Since the appearance of the stem borer Chilo partellus (Swinhoe) on the African continent in 1932, it has continuously expanded its distribution in the warm, low-altitude regions of eastern and southern Africa. The current study revealed that C. partellus is expanding its distribution into the high elevations of the eastern Highveld region of South Africa. The only stem borer already found there (elevation 1,600 m) is Busseola fusca (Fuller). C. partellus, having invaded the region, rapidly increased its share of the total borer population every year. On maize, it reached 32% of the total borer population within 6 years and on grain sorghum 59% within 7 years. The most rapid population increase by C. partellus occurred on the ratoon grain sorghum crop. Within 2 years it became the predominant borer, constituting ± 90% of the total stem borer population.

Chilo partellus has proven to be a very efficient colonizer, and it seems to be displacing the indigenous B. fusca. C. partellus survives the dry winters (with 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 ± 1 month earlier than B. fusca. This enables C. partellus to infest the grain sorghum ratoon crop before B. fusca, thus becoming the predominant borer in this niche. The life cycle of C. partellus is 3 weeks shorter than that of B. fusca, which gives it a further competitive advantage because of its higher potential rate of increase.

For more detail see:

Kfir, R. 1997. Competitive Displacement of Busseola fusca (Lepidoptera: Noctuidae) by Chilo partellus (Lepidoptera: Pyralidae). Annals of the Entomological Society of America 90: 619-624