<p>In this thesis the isolation and identification of infochemicals which are involved in <em>Cotesia-Pieris-Brassica</em> relationships with the prospect of their eventual use in cabbage crop protection, are described. The study focuses on two topics: regulation of <em>Pieris</em> oviposition behaviour and host selection behaviour of parasitoids of <em>Pieris</em> larvae.<p>A general introduction about relationships between plants/insects and insects/insects, and more specifically the relationship between Crucifers and their associated insect herbivores as well as phytochemical information about Cruciferae is given in chapter 1.<p>In chapter 2 the isolation and identification of the oviposition stimulant for the large cabbage white butterfly, present in the leaf surface of Brussels sprout plants, is described. The oviposition stimulant could be identified as glucobrassicin (3-indolyl-methyl-glucosinolate), a secondary plant compound belonging to the glucosinolates which are characteristic for the genus <em>Brassica.</em><p>The identification of oviposition deterrents from the eggs of the large cabbage white is described in chapter 3. Three compounds, responsible for the oviposition deterring activity of an egg wash when sprayed onto a cabbage leaf, were isolated and identified as <em>trans</em> -2-[3-(3,4,5-trihydroxy- phenylpropenoyl)-amino]-3,5-dihydroxy-benzoic acid (miriamide), <em>trans</em> -2-[3-(3,4-dihydroxyphenylpropenoyl)amino]-3,5-dihydroxybenzoic acid and <em>trans</em> -2-[3-(3,4-dihydroxy-5-β- glucopyranose-phenylpropenoyl)amino]-3,5-dihydroxybenzoic acid. The synthesis of the first two compounds is also described. The three previously unknown avenanthramide alkaloids (amides of derivatives of anthranilic and cinnamic acid) form a group of compounds that have not been reported from the animal kingdom before.<p>The structure-activity relationship of the isolated avenanthramide alkaloids (described in chapter 3) and eight related synthesized compounds, as oviposition deterrents for <em>P.</em><em>brassicae</em> L., is studied in chapter 4. For ten of the tested compounds, the effective dosis at which an oviposition deterring index of 50 % (ED <sub><font size="-2">50</font></sub> ) occurred, has been calculated. At least three groups with different activity levels were found. Changes in the way both ring systems were connected had no influence on the deterrent activity, while modifications of groups linked to the anthranilic part of the molecule led to a reduction of activity compared to miriamide. Mono- and dihydroxy substituted cinnamic parts of the molecule increased its effectiveness. <em>trans</em> -(4-Hydroxyphenylpropenoyl)amino]-3,5-dihydroxybenzoic acid was found to be significantly more active than miriamide.<p>In chapter 5, the question whether ovipositing female butterflies, after landing on the upper surface of a cabbage leaf, can perceive the host marking pheromone (HMP) present on the eggs deposited on the lower side of the leaf is studied. The strongly oviposition deterring avenanthramide alkaloids could not be detected in leaf surface extracts from leaves from which egg batches had been removed. Thus the isolated avenanthramide alkaloids are not directly responsible for the HMP effect. Evidence is obtained that cabbage leaves themselves produce oviposition deterrents in response to oviposited egg batches, thus making the use of the terni HMP disputable. Fractions containing potent oviposition deterrents were isolated from surface extract of leaves from which previously laid egg batches had been removed.<p>In chapter 6 headspace analysis of intact cabbage plants and cabbage plants infested with larvae of the small cabbage white and the large cabbage white is described. The volatile production of intact cabbage plants shows a seasonal fluctuation with the highest production rate in the summer period.<p>Major differences in the headspace profile of intact and caterpillar damaged plants were revealed for hexyl acetate, <em>cis</em> -3-hexenyl acetate, myrcene, sabinene and 1,8-cineole. No significant quantitative differences were found between the headspace of cabbage plants infested by one or the other caterpillar species. In a windtunnel bioassay (dual-choice), it was found that the solitary parasitoid of the small cabbage white and a gregarious parasitoid of the large cabbage white distinguish between intact cabbage plants and cabbage plants infested with their preferred hosts.