COMPETITIVE DISPLACEMENT OF BUSSEOLA-FUSCA (LEPIDOPTERA, NOCTUIDAE) BY CHILO-PARTELLUS (LEPIDOPTERA, PYRALIDAE)

Since the appearance of the stem borer Chilo partellus (Swinhoe) on th e African continent in 1932, it has continuously expanded its distribu tion in the warm, low-altitude regions of eastern and southern Africa. The current study revealed that C. partellus is expanding its distrib ution into the high elevations of the eastern Highveld region of South Africa. The only stem borer already found there (elevation 1,600 m) i s Busseola fusca (Fuller). After C. partellus invaded the region, it r apidly increased its share of the total borer population every pear. O n maize, it reached 32% of the total borer population within 6 yr and on grain sorghum 59% within 7 yr. The most rapid population increase b y C. partellus occurred on the ratoon grain sorghum crop. Within 2 yr it became the predominant borer, constituting approximate to 90% of th e total stem borer population. C. partellus has proven to be an effici ent colonizer, and it seems to be displacing the indigenous B. fusca. C. partellus survives the dry winters (at subzero temperatures) of the Highveld region in South Africa by diapausing low in the dry stalks, often beneath the soil. Hibernating larval populations of C. partellus terminate diapause and emerge as moths approximate to 1 mo earlier th an B. fusca. This enables C. partellus to infest the grain sorghum rat oon crop before B. fusca, thus becoming the predominant borer in this niche. The life cycle of C. partellus is 3 wk shorter than that of B. fusca, which gives it a further competitive advantage because of its h igher potential rate of increase.