Biosystematic studies of the gramineous stemborers, Chilo partellus (Swinhoe) and Chilo orichalcocitiettus (Strand) (Lepidoptera:Pyralidae).

Abstract

On the Kenya coast, the indigenous coastal stemborer, Chilo orichalcociliellus (Strand), and the exotic spotted stemborer Chilo partellus (Swinhoe), are pests of cereals. The two species occur together occupying similar ecological niches. Earlier studies have shown that their larvae are difficult to distinguish and that the exotic C. partellus may be gradually displacing the indigenous species. During this study, comparative larval development investigations in the laboratory suggested that the shorter generation time could explain the superior competitiveness of the exotic C. partellus over the indigenous C. orichalcociliellus. A study of morphological characters showed that the structure of spiracles, mandibular structures, the arrangement of setae and number of crochets on the abdominal and caudal prolegs could not be reliably used for distinguishing the two species. Asetose tubercles were shown to be useful characters in separating larvae of the two species. However, the use of asetose tubercles was limited since they were not present in all larval stages. The character therefore became unreliable for species separation. Hexane soluble cuticular components were studied using gas chromatography. Chromatograms of the components did not show any qualitative differences in the peak patterns, nevertheless, quantitative differences were observed. Analysis of the quantitative differences suggested that the cuticular components of these species were highly influenced by the diet and/or the environment, and as such, were not a reliable character for their identification. Electrophoretic banding patterns to three enzyme systems were found to be diagnostic. Glucose phosphate isomerase (GPI), Lactate dehydrogenase (LDH) and Glutamate oxaloacetate transaminase (GOT) on thin starch gel electrophoresis gave distinct separation between the larvae of the two species. These results showed that starch gel electrophoresis could be used as a reliable alternative technique for distinguishing larvae of these two stemborer species, especially in cases where morphological characters cannot be used.