1. Indirect induced plant defence via emission of herbivore-induced plant volatiles (HIPV) to recruit natural enemies of herbivores is a ubiquitous phenomenon, but whether and how emission of above-ground HIPVs is adaptively modulated by below-ground mutualistic micro-organisms is unknown.
2. We investigated the effects of the mycorrhizal fungus Glomus mosseae on chemical composition of HIPVs emitted by bean plants Phaseolus vulgaris attacked by spider mites, Tetranychus urticae, using proton-transfer mass spectrometry, and attraction of the spider mites’ natural enemy, the predatory mite Phytoseiulus persimilis, to these HIPVs using a Y-tube olfactometer.
3. Mycorrhiza significantly changed the HIPV composition. Most notably, it increased the emission of β-ocimene and β-caryophyllene, two compounds synthesized de novo upon spider mite attack. The constitutively emitted methyl salicylate was increased by spider mite infestation but decreased by mycorrhiza.
4. The predators responded strongly to HIPVs emitted by plants infested for 6 days and preferred HIPVs of mycorrhizal plants to those of non-mycorrhizal plants. In contrast, they were less responsive and indiscriminative to mycorrhization when exposed to volatiles emitted by non-infested plants and plants infested by spider mites for 1 or 3 days.
5. Our study provides a key example of an adaptive indirect HIPV-mediated interaction of a below-ground micro-organism with an above-ground carnivore.