Few studies have linked density dependence of parasitism and the tritrophic environment within which a parasitoid forages. In the non-crop plant-aphid, Centaurea nigra-Uroleucon jaceae system, mixed patterns of density-dependent parasitism by the parasitoids Aphidius funebris and Trioxys centaureae were observed in a survey of a natural population. Breakdown of density-dependent parasitism revealed that density dependence was inverse in smaller colonies but direct in large colonies (>20 aphids), suggesting there is a threshold effect in parasitoid response to aphid density. The CV2 of searching parasitoids was estimated from parasitism data using a hierarchical generalized linear model, and CV2>1 for A. funebris between plant patches, while for T. centaureae CV2>1 within plant patches. In both cases, density independent heterogeneity was more important than density-dependent heterogeneity in parasitism. Parasitism by T. centaureae increased with increasing plant patch siz! e. Manipulation of aphid colony size and plant patch size revealed that parasitism by A. funebris was directly density dependent at the range of colony sizes tested (50-200 initial aphids), and had a strong positive relationship with plant patch size. The effects of plant patch size detected for both species indicate that the tritrophic environment provides a source of host density independent heterogeneity in parasitism, and can modify density-dependent responses. [copyright] 2007 Gessellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved. Uroleucon jaceae (Aphidoidea): Parasite; Aphidius funebris (Aphidiidae): Host. Aphidius funebris: Food plants, Centaurea nigra, Patch size, hymenopteran parasitism risk, Hemipteran parasites, Uroleucon jaceae, Parasitism risk, food plant patch size, Population density, England, Hertfordshire; Uroleucon jaceae: Hymenopteran hosts, Aphidius funebris, Parasitism risk, England, Hertfordshire. Arthropods; Hymenopterans; Insects; Invertebrates; True Bugs. none