Exogenous application of liquid diet,previously fed upon by pea aphids Acyrthosiphon pisum (Harris), to broad bean leaves induces volatiles attractive to the specialist parasitic wasp Aphidius ervi (Haliday)

Proof of the existence of aphid-derived molecules that induce emission of volatile organic compounds as indirect defensive responses in plants (hereafter called elicitors) has not yet been obtained. We showed here the presence and some aspects of the chemical nature of these elicitors. Exogenous application of the fed diet (liquid diet previously fed on by the Pea aphid, Acyrthosiphon pisum) on the damaged part of leaves of broad bean plants, made the plants more attractive to a parasitic wasp, Aphidius ervi, than application of the unfed diet. This result suggested that elicitors existed in the fed diet. Chemical characterization using ultrafiltration and subsequent bioassay revealed that the responsible factor was larger than 3 kDa. Heating of the fed diet did not inactivate its ability to induce attractive volatiles, which suggested thermotolerance of the elicitor. These results contribute to the mechanistic understanding of plant defenses against aphids.