Experimental maize hybrids with various levels of resistance to the maize stalk borer,Busseola fusca(Fuller), were evaluated at different infestation levels during seasons of favourable and unfavourable rainfall. Limited plant resistance sufficed to reduce plant damage and yield losses significantly. In spite of plant damage being more severe during the unfavourable season, yield losses were less severe, which was attributed to a greater seasonal effect on undamaged than on damaged plants. Yield reduction by seasonal effects was shown to manifest as a constant proportion of attainable yield in the favourable season, independent of infestation level. The significance of the results is discussed in relation to the development of differentiated economic thresholds, in order to improve the predictive estimation of yield losses for various categories of yield potential.