Betraying its presence: identification of the chemical signal released by Tuta absoluta-infested tomato plants that guide generalist predators toward their prey

Plants respond to insect herbivory with the production of volatiles that attract carnivorous enemies of the herbivores, a phenomenon called indirect defence or 'plants crying for help'. Plants are under selection to maximize Darwinian fitness, and this can be done by making the right 'decisions' (i.e. by responding to environmental stress in ways that maximize seed production). Plant decisions related to the response to herbivory in terms of the emission of herbivore-induced volatiles include 'to respond or not to respond', 'how fast to respond', 'how to respond' and 'when to stop responding'. In this review, the state-of-the-art of the research field is presented in the context of these decisions that plants face. New questions and directions for future research are identified. To understand the consequences of plant responses in a community context, it is important to expand research from individual interactions to multispecies interactions in a community context. To achieve this, detailed information on underlying mechanisms is essential and first steps on this road have been made. This selective review addresses the ecology of herbivore-induced plant volatiles (HIPVs) by integrating information on mechanisms and ecological functions. New questions are identified as well as challenges for extending current information to community ecology.

Fifteen synthetic herbivore-induced plant volatiles (HIPVs) were field-tested for attractivity to beneficial insects in two experiments conducted in an open field and a hop yard in Washington State. Eleven insect species or families showed significant attraction to 13 HIPVs. The ladybeetle, Stethorus punctum picipes, was attracted to sticky traps baited with methyl salicylate (MeSA), cis-3-hexen-1-ol (He), and benzaldehyde (Be). The minute pirate bug, Orius tristicolor, was attracted to traps baited with MeSA, He, Be, and octyl aldehyde (Oa), and the bigeyed bug, Geocoris pallens, responded to MeSA, indole, and trans-2-hexen-1-al. The mymarid wasp, Anagrus daanei, was attracted to He, Oa, and farnesene. The chloropid fly, Thaumatomyia glabra, was highly attracted to methyl anthranilate. Insect families responding to HIPVs included Syrphidae (MeSA, He), Braconidae ((Z)-3-hexenyl acetate, He, cis-jasmone (J), methyl jasmonate (MeJA), methyl anthranilate (MeA)), Empididae (MeSA), Sarcophagidae (MeSA, Be, J, nonanal and geraniol), Tachinidae (Be), and Agromyzidae (MeSA). Micro-Hymenoptera (primarily parasitic wasp families) were attracted to MeSA, He, and indole. These results are discussed with respect to known properties and bioactivity of the tested HIPVs and to their potential as tools for recruiting natural enemies into agroecosystems.