The co-evolution of aphids and their indigenous natural enemies means that, on field crops, biological control to a grower-acceptable level will occur only sporadically in the absence of manipulative interventions. Such interventions should focus on raising the natural enemy:aphid ratio. This ratio is far more important than the absolute number of natural enemies present. The main interventions for improving the ratio are habitat modifications, advancing in time the activity of natural enemies on the crop, reducing aphid multiplication through genetically based or induced partial plant resistance, and ingenious use of pesticide to build in relative selectivity of kill. The interactions between biological control and some of the other interventions offer exciting opportunities for managing natural enemies.