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1 Field experiments were conducted on maize and sorghum at three locations in the Amhara state of Ethiopia to determine the effects of mixed cropping on stem-borer infestation, borer natural enemies and grain yields. In the cool-wet ecozone of western Amhara, sole maize was compared with maize intercropped with faba bean, mustard, potatoes and cowpea. In the semi-arid ecozone of eastern Amhara, the trial was conducted on both maize and sorghum with the companion crops haricot bean, sesame, cowpea and sweet potatoes.

2 The results showed that the predominant borer species in western and eastern Amhara were, respectively, Busseola fusca and Chilo partellus. In Addis Zemen, western Amhara, maize intercropped with mustard and potatoes had significantly lower pest numbers and percent tunnelling than other intercrops and the maize monocrop during the vegetative stage. In eastern Amhara, the cropping system did not significantly affect pest densities but damage to stem, ear or heads tended to be greatest when cereals were intercropped with sweet potatoes.

3 Parasitism of C. partellus by the braconid Cotesia flavipes was greater on maize than sorghum, and on maize it was greater with sweet potatoes than in other intercrops or sole maize. Cocoon mass number per plant did not vary significantly between treatments.

4 There were significant differences between treatments in yields of both sorghum and maize (per plant and per unit area) with the lowest yields observed when they were intercropped with a tuber crop.

5 The results suggest that simultaneous planting of the crop species selected has little advantage over monocropped maize.