Host-parasitoid interactions of Eldana Saccharina (Lepidoptera: Pyralidae) in Cyperus Papyrus.

Abstract

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.