In the present study the nestedness between bipartite networks of plants, aphids and parasitoids species of an organic citrus grove located at La Selva del Camp (Tarragona, NE Spain) was determined to analyze the dynamics of the plant-aphid-parasitoid system and establish the possible reservoir of citrus aphids and their rates of parasitism. The results showed a low nested in plant-aphid assemblages, because the aphids are specialized forms to exploit certain types of plants, while the aphid-parasitoid assemblages were clearly nested because there are species of parasitoids generalists; such is the case of the parasitoid Lysiphlebus testaceipes, one of the most common and abundant on this type of crop, which interacts with various species of aphids. This parasitoid produced a high rate of parasitism in two species of aphids on two species of plants associated with citrus, Dysaphis pyri on Pyrus communis and Sipha maydis in Avena barbata. This positive nesting is also due to the heterogeneity of aphids attacked, because if there are few species of aphids, would lead to a competitive exclusion and therefore fewer species of parasitoids.