Indirect interactions where one species mediate interactions between two other species are common in food webs. Not surprisingly, research on such indirect interactions are common, but few studies recognize that such interactions are often evolutionary unstable and that the occurrence of such interactions therefore may vary over time. In evolutionary time, speciation and diet changes in either predator or prey may cause indirect interactions to either appear or disappear. In ecological time, range shifts or species exclusions may have similar consequences. In order to understand the temporal variation in predator-mediated interactions, we need to understand the evolution or species traits connected to the interaction as a consequence of the interaction and how this in turn affects the topology of the food web. In host-parasitoid system, the key traits involved include traits connected to parasitoid host selection, host immune responses and parasitoid virulence. Consequently, in order to understand the underlying mechanisms, it is necessary to connect ecology, evolution and immunology in studies of the same system. In this project, we will use a model system that is very promising for these studies. First, we have previously shown that the parasitoid Asecodes parviclava may mediate indirect interactions between its host species, larvae of three beetle species (Galerucella calmariensis, G. pusilla and G. tenella). Second, we have also characterised the immune system of the larvae, particularly the encapsulation response to parasitoid eggs, and shown fairly large differences in immunocompetence between the host species. Third, we have identified geographic variation in parasitoid virulence and parasitoid host race formation suggesting on-going evolution of relevant traits. Fourth, we discovered that female parasitoids have at least limited capacities to identify host larval odors. Finally, we have identified candidate genes explaining variation in host immunocapacity. Within this project, we will study evolutionary changes among wasp populations depending on host use. For this purpose, we have created a reference genome and then performed whole-genome resequencing from four different wasp populations in a factorial design. The populations were selected based on previous phenotypic differences in host use and capacity to overcome host immune defenses. One factor is host use (different Galerucella species) and the second factor is geography (Stockholm vs. Umeå). The second factor is connected to differences in host distributions where one species with high immunocompetence is absent from Umeå. Thus, we have wasps hatched from G. tenella from Stockholm and Umeå and wasps hatched from G. calmariensis/G. pusilla from Stockholm and from G. calmariensis from Umeå. To understand the evolutionary process, we will compare two gene classes, those coding form venoms and those coding for chemosensory traits. The venom genes are hypothesized to affect the capacity of the wasp to overcome how immune systems, whereas chemosensory genes are more involved in host finding and/or mate finding. The latter will then also allow us to better understand the population differentiation process.