The stages of bush fly Musca vetustissima Walker, killed by the dung beetles, Onthophagus binodis Thunberg and Onitis alexis Klug, at low and high densities were measured in cattle dung of low nutritive value. A high level of M. vetustissima mortality occurred in the presence of Onitis alexis and Onthophagus binodis in experiments both in the laboratory at a constant 25°C, and exposed to summer fluctuating temperatures outside. The K values for total eggpuparia mortality increased curvilinearly with increasing log10 beetle numbers. The mortality of older larvae, k3, was most closely correlated with total mortality, K, where b = 0.720 and r2 = 0.931, while a lesser but significant correlation was obtained with mortality of young larvae, k2, where b = 0.291 and r2 = 0.645, but no correlation was obtained with egg mortality, k1, where b = −0.010 and r2 = 0.058. It is predicted that in unfavourable dung which occurs during summer, mortality of older bush fly larvae will be the key mortality caused by scarabaeine dung beetles.