Indirect Defence — Recent Developments and Open Questions

Abstract

Many plants defend themselves indirectly against herbivores via the attraction of carnivores. Carnivores can be provided with resources such as cellular food bodies, domatia, or extrafloral nectar (EFN), and they can be attracted by herbivore-induced volatile organic compounds (HI-VOCs) that signal the presence of prey. Both HI-VOCs and EFN are usually induced in response to herbivore attack. HI-VOCs have been intensively studied with respect to their (bio)chemistry and the signalling events leading to their induction, but field studies on the role of HI-VOCs in the plant’s indirect defence via tritrophic interactions are scarce. In contrast, EFN has been described for hundreds of species, while we lack detailed knowledge on its chemical ecology and the physiology of EFN secretion. Food bodies and domatia have been almost ignored by researchers in the past ten years. Tritrophic interactions are increasingly being discussed as environmentally friendly tool in crop protection, but current research is highly unbalanced in focusing on entirely different aspects of the different defensive traits. Future studies should consider what is already known on other traits in order to gain a more complete understanding of a plant’s possibilities to defend itself by establishing mutualisms with the third trophic level.