Seasonal Abundance of the Stem Borer Chilo partellus (Lepidoptera: Pyralidae) and its Parasites on Summer Grain Crops

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The seasonal abundance of a lepidopteran stem borer, Chilo partellus (Swinhoe), and its parasites was studied at Brits in maize and grain sorghum from 1985 to 1989, and from 1985 to 1986 at Warmbaths, South Africa. C. partellus generations overlapped, and all developmental stages were present throughout most of the summer (November- March). The borer completed two and a half generations per season. The first generation occurred during October-November, infesting only early grain sorghum tillers from a ratoon crop. C. partellus produced an additional one and a half generations on the commercial crops that emerged later. In March-April, fifth and sixth instars of the third generation entered diapause in the dry stalks. During the second part of August, the larvae started to emerge from diapause and all had pupated or emerged as adults by October-November. Diapausing larvae were present during most of the year. An increase in the proportion of young larvae indicated an increase in the population, and an increase in the proportion 'of pupae indicated a decline. Infestation by borers started on the funnel leaves and then moved down in the stalks as the season progressed. The majority of winter larvae diapaused in the lower parts of stalks. Pupal parasitism generally peaked at 100% and this corresponded with a decline of the first-generation larvae. The most abundant pupal parasites were Dentichasmias busseolae Heinrich at Brits, and Pediobius furous (Gahan) at Warmbaths. Larval parasitism lagged behind peak larval populations and peaked at 80% parasitism. The most abundant larval parasite of C. partellus was Cotesia sesamiae (Cameron). Most recorded parasites are indigenous to Africa and their association with the exotic C. partellus is relatively new. Although they reduced densities of C. partellus, parasites did not reduce densities below economic damage levels.

Document Type: Research Article

Publication date: April 1, 1992

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