From plant exploitation to mutualism

Phytophagous insects have developed mechanisms of various complexity levels to utilize plants in spite of the barriers that plants have built to resist aggressions. Plant exploitation, the simplest level, is the use of plant defence chemicals for the benefit of insects. It is illustrated by the use of plant toxins for defence against predators. The energetic cost of that defence strategy is discussed according to the toxicity of the chemicals and the necessity of protecting the herbivore, and the modes of action on predators are presented. Furthermore, manipulation of the plant can reorient the plant metabolism to satisfy insect needs. Drastic remodelling of the host plant can occur, from ultrastructure to anatomy levels, with alteration of both its nutritional quality and secondary metabolism. The mechanisms involved are being investigated. Outcomes concern optimization of the nutritional value of the host plant and protection from adverse abiotic and biotic (natural enemies, competition) conditions. Cooperation with conspecifics or microorganisms often interferes. At the highest level of complexity, mutualism is the result of a compromise between insect and plant where each partner benefits from the association. Pollination is a typical example. Pollinators vary from generalists to specialists and belong to a community of insect linked to a community of plants. In the fig-fig wasp mutualism, the various mechanisms involved in situations of monoecy and dioecy are discussed, as well as the existence of coadaptations and cospeciations. The chapter ends with a presentation of research perspectives for improving crop productivity.