanne and Klink 1975). Nevertheless C. sordidus is always likely
to be of importance in areas that experience strong winds. The abundance of
adult weevils can be estimated by counting the number attracted to cut
segments of pseudostem and of larvae by estimating the area damaged and
counting the number of galleries exposed by slitting the rhizome or the
pseudostem very near to its base (Vilardebo 1973; Delattre 1980; Mesquita
1985; Smith 1993). In spite of this, an adequate relationship between adult
and larval abundance and economic loss remains to be established. It must
be added, however, that there continues to be a widespread view that
C. sordidus is a major pest.
The weevil attacks all banana ( Musa sapientum)
cultivars, including
plantain, and also Manila hemp ( Musa textilis).
It has been recorded in
earlier days (but not in recent years) from plants in other Orders, but these
reports are almost certainly in error.
Natural enemies
Although many predators are known to attack C. sordidus eggs, larvae and
pupae (Table 4.5.1), it is extraordinary that, with one possible exception, not
a single parasitoid of any life history stage has been recorded. That exception
is the early report from the Philippines of Cendana (1922), who found a
chalcidid wasp in one of his C. sordidus breeding cages, but it may not have
been parasitising the weevil. It is, perhaps, less surprising that the heavily
sclerotised adult weevil has very few enemies. It is true that some weevils
are effectively attacked by hymenopterous parasitoids and that some
tachinid parasitoids are able to attack certain weevils by laying eggs in their
food or beneath their mouth when they are feeding (Jacobs and Renner
1988), but neither has been observed for C. sordidus.
Koppenhšfer
(1993a,b) estimated that 58% of the eggs were accessible to predators and
presumably at least as many should be available to parasitoids if there were
any. Most eggs were found in the surface layer of rhizomes, particularly in
the crown, but some are also laid at the base of pseudostems and in the walls
of abandoned larval tunnels in both pseudostems and rhizomes. As soon as
eggs hatch the young larvae immediately tunnel deeper into the plant tissue
and thus become far less available to natural enemies.
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