Summary1. 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 microorganisms 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 b-ocimene and b-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.

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Fears of terrorist attacks have led to the development of various technologies for the real-time detection of explosives, but all suffer from potential ambiguities in the assignment of threat agents. Using proton transfer reaction mass spectrometry (PTR-MS), an unusual bias dependence in the detection sensitivity of 2,4,6 trinitrotoluene (TNT) on the reduced electric field (E/N) has been observed. For protonated TNT, rather than decreasing signal intensity with increasing E/N, which is the more usual sensitivity pattern observed in PTR-MS studies, an anomalous behavior is first observed, whereby the signal intensity initially rises with increasing E/N. We relate this to unexpected ion−molecule chemistry based upon comparisons of measurements taken with related nitroaromatic compounds (1,3,5 trinitrobenzene, 1,3 dinitrobenzene, and 2,4 dinitrotoluene) and electronic structure calculations. This dependence provides an easily measurable signature that can be used to provide a rapid highly selective analytical procedure to minimize false positives for the detection of TNT. This has major implications for Homeland Security and, in addition, has the potential of making instrumentation cost-effective for use in security areas. This study shows that an understanding of fundamental ion−molecule chemistry occurring in low-pressure drift tubes is needed to exploit selectivity and sensitivity for analytical purposes.

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