Since becoming a pest in graminaceous crops in Africa, the African sugarcane
stalkborer Eldana saccharina Walker has been the subject of much study. Its
very cryptic habits have precluded more commonly available control measures
being used against it. Biological control is regarded as a viable control option,
but the apparent paucity of parasitoids in graminaceous crops leads to E.
saccharina being regarded as lacking parasitoids, and thus not a good candidate
for biological control in the classical sense. In contrast, this project argues that
interactions in indigenous hosts of E. saccharina had been ignored, and that
classical biocontrol principles and basic ecological theory could be applied by the
discovery, collection and introduction of parasitoids from its indigenous habitat
to its newly adapted habitat, sugarcane.
The habitat offered by Cyperus papyrus L. was shown to be heterogenous both
temporarily and spatially. Umbels, from young through mature to senescent,
were available in the same proportion for colonisation throughout the year.
Umbels with sexual reproductive stages (seeds) were present from early spring
into late summer, and provided an additional component to the already
heterogenous environment. Young umbels, in addition, developed from rhizomes
in an environment regarded as sub-optimal for photosynthesis, until they reached
the canopy.
All stages of umbels were attacked by E. saccharina, but larvae were only found
in rays of umbels and in the apex of the culm, which was the meristematic area
for rays, both high nutrient areas. Young umbels were never found with borer
pupae, only smaller larvae, indicating that E. saccharina development matched
growth of young umbels until they reached canopy height. Also, the majority of
borings found were occupied, indicating that infestation of young umbels was
recent. All stages of E. saccharina development were found in mature umbels,
which were also most abundant at anyone time. Numerous empty borings were
found in addition to those occupied, indicative of past occupation by E. saccharina. Very few young larvae were found in old umbels, the majority of life
stages found being pupae or empty pupal cases, and also many empty borings,
showing that old umbels were not suitable for E. saccharina development.
A guild of parasitoids which comprised Orgilus bifasciatus Turner, the most
common parasitoid of small and smaller medium E. saccharina larvae, Goniozus
indicus Ashmead the most common parasitoid of larger medium and large larvae,
and an entomogenous fungus Seauveria bassiana (Sals.) Vuill. attacking all life
stages of E. saccharina was found. Three uncommon parasitoids of smaller
saccharina were also found, viz. Sassus sublevis (Granger), Iphiaulax sp. and
Venturia sp. The former three natural enemies were instrumental in depressing
a major outbreak within two months of it being observed and then maintaining
the host population at a lower level in C. papyrus. G. indicus and B. bassiana
were most effective during the summer and autumn months, and O. bifasciatus
most effective during the winter months.
This study supports the hypotheses that the apparent paucity of parasitoids and
lack of biological control success thus far against E. saccharina in sugarcane has
been because very little was known about its ecology and biology in its
numerous indigenous host plants, and that studies of the latter factors coupled
with ecological theory could enhance biological control programmes against this
borer. As more indigenous host plants are investigated in the same way as has
been done with C. papyrus, more will become known of natural enemies of E.
saccharina. Parasitoid guilds could be selected, even from rare parasitoids in the
more stable indigenous habitats, which would provide control in the unstable
habitat of sugarcane. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1994.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10870 |
| Date | January 1994 |
| Creators | Conlong, Desmond Edward. |
| Contributors | Carnegie, A. J. M., Samways, Michael J. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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