Postingestive Effects of Phytochemicals On Insects: On Paracelsus and Plant Products

The eating habits of herbivorous insects have long been of more than passing interest to humans, particularly since the domestication of plants some 8000 years ago, but the precise relationship between herbivorous insects and their host plants has persistently defied characterization. That plant chemicals, rather than nutritive substances, determine at least in part the peculiar likes and dislikes of insects, as they do for humans, is an idea of rather recent origin. The suggestion that the diversity of plant chemistry might be defensive in function was introduced when M. Leo Errera, in an address to the Royal Botanical Society of Belgium in 1886, remarked that, “many of the chemical compounds may serve the plant as means of defense against animals, and when we camphorize our furniture and poison our flower-beds, we are only imitating and reinventing what the plants practiced before the existence of man” (Abbott, 1887). The importance of insects as selective agents in determining patterns of plant chemistry in the “parallel evolution” (Brues, 1920) of insects and plants was not articulated until Fraenkel (1959) suggested that “reciprocal adaptive evolution” determined patterns of host-plant use by insects. Ehrlich and Raven (1964) coined the term “coevolution” to describe the stepwise process by which plants elaborate chemical defenses and insects evolve mechanisms of resistance or tolerance to those defenses.