Resistance is seen as a manifestation of the most basic property of life, namely the capacity of organisms to evolve in response to changing environmental conditions and to adapt to new circumstances. Despite the detection of severe resistance as far back as the l940's-60's, the relative abundance of new insecticides in the 1950's, 60's and 70's circumvented the need to delay or prevent resistance. New chemicals were always available.to replace those to which the pests had become resistant. Only recently has the realisation emerged that the discovery of new insecticides will not keep pace with the development of resistance in many of our major insect pest species.