Plant-derived secondary metabolites as defensive chemicals in herbivorous insects: a case study in chemical ecology

In terrestrial communities, plant photosynthesis is the first step in a chain of trophic relationships involving many species of herbivores, pathogens and their ene mies. In the evolutionary course of their adaptations to the pressure of herbivores and pathogens, plants developed a diverse array of defenses. In sessile ter restrial plants, chemical defense strategies play an outstanding role, well documented by the immense diversity of plant secondary metabolism with some 200,000 known compounds. Some of these defenses are apparently so powerful that specialized herbivorous insects adopted them for their own protection against predators, parasitoids and pathogens. Such examples not only affirm the importance of plant chemical de fenses but also provide an opportunity to trace a plant derived defensive compound to the next trophic level and realize its impact. Recent research confirmed that recruitment of plant secondary metabolites by special ized insects is associated with specific mechanisms affecting physiological, morphological and behavioral functions. An example is offered by the pyrrolizidine alkaloids (PAs), which are utilized by a number of herbivorous insects (for detailed review, see Hartmann 1999; Hartmann and Ober 2000). This Progress Report focuses on the impact of PAs on biochemical, sensory and behavioral adaptations, exemplified by arctiid moths (Lepidoptera) and chrysomelid leaf-beetles (Coleoptera).