The extent to which insecticides are needed is influenced by the level of resistance in the host plant. Plant breeders control this level through the choice of parents and selection procedures. If plants are exposed to insects during selection, resistance can be recognised and selected for. When, on the other hand, plants are fully protected from insects, e.g. through application of insecticides, the resistant phenotype goes unnoticed. In such a setting, genes for resistance have no selective advantage and may be lost.