Lepidoptera and pyrrolizidine alkaloids: Exemplification of complexity in chemical ecology

Pyrrolizidine alkaloids (PAs) are defensive secondary metabolites found in numerous plant groups. Various insects belonging to different orders have special requirements for these compounds and sequester them from such plants for their own defense and often as pheromone precursors. The fitness of these insects depends on PAs and, in some cases, PAs even act as regulators of androconial organ development. This article discusses selected behavioral, chemical, physiological, and phylogenetic aspects of insect-PA relationships, and raises questions about the complex interactions of the variety of PA-related adaptations as they occur among a diverse array of species. Although many superficial similarities are recognized, few generalizations can yet be drawn. However, insect-PA relationships not only exemplify basic features of chemical ecology but illustrate a multiplicity of aspects and adaptations, which we should expect to find in any thorough study of insect-plant relationship.