In the absence of equivalent research on genetically modified (GM) pest-resistant crops, their impact in pest management can be predicted from experience with traditionally bred varieties which share with GM crops the characteristic that the resistance is based on high expression of a single toxin. Such varieties lead to the rapid selection of tolerant pest strains, damage biological control and induce tolerance to synthetic pesticides. By contrast, polygenic and more broadly based resistant varieties will maintain their resistance for longer, and often synergise beneficially with biological control. The pests also become more susceptible to insecticides, giving the opportunity for applications which are selective in favour of natural enemies. However, although GM crops compare badly with traditional pest-resistant varieties, they compare favourably with insecticides, the technology they are most likely to replace.