Chemical warfare between plants and their herbivores and pathogens was first brought to our attention by the publication 25 years ago of the paper by Fraenkel in Science. There, he pointed out that most plants have similar nutritional characteristics so that the selection of plants by insect herbivores must depend on the relative toxicity of secondary compounds. This led, rather gradually, to a host of papers on plant-herbivore interactions. More or less at the same time, insect physiologists and ecologists were starting to realise the importance of chemical communi cation systems in...