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 determining sexual and other
characteristics of insect behaviour. Nine years ago the Phytochemical
Society of North America published their Symposium on 'Biochemical
Interaction Between Plants and Insects' in which the plant apparency
theory was expounded by both Paul Feeny and Rex Cates and David Rhoades.
This stated that plants which are apparent usually contain secondary
components which reduce digestibility (tannins and lignins) while
ephemeral plants have more toxic, and perhaps less costly, compounds
such as alkaloids. These papers stimulated much research on biochemical
ecology. The recognition of the importance of the biochemical factors in
such interactions is not just of scientific interest. It is vitally
important in programs for the production of new varieties of cultivated
plants, especially in tropical countries where about one-third or more
of the crops are lost to predation or disease.