Short-range allelochemicals from a plant-herbivore association: a singular case of oviposition-induced synomone for an egg parasitoid

Oviposition-induced plant synomones are semiochemical cues used by egg parasitoids during host selection, and are therefore considered important elements of plant defence. In this paper we show that, in the tritrophic system Brassica oleracea-Murgantia histrionica-Trissolcus brochymenae, the latter responded in a closed arena and in a static olfactometer to induced chemicals that are perceived from a very short range and after parasitoid contact with the leaf surface opposite the treated surface. An additive or synergistic effect due to (1) egg deposition, (2) feeding punctures and (3) chemical footprints of M. histrionica was observed. When all three phases were present, the parasitoid reacted to the induced synomone locally on the treated leaf area, at a close distance to the treated area, and on the leaf above the treated one, showing that there is also a systemic effect. When plants with host footprints combined with feeding punctures or with oviposition were tested, responses were obtained both locally and at a close distance, whereas in the remaining assays only local responses were observed. Induction time was less than 24 h, whereas signal duration was apparently related to the suitability of the host eggs, as parasitoids did not respond to plants carrying old or hatched eggs. These oviposition-induced short-range plant synomones might have an important role in the host location process after parasitoid landing on the plant, in different combinations with the host kairomones involved in the system studied here.