1849 Both insect egg deposition and feeding by herbivorous arthropods is well-known to induce plant defensive responses. The plant’s response may have direct detrimental effects on the eggs, the ovipositing female or the feeding herbivore (Hilker et al., 2002a). Furthermore, the plant is able to respond to egg deposition and feeding by recruiting predators and parasitoids of the herbivores (Baldwin and Preston, 1999; Dicke and van Loon, 2000; Hilker and Meiners, 2002; Turlings et al., 2002). In particular, many plants produce volatile substances in response to egg deposition or feeding that have been shown to attract enemies of the assailant (Boland et al., 1999; Colazza et al., 2004a,b; Dicke and Hilker, 2003; Hilker and Meiners, 2002; Mumm et al., 2003, 2005). While the plant responding to feeding herbivores acts in response to the damage, the plant responding to insect egg deposition acts prior to being damaged by feeding larvae. Thus, Hilker and Meiners (2002) interpreted the plant’s defensive response to insect egg deposition as a preventive induced defense mechanism. Egg deposition by the phytophagous sawfly Diprion pini has been shown to induce the plant to release volatiles attracting Chrysonotomyia ruforum, an egg parasitoid of D. pini (Hilker et al., 2002b). This induction of volatiles is not restricted to the oviposition site, but also occurs in surrounding tissue that is not damaged (systemic effect). During egg deposition, D. pini females make a tangential slit in the pine needles with their sclerotized ovipositor valves, and insert the eggs into the wounds in the needles. Finally, eggs are covered on the top by a mixture of a frothy secretion and needle tissue that hardens within a few hours (Eliescu, 1932). The elicitor inducing the pine’s response was found to be located in the oviduct secretion coating the eggs, since application of the oviduct secretion into artificially wounded pine needles also resulted in the induction of volatiles attractive to the egg parasitoid, whereas artificial wounding alone did not (Hilker et al., 2002b). For application of the oviduct secretion to artificially wounded pine needles, oviducts were dissected from sawfly females and secretion was The Journal of Experimental Biology 208, 1849-1854 Published by The Company of Biologists 2005 doi:10.1242/jeb.01578 Egg deposition by the phytophagous sawfly Diprion pini L. (Hymenoptera, Diprionidae) is known to induce locally and systemically the emission of volatiles in Scots pine (Pinus sylvestris L.) that attract the egg parasitoid Chrysonotomyia ruforum Krausse (Hymenoptera, Eulophidae). The egg parasitoids kill the eggs and thus prevent damage to the plant from feeding sawfly larvae. The elicitor inducing the pine’s response is known to be located in the oviduct secretion which the female sawfly applies to the eggs when inserting them into a slit in the pine needle using the sclerotized ovipositor valves. In this study we have characterized the elicitor. The elicitor was still active when isolated from the oviduct and applied directly to slits made in the pine needles. However, as soon as the oviduct secretion was dissolved in Aqua dest. and stored for 3·h at room temperature or kept frozen at –80°C, its activity was lost. In contrast, oviduct secretion kept its eliciting activity, when dissolved in Ringer solution (pH 7.2) both after storage at room temperature and after freezing. The activity of the elicitor vanished after treatment of the oviduct secretion with proteinase K, which destroyed all proteins. This suggests that the elicitor in the oviduct secretion is a peptide or protein, or a component bound to these. SDS-PAGE revealed a similar, but not identical protein pattern from hemolymph and oviduct secretion. Hemolymph itself has no eliciting effect. The elicitor in the oviduct secretion is only active when transferred to slit pine needles, since its application on undamaged needles did not induce the emission of attractive volatiles. Key words: elicitor, egg deposition, egg parasitoid, induction, oviduct secretion, sawfly, Diprion pini, volatiles. Summary Introduction Insect egg deposition induces defence responses in Pinus sylvestris: characterisation of the elicitor Monika Hilker 1, *, Claudia Stein 1,† , Roland Schröder 1 , Martti Varama 2 and Roland Mumm 1,‡ 1 Institute of Biology, Freie Universität Berlin, Haderslebener Str. 9, D-12163 Berlin, Germany and 2 Finnish Forest Research Institute, Vantaa Research Centre, PO Box 18, FIN-01301, Vantaa, Finland *Author for correspondence (e-mail: hilker@zedat.fu-berlin.de) † Present address: Department of Community Ecology, Centre for Environmental Research (UFZ), Theodor-Lieser Str. 4, D-06120 Halle, Germany ‡ Present address: Laboratory of Entomology, Wageningen University, Binnenhaven 7, NL-6709 PD Wageningen, The Netherlands Accepted 2 March 2005 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY