Parasitic wasps, that search for their hosts, may use chemical information derived from plants, the first trophic level, and the herbivorous hosts, the second trophic level. These chemical stimuli may contain general and specific components. To test our hypothesis, that the host-specificity of a parasitoid species determines the searching strategy, a comparative research was initiated on the tritrophic system cabbage (Brassicaceae), cabbage-white caterpillars (Pieris sp.) and two closely related parasitoid species of these herbivores, the generalist Cotesia glomerata and the specialist C. rubecula. A theoretical outline will be given here, illustrated with preliminary results.