none This thesis examines effects of plant fertilisation regime on herbivore and parasitoid performance for two aphids, Macrosiphum euphorbiae and Myzus persicae, and one parasitoid wasp, Aphidius ervi. Herbivore performance was measured as adult fresh weight and intrinsic rate of increase. Parasitoid performance was measured as length of hind tibia, sex ratio, and in a series of choice experiments also parasitoid development time and parasitization rate was measured. Comparisons were made between the performance of M. euphorbiae on two different plant species, petunia and sweet pepper, between the performance of the aphids M. euphorbiae and M. persicae on sweet pepper, and between the performance of A. ervi in M. euphorbiae and in M. persicae on sweet pepper. The performance of M. euphorbiae was better on petunia than on sweet pepper. On petunia a positive effect of plant content of potassium and indications of a positive effect of nitrogen fertilisation were found while on sweet pepper both nitrogen and of potassium had negative influence. On both plant species, however, a negative influence of sulphur was found. The two aphid species reacted differently to fertilisation and plant content of nutrients. For M. euphorbiae fresh weight was decreased by fertilisation while for M. persicae the influence varied with experiment. For M. persicae a high N:K ratio was favourable while for M. euphorbiae plant contents of N and K were both negative. Plant content of sulphur and potassium influenced tibia length of A. ervi positively in M. euphorbiae. In M. persicae plant fertilisation was positive but the composition of the plant fertilisation did not seem important for the parasitoid. Plant fertilisation enhanced parasitization rate in M. euphorbiae but not in M. persicae. Parasitoid performance was, however, more improved by fertilisation in M. persicae than in M. euphorbiae. The parasitoid both preferred, and developed faster in, M. persicae compared to M. euphorbiae. The results of this study show that it is possible to influence both aphid performance and performance of A. ervi with plant fertilisation but the influence varies with both plant and aphid species. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology Aphidius ervi (Aphidiidae); Hemipteran hosts; Macrosiphum & Myzus; Effects of plant fertilization regime on parasite performance Macrosiphum euphorbiae; Myzus persicae (Aphidoidea ); Hymenopteran parasites; Aphidius ervi; Effects of plant fertilization regime on parasites; Plant hosts; Capsicum annuum & Petunia axillaris; Effects of plant fertilization regime on herbivore performance & hymenpteran parasites; Population dynamics