THE IMPACT on sugarcane yield of two key stem borer species, Chilo sacchariphagus and Eldana saccharina, were investigated over a period of 10 years in Réunion Island and South Africa. Replicated and randomised field plot trials were conducted. Treatments consisted of pest exclusion using concentrated and repeated chemical applications, of natural infestations, and of artificial inoculations to enhance these infestations. The relationship between borer injury (measured as percent bored internodes) and the corresponding stalk length and diameter, biomass, fibre and sugar content were determined. Borer injury impaired the growth and reduced quality of sugarcane stalks. C. sacchariphagus decreased stalk biomass to a greater extent than sucrose content. E. saccharina injury reduced sucrose content and increased fibre level, and affected to a lesser extent stalk biomass. Since E. saccharina typically attacks sugarcane early during the main period of biomass accumulation and C. sacchariphagus attacks later during the maturation phase, the timing of borer infestations might explain these results. Numerous components of stalk quality were negatively correlated to injury from both species. Chilo sacchariphagus impacted mostly sugarcane biomass while E. saccharina decreased sucrose content. Crop loss models, as well as the formulation of any IPM recommendation, would need to be specific to the borer species.