Effect of dual biotic stress on plant volatile synomones used by egg parasitoids

Studies on semiochemical communication have demonstrated that broad bean plant, Vicia faba, emits volatile synomones induced by feeding and oviposition activities of the southern green stink bug, Nezara viridula, which recruit the egg parasitoid Trissolcus basalis. However plants growing in agro-ecosystems can be attacked by several herbivore species that could affect both above and belowground plant tissues with possible consequences for parasitoid recruitment. For example, broad bean plants can also be attacked by the leaf weevil, Sitona lineatus, and simultaneous attacks by the southern green stink bug and the leaf weevil can occur in agro- ecosystems. The purpose of this work was to investigate the effects of dual biotic stresses, the host N. viridula and the non-host S. lineatus, on V. faba volatile synomones that recruit T. basalis. The response of wasp females to V. faba volatiles was investigated in a Y-tube olfactometer testing the following treatments: (1) plant damaged by N. viridula feeding and oviposition; (2) plant damaged by S. lineatus feeding; (3) plant mechanically damaged to resemble S. lineatus feeding; (4) plants damaged by N. viridula feeding and oviposition and by mechanical damages; (5) plants damaged by N. viridula feeding and oviposition and by S. lineatus feeding; (6); healthy plants. Volatile organic compounds emitted from tested plants were also chemically analyzed by Gas Chromatography – Mass Spectrometry (GC-MS). The results showed that dual biotic stresses affect V. faba volatile synomones decreasing their attractiveness towards T. basalis. Chemical analysis indicated qualitative differences between volatiles emitted by V. faba plants in response to N. viridula feeding and oviposition and volatile emitted as consequence of dual insect infestation. The ecological consequences of these results in terms of multi-trophic interactions are discussed.