Recent studies on the leek Allium porrum L. demonstrate that a sharp increase in the release of volatile and nonvolatile sulphur compounds occurs in response to attack by the specialist phytophage, the leek moth Acrolepiopsis assectella (Zeller). The consequences of that response on the developmental capacity and behaviour of the leek moth and on the host-searching behaviour of the parasitoid Hymenoptera Diadromus pulchellus Wesmael have been studied. No immediate adverse effect on larval development is seen, but the duration of male development is significantly longer. In addition, an almost 20% decrease is observed in the number of oocytes produced by adult females that developed on attack-damaged leek plants. Leek moths do not avoid damaged plants systematically. On attacked plants, larvae tend to consume more and females to lay more eggs, although the latter tendency disappears after intensive attack. Concerning host-searching by D. pulchellus, the data indicate that females make use of the induced leek response. Parasitoids are attracted more strongly to attacked plants than to undamaged plants and mechanically damaged plants. The results of this study in a tritrophic system involving a biannual plant suggest a complex defence system combining direct and indirect mechanisms. Sulphur compounds may act by reducing second-generation pests, thereby preventing the specialist insect population from exploding at the expense of the leek. none